October 15, 2014 EXISTENCE OF FINITE BASES FOR QUASI-EQUATIONS OF UNARY ALGEBRAS WITH 0
|
|
- Daniella Bryant
- 5 years ago
- Views:
Transcription
1 October 15, 2014 EXISTENCE OF FINITE BASES FOR QUASI-EQUATIONS OF UNARY ALGEBRAS WITH 0 D. CASPERSON, J. HYNDMAN, J. MASON, J.B. NATION, AND B. SCHAAN Abstract. A finite unary algebra of finite type with a constant function 0 that is a one element subalgebra, and whose operations have range 0, 1}, is called a 0, 1}-valued unary algebra with 0. Such an algebra has a finite basis for its quasi-equations if and only if the relation defined by the rows of the non-trivial functions in the clone form an order ideal. 1. Introduction An ongoing question is to determine which finite algebras have a finite basis for their equations or quasi-equations. G. Birkhoff s early work [2] shows that a finite unary algebra with finite type has a finite basis for its equations. More recently, V. K. Kartashov considered commutative unary algebras, that is, unary algebras where every pair of basic operations commute. He showed that every variety of commutative unary algebras of finite type has a finite basis of equations [9]. Note that Kartashov s result includes non-finitely-generated varieties. Mal cev [10] proved that every variety of mono-unary algebras (unars) has a one-element basis of equations. Kartashov has also shown (see [8]) that every finite mono-unary algebra has a finite basis of quasi-equations. Non-existence of finite basis results include that of I. P. Bestsennyi, who showed in [1] that a 3-element unary algebra of finite type does not have a finite basis for its quasi-equations if and only if it has as a term reduct one of three bad algebras. Since then, Hyndman [6] showed that any finite unary algebra of finite type with a pp-acyclic relation does not have a finite basis for its quasi-equations. The connection between these results is that the three bad algebras of Bestsennyi all have a pp-acyclic relation. Continuing with the flavour of non-existence of a finite basis, Casperson and Hyndman in [5] show that if the graph of a group operation can be defined using positive primitive formulas, then a finite unary algebra of finite type does not have a finite basis for its quasi-equations. When working with a finite unary algebra, the clone of non-trivial operations can be presented as a table of elements. Properties of the rows of this table can be used to determine if the algebra has a finite basis for its quasi-equations. For particular finite unary algebras that we call 0, 1}-valued unary algebra with 0, and define in the next section, we show that the rows form an order ideal if and only if the algebra has a finite basis for its quasi-equations. 2. 0, 1}-Valued Unary Algebras with 0: Definitions and Main Result Consider a finite unary algebra of finite type, M, with constant 0 that is a oneelement subalgebra and such that the range of all basic operations is included in 0, 1}. Assume that the clone of functions of M is f 0, f 1,..., f s, f s+1 } where f 0 is 1
2 2 CASPERSON, HYNDMAN, MASON, NATION, AND SCHAAN the constant 0 function and f s+1 is the identity function. We call such an algebra a 0, 1}-valued unary algebra with 0. These algebras are by definition finite and of finite type, and hence generate locally finite varieties. See Figure 1. f 0 f 1... f s f s or or m 0 0 or or 1 m... Figure 1. A generic 0, 1}-valued unary algebra with 0. Let M be a 0, 1}-valued unary algebra with 0. For c M define row(c) 0, 1} s to be the tuple f 1 (c),..., f s (c). Let Rows(M) be the s-ary relation Rows(M) = row(c) c M}. This relation is referred to as the rows of M. Relations defined by positive primitive formulas (see Section 6) are important to the quasi-equational theory of an algebra. We note in passing that Rows(M) is a positive primitively defined relation. For ζ Rows(M) and for c in M, when ζ = row(c) we say that ζ is witnessed by c. Note that there may be multiple witnesses for ζ. A partial order on Rows(M) is induced by the order on 0, 1} with 0 < 1. Rows(M) is an order ideal if for every ζ in Rows(M) and every σ 0, 1} s with σ ζ, the row σ is also in Rows(M). This order is important, and indeed the main result of this article is Theorem 31, which has the following as an immediate consequence.... Corollary 32. Let M be a finite 0, 1}-valued unary algebra with 0. Then M has a finite basis for its quasi-equations if and only if the rows of M form an order ideal. 3. Quasicritical Algebras One direction of the proof of Theorem 31 consists of showing that algebras whose rows form an order ideal have finite bases for their quasi-equations. To show that a finite algebra has a finite basis for its quasi-equations we use the concept of quasicritical algebras. Let Q(M) = ISP(M) be the quasivariety generated by M, and let Q n (M) be the quasivariety of all algebras in HSP(M) that satisfy the at most n-variable quasi-equations of M. For M finite, of finite type, and in a locally finite variety, for a given N there are only finitely many quasi-equations with at most N variables. To show that such an M has a finite basis for its quasiequations, it suffices to show that there exists an N such that for all algebras E we have E Q N (M) implies E ISP(M). In fact, we can restrict our attention to algebras E that are finite and quasicritical. We first define quasicriticality and explore the concept generally, and then show how it applies to 0, 1}-valued unary algebras with 0.
3 3 A finite algebra E is quasicritical if it is not isomorphic to any subdirect product of its proper subalgebras. V. A. Gorbunov shows in [3, 4] that the lattice of subquasivarieties of a locally finite quasivariety V is finite if and only if the number of quasicritical algebras in V is finite. The proof of this utilizes the facts that finite subdirectly irreducible algebras are quasicritical and that distinct quasicritical algebras generate distinct quasivarieties. In fact, given two quasivarieties U and V of the same type, if U is a proper sub-quasivariety of V, then there is a quasicritical algebra in V that is not in U. This allows a proof technique to show that two quasivarieties are equal by showing that all quasicritical algebras in one are already in the other, and vice versa. Thus counting (as in [1]) or classifying the quasicritical algebras can assist with determining the existence of a finite basis. Here we develop techniques that give explicit embeddings of appropriate finite quasicritical algebras into powers of M. The next lemma indicates that, for locally finite varieties, looking at the finite quasicritical algebras is sufficient. This lemma underpins the proof of Theorem 8. Lemma 1. Let M be a finite algebra in a locally finite variety. If E is an algebra in HSP(M) such that E satisfies the n-variable quasi-equations of M but not all of the (n + 1)-variable quasi-equations of M, then there is a finite quasicritical subalgebra of E that satisfies the n-variable quasi-equations of M but not all of the (n + 1)-variable quasi-equations of M. Proof. Let E satisfy the n-variable quasi-equations of M, and let Υ be an (n + 1)- variable quasi-equation of M that E does not satisfy. There are elements a 0, a 1,..., a n of E that invalidate Υ. Consider E, the subalgebra of E generated by a 0, a 1,..., a n. This finite subalgebra will satisfy all quasi-equation that E satisfies, in particular, E satisfies the n-variable quasi-equations of M. However, E does not satisfy Υ. Now choose some Ê E minimal with respect to satisfying the n-variable quasiequations of M but not all of the (n + 1)-variable quasi-equations of M. By the above Ê is finite. If Ê is not quasicritical, then it is a subdirect product of proper subalgebras of itself. By minimality of Ê each of these subalgebras satisfies the (n + 1)-variable quasi-equations of M. As Ê is isomorphic to a subalgebra of a product of algebras satisfying the (n + 1)-variable quasi-equations of M, we get, for a contradiction, that Ê satisfies the (n + 1)-variable quasi-equations of M. Lemma 2. Let M be a finite algebra of finite type in a locally finite variety. If, in the variety generated by M, every finite quasicritical algebra satisfying the n- variable quasi-equations of M is in the quasivariety generated by M, then the n- variable quasi-equations form a basis of the quasi-equations of M. Proof. We prove the contrapositive. Assume that the n-variable quasi-equations do not form a basis of the quasi-equations of M. As the quasivariety generated by M is different from the quasivariety determined by the n-variable quasi-equations, there is a quasicritical algebra E that satisfies the n-variable quasi-equations of M but not all of the quasi-equations of M. There must be an s n such that E satisfies all of the s-variable quasi-equations of M but not all of the (s + 1)-variable quasi-equations of M. By Lemma 1, we may assume that E is finite. Choose an (s+1)-variable quasi-equation Υ of M that does not hold in E. Since s n, the finite algebra E satisfies the n-variable quasi-equations of M but is not in the quasivariety generated by M.
4 4 CASPERSON, HYNDMAN, MASON, NATION, AND SCHAAN Corollary 3. Let M be a finite algebra in a locally finite variety. If there are only finitely many finite quasicritical algebras in the variety generated by M, then there is a finite basis for the quasi-equations of M Quasicritical Unary Algebras. For finite unary algebras with a one-element subalgebra 0}, there are restrictions on what the structure of a quasicritical algebra can be. Lemma 4. Assume E is a unary algebra that has a one-element subalgebra 0}. If E = A B where A B = 0} and A and B are proper subalgebras of E, then E is not quasicritical. Proof. Let α: E A B be given by (x, 0) if x A, α(x) = (0, x) if x B. Then α is a subdirect embedding. An irredundant generating set of an algebra E is a subset D such that D generates E but no proper subset of D does. For A any subset of a unary algebra E, the subalgebra generated by A is Sg E (A) = a A SgE (a}). This implies that when D is a generating set of E for d D we have E = Sg E (D) = Sg E (D \ d}) Sg E (d}). Thus, when D is an irredundant generating set and d f(d) for all non-identity functions f in the clone, the set E \ d} is a subalgebra of E. Lemma 5. Assume E is a unary algebra that has an irredundant generating set that contains distinct a, b, and c. If f(a) = f(b) = f(c) for all terms f that are not the identity map, then E is not quasicritical. Proof. Let D be the irredundant generating set containing a, b, and c. Let A = E \ c}. If c = f(c) for some non-identity term f then c = f(a) which contradicts the minimality of the generating set. Thus A is a proper subalgebra of E. Embed E into A A via (x, x) if x A, α(x) = (a, b) if x = c. As f(α(c)) = f((a, b)) = (f(a), f(b)) = (f(c), f(c)) = α(f(c)) for any non-identity term f, the map α is a subdirect embedding Quasicritical: 0, 1}-Valued Unary Algebras with 0. We now turn our attention to the nature of finite quasicritical algebras in the variety generated by M, a 0, 1}-valued unary algebra with 0. The following observations are used frequently. Because 0 forms a one-element subalgebra, the constant 1 is not a function in the clone, while the constant 0 function is f 0. Thus the range of each f i is 0, 1} for 1 i s. For i s and j s, we have on M that f 0 if f i (1) = 0, (1) f i f j = f j if f i (1) = 1.
5 5 This holds as 0 forms a one-element subalgebra, so that f i (0) = 0. Equation (1) implies that the non-trivial functions in the clone must correspond to basic operations of the algebra. As we continually are concerned with whether f i (1) = 1 or not, we partition 0, 1, 2..., s} as I 0 I 1 such that for a in 0, 1} and i in I a we have f i (1) = a. Thus for i 0 I 0 and i 1 I 1 the following equations hold in M for 0 j s: (2) f i0 (f j (x)) f 0 (x) 0 and f i1 (f j (x)) f j (x). Note that (3) row(1)(i) = 1 if i I 1, 0 if i I 0. Lemma 6. Let M be a 0, 1}-valued unary algebra with 0. Let E be a finite quasicritical algebra in the variety generated by M, with E having at least three elements. Let D be an irredundant set of generators of E and set C = E \ (D 0 E }). For every c C and every d D, we have f j (d) j s} C 0 E } and c if i I 1, f i (c) = 0 E if i I 0. Moreover, for each d D there exists an i s with f i (d) C, and consequently C is non-empty. Proof. Suppose that for some d D and some j s, we have d = f j (d). Then for all i s we have f i (d) = f i (f j (d)) 0 E, f j (d)} = 0 E, d} by Equation (2). Thus 0 E, d} is a subalgebra of E, and, as E has at least three elements, 0 E, d} is a proper subalgebra. As D is irredundant d Sg E (D \ d}). This fact and Lemma 4 imply that E is not quasicritical, which is a contradiction. Thus we have d f j (d) for every d D and every j s. Together with irredundancy of D this implies D f j (D) = for j s. Hence f j (D) C 0 E } for each j s. However, if for some d D we have f i (d) = 0 E for every i s then Sg E (d) = 0 E, d} and, by Lemma 4, E is not quasicritical. Thus we may assume for each d D there is an i s with f i (d) C. For c C there exist j s and d D with c = f j (d). For i s we have f i (c) = f i (f j (d)) 0 E, f j (d)} = 0 E, c} with f i (c) = c when i I 1. The next lemma gives us a complete description of the quasicritical algebras in the quasivariety generated by a 0, 1}-valued unary algebra with 0 when there is at most one non-trivial function in the clone. Thus, after this lemma we freely assume that there are at least two non-trivial functions. Lemma 7. Let M be a 0, 1}-valued unary algebra with 0 with at most one nonzero, non-identity function in its clone. Then HSP(M) has finitely many quasicritical algebras, and therefore ISP(M) has a finite basis for its quasi-equations. Proof. If M has no non-trivial functions in its clone, then the only quasicritical algebra in HSP(M) is the two-element algebra. Thus we may assume that there is a non-trivial function. Let f be the non-zero, non-identity function in the clone of M. Either f 2 = f or f 2 = f 0, the constant 0-valued function. Let E be a finite quasicritical algebra in the variety generated by M with at least 3 elements. Fix D an irredundant set
6 6 CASPERSON, HYNDMAN, MASON, NATION, AND SCHAAN of generators of E, and set C = E \ (D 0 E }). By Lemma 6, C is non-empty; and for each d in D there is some non-trivial function that maps d into C. As the only such function is f, we have that f(d) is in C. In addition, for each c C there exists d D with f(d) = c. Suppose there are at least two elements in C. Pick one, say c, and let D c = d D : f(d) = c}. Then D c 0 E, c} is a subalgebra of E as is E \ (D c c}). Their intersection is 0 E }, so E is not quasicritical by Lemma 4. Thus C is a singleton. By Lemma 5, D has at most two elements. Thus every finite quasicritical algebra E has at most four elements. By Corollary 3, M has a finite basis for its quasi-equations. 4. When the Rows Form an Order Ideal Consider M 1 whose operations are those shown in Figure 2. This 0, 1}-valued f 0 f 1 f 2 f Figure 2. Rows(M 1 ) form an order ideal. unary algebra with 0 has rows which form an order ideal. The next theorem applies to algebras like M 1 to show that they have a finite basis for their quasi-equations. Theorem 8. Let M be a finite 0, 1}-valued unary algebra with 0 whose rows form an order ideal. Then M has a finite basis for its quasi-equations. The remainder of this section is the proof of Theorem 8. Recall that the clone of functions of M is f 0, f 1,..., f s, f s+1 } where f 0 is the constant 0 and f s+1 is the identity function. By Lemma 7 we may assume s 2. Set N = 2 + M ( M 1). As M has at least two elements N 4. We wish to show that the N-variable quasi-equations form a basis. Let E be an arbitrary finite quasicritical algebra that satisfies the N-variable quasi-equations of M. By Lemma 2 it suffices to show that E is in ISP(M). If E has at most N elements and satisfies the N-variable quasiequations, then E satisfies all quasi-equations of M and hence is in ISP(M). Thus we also assume that E has at least N elements. The proof starts with partitioning of the elements of the quasicritical algebra. Let D be an irredundant set of generators of E. Set C = E \ (D 0 E }). By Lemma 6, C is non-empty and for every c C and every d D we have f j (d) j s} C 0 E }, f i (c) i s} 0 E, c}, and for every d D there is some j s with f j (d) C. To simplify notation in various situations we let f(x) denote the tuple f 1 (x),..., f s (x). Throughout the remainder of Section 4, where E denotes a finite quasicritical algebra, we utilize the above notation and assumptions.
7 The Quasi-equational Order Ideal Property. In this subsection we develop the concept of the quasi-equational order ideal property and show that it is equivalent to Rows(M) being an order ideal. For i 0 and T chosen such that i 0 T 1, 2,..., s}, let Σ T i 0 be the one-variable quasi-equation [ ] i,j T & f i (x) f j (x) f i0 (x) 0, and let Γ T i 0 be the associated (at most) s-variable quasi-equation [ ] i,j T & z i z j z i0 0. Notice that for a M we have that row(a) = f 1 (a), f 2 (a),..., f s (a) satisfies Γ T i 0 if and only if a satisfies Σ T i 0. For i 0 T the operation f i0 is not the constant 0 map, so if M satisfies some Σ T i 0 then T 2. We say that M satisfies the quasi-equational order ideal property when for every tuple σ in 0, 1} s \ Rows(M), there exist T 1, 2,..., s} and i 0 T such that M satisfies Σ T i 0 but σ does not satisfy Γ T i 0. Within algebras that satisfy this property we look for failure of Γ T i 0 rather than prove directly that Σ T i 0 holds. The next two lemmas guarantee the existence of particular rows in Rows(M) when M satisfies the quasi-equational order ideal property. Lemma 9. Suppose that M satisfies the quasi-equational order ideal property. Let E be an algebra in HSP(M) satisfying the 1-variable quasi-equations of M. For all ζ 0, 1} s if there exists e E, and c E with c 0 E, such that then we have ζ Rows(M). ζ(i) = 1 implies f i (e) = c, Proof. Let ζ 0, 1} s and assume that c, e E with c 0 E have the property that ζ(i) = 1 implies f i (e) = c. Note that if c D, then, as f i (e) D, we have that ζ is the zero tuple which is in Rows(M), so we may assume that c C. If for every Σ T i 0 that holds in M we obtain Γ T i 0 holding for ζ, then we must have ζ in Rows(M), as otherwise we would have a contradiction to the quasi-equational order ideal property. Suppose that Σ T i 0 holds in M and hence in E. We show that Γ T i 0 holds for ζ. Consider what happens when we set x = e in Σ T i 0. Either the hypothesis of Σ T i 0 holds, in which case the result holds, to wit, f i (e) i T } = 0 E } so ζ(i) i T } = 0}; or the hypothesis of Σ T i 0 fails and f i (e) i T } is not the singleton c}. Since ζ(i) = 1 implies f i (e) = c it follows that ζ(i) i T } is not the singleton 1}. Thus, either ζ(i) i T } = 0} or the hypothesis of Γ T i 0 fails. In either case Γ T i 0 is satisfied by ζ. By the first paragraph, ζ is in Rows(M). Corollary 10. Suppose that M satisfies the quasi-equational order ideal property. Let E be an algebra in HSP(M) satisfying the 1-variable quasi-equations of M. Then for c, e E, with c 0 E, the tuple ζc e defined by ζc e 1 if f i (e) = c, (i) = 0 otherwise is in Rows(M).
8 8 CASPERSON, HYNDMAN, MASON, NATION, AND SCHAAN Lemma 11. For M a 0, 1}-valued unary algebra with 0, we have that Rows(M) is an order ideal if and only if M satisfies the quasi-equational order ideal property. Proof. Assume that Rows(M) is an order ideal. Suppose that σ 0, 1} s but σ is not in Rows(M). Let T = i: σ(i) = 1}. As σ row(0) the set T is non-empty. For any i 0 in T the quasi-equation Σ T i 0, which is [ ] i,j T & f i (w) f j (w) f i0 (w) 0, is satisfied by M. To see this, note that the failure of this quasi-equation to hold in M implies the existence of an element m in M with f i (m) = 1 for i T. Since σ row(m) for any such m and Rows(M) forms an order ideal, we have σ in Rows(M), a contradiction. However, the corresponding Γ T i 0 fails for any τ σ, in particular for σ itself. Thus M satisfies the quasi-equational order ideal property. Now assume that M satisfies the quasi-equational order ideal property. Suppose that m M, and ζ 0, 1} s is such that ζ row(m). Set E = M, e = m, and c = 1, so that ζ(i) = 1 implies f i (m) = 1, that is, f i (e) = c. By Lemma 9, we have that ζ is in Rows(M). Thus Rows(M) is downward closed, that is, an order ideal Homomorphisms. To show that E ISP(M) it is sufficient to show that for any pair of distinct elements in E there is a homomorphism separating them, that is show that for every pair (e 1, e 2 ) E 2 \ E, there is a homomorphism h: E M such that h(e 1 ) h(e 2 ). That E ISP(M) follows, as then the map E M Hom(E,M) given by e h(e) : h Hom(E, M) is an embedding. Under the hypotheses of Theorem 8 and the quasicriticality of E, we now construct these homomorphisms. In what follows, let and J 1 = (a, b) D 2 : a b and f(a) = f(b)} J 2 = (c, d) C D : f(c) = f(d)}. The overview for the remainder of this section is as follows. Lemma 12 shows that J 1 J 2 has at most 1 element. Corollary 14 shows that for c C, we can construct a homomorphism h c : E M such that h c (c) h c (c ) for c E \ D. For (a, b) J 1, Lemma 17 provides a homomorphism h: E M such that h(a) h(b). For (c, d) J 2, Lemma 22 provides a homomorphism h: E M such that h(c) h(d). Lemma 23 assembles this information to show that there is a homomorphism separating any pair of distinct elements of E, thereby completing the proof of Theorem 8. Lemma 13 and Corollary 14 construct homomorphisms from E to M that separate elements of C. Note that Lemma 13 does not assume that M has the quasiequational order ideal property, but Corollary 14 does. The next lemma demonstrates that, in a quasicritical algebra, there are very few pairs of elements (u, v) with f(u) = f(v). We would like to thank the anonymous referee for this observation. Lemma 12. The set J 1 J 2 has at most one element. Proof. Suppose that (a, b) and (u, v) are in J 1 J 2. Thus b and v are in D. The sets A = E \ b} and B = E \ v} are proper subuniverses of E as D is a minimal
9 9 generating set. Embed E into A B via (x, x) if x A B, α(x) = (a, b) if x = b, (v, u) if x = v. As f i (h(b)) = f i ((a, b)) = (f i (a), f i (b)) = (f i (b), f i (b)) = h(f i (b)) and f i (h(v)) = f i ((v, u)) = (f i (v), f i (u)) = (f i (v), f i (v)) = h(f i (v)) for 0 i s, the map α is a subdirect embedding, contradicting the quasicriticality of E. Lemma 13. Let M be a 0, 1}-valued unary algebra with 0, and let E HSP(M) be quasicritical, have at least three elements, and satisfy the 1-variable quasi-equations of M. Let D be an irredundant set of generators of E. Set C = E \ (D 0 E }), and fix an element c C. For each d D, let ρ d 0, 1} s be defined by ρ d (j) = 1 if and only if f j (d) = c. Suppose that h: D M is a map such that h(d) witnesses ρ d for each d D, that is, row(h(d)) = ρ d. Then (1) h extends uniquely to a homomorphism ĥ: E M; (2) ĥ(c) = 1; and (3) ĥ(c ) = 0 for c / D c}. Proof. As D is a generating set, any homomorphism that extends h is unique. To show that such a homomorphism exists, set h(e) when e D; ĥ(e) = 1 when e = c; 0 otherwise. We now show for 0 j s + 1 and d D that (4) ĥ(f j (d)) = f j (h(d)). Consider any d in D. For j = 0 we have ĥ(f 0(d)) = ĥ(0e ) = 0 = f 0 (h(d)). For j = s + 1 we have ĥ(f s+1(d)) = ĥ(d) = h(d) = f s+1(h(d)). We now consider the cases where 1 j s. Since row(h(d)) = ρ d, we have f i (h(d)) = ρ d (i) for 1 i s. First, suppose that f j (d) = c. Then ĥ(f j(d)) = ĥ(c) = 1, and f j (h(d)) = ρ d (j). But ρ d (j) = 1 exactly when f j (d) = c. Now suppose f j (d) c, so that f j (d) / c} D and ĥ(f j(d)) = 0. On the other side we have f j (h(d)) = ρ d (j) = 0. The last equality holds because f j (d) c. Thus (4) holds and we use it to show that ĥ is a homomorphism. We wish to compute ĥ(f i(e)) for any e E and i with 0 i s + 1. Pick any d D, and any j with 0 j s + 1 such that e = f j (d). There is an r with 0 r s + 1 such that f i f j = f r. Now we compute ĥ(f i(e)) = ĥ(f i(f j (d))) = ĥ(f r (d)) = f r (h(d)) = f i (f j (h(d))) = f i (ĥ(f j(d))) = f i (ĥ(e)), showing that ĥ is a homomorphism. Corollary 14. Let M be a 0, 1}-valued unary algebra with 0 satisfying the quasiequational order ideal property, and let E HSP(M) be quasicritical, have at least three elements, and satisfy the 1-variable quasi-equations of M. Let D be an irredundant set of generators of E. Set C = E \ (D 0 E }).
10 10 CASPERSON, HYNDMAN, MASON, NATION, AND SCHAAN Then, for c C, there exists a homomorphism h c : E M such that for c E\D In particular, h c (c) 0. h c (c) = h c (c ) if and only if c = c. Proof. Fix c C. For d D, consider ρ d 0, 1} s defined by ρ d (j) = 1 if and only if f j (d) = c. By Corollary 10, ρ d is in Rows(M). Define h: D M by setting h(d) to be any witnesses that ρ d is in Rows(M). Then by Lemma 13 there is a homomorphism h c : E M extending h such that h c (c) = 1 and for c c, with c / D, we have h(c ) = 0. Under certain circumstances Corollary 14 suffices to prove Theorem 8. If M satisfies the two-variable quasi-equation f(x) f(y) x y, then so does E. The quasi-equation f(x) f(y) x y holding in M says that row(c 1 ) = row(c 2 ) implies that c 1 = c 2. In this case we say that the relation Rows(M) is uniquely witnessed. When Rows(M) is uniquely witnessed, we have J 1 J 2 = and we shall see in Lemma 23 that h c c C} separates points in E. In general we need more homomorphisms. Recall that J 1 = (a, b) D 2 : a b and f(a) = f(b)}. Lemma 17 states that if (a, b) in J 1, then there is a homomorphism h: E M such that h(a) h(b), that is, there are homomorphisms to separate the elements in a pair in J 1. From now through the proof of Lemma 17 we assume that (a, b) J 1 and use the following algebras and formulas. Let E 0 = Sg E (a, b}), and let E ω be the algebra whose universe is given by E ω = e E : Sg E (e}) E 0 = 0 E }}. By Lemma 6, C E 0 E ω. We now define various formulas that relate to the existence of appropriate homomorphisms. Let ψ 0 (w a, w b ) be the two variable formula, and for e E let ψ e (w a, w b, w e ) be the three variable formula, defined by: ψ 0 (w a, w b ): f(w a ) f(w b ) & & f i (w a ) f j (w a ); f i(a)=f j(a) ψ e (w a, w b, w e ): ψ 0 (w a, w b ) & & f i (w e ) f j (w a ). f i(e)=f j(a) The formula ψ 0 identifies elements that behave like a and b, while ψ e captures how e interacts with a and b. Note that for (a, b) J 1 and e E, the formulas ψ 0 (a, b) and ψ e (a, b, e) hold in the algebra E. Let H 1 be the set of homomorphisms H 1 = h Hom(E 0 E ω, M): h(a) h(b) and h Eω 0}. We now prove two technical lemmas about homomorphisms before proceeding to the proof of Lemma 17. Lemma 15. The set H 1 is non-empty. Moreover, for every pair (m a, m b ) of distinct elements in M 2 such that ψ 0 (m a, m b ) holds in M, there is exactly one h H 1 such that h(a) = m a and h(b) = m b.
11 11 Proof. The quasi-equation ψ 0 (w a, w b ) w a w b fails in E 0 (i.e, in E) with w a = a and w b = b, so it must also fail in M. This means that there exist distinct m a, m b M with ψ 0 (m a, m b ) holding. We demonstrate one-to-one correspondences between H 1, h E0 h H 1 }, and (h(a), h(b)) h H 1 }. Let H 0 = h Hom(E 0, M): h(a) h(b)}. The map ext : H 0 H 1 given by [ext h 0 ](x) = h 0 (x) x E 0, 0 x E ω ; is the inverse of the restriction map E0 : H 1 H 0, so H 0 and H 1 are in one-to-one correspondence. For h H 0, as a, b} generates E 0 it is clear that (h(a), h(b)) completely defines h in H 0 and ext h in H 1. Pick m a m b such that ψ 0 (m a, m b ) holds in M. This allows us to construct a homomorphism h H 0. To start, set h(a) = m a and h(b) = m b. As E 0 is generated by a, b} and f(a) = f(b), for every e E 0 \ a, b} there is a non-identity term f i with i 0, 1,..., s} such that f i (a) = f i (b) = e. Set h(e) = h(f i (a)) = f i (m a ). If f i (a) = f j (a) then ψ 0 (m a, m b ) implies that f i (m a ) = f j (m a ), so h is a well-defined homomorphism. By the preceding paragraph this homomorphism extends uniquely to H 1. The following lemma connects the formula ψ e (w a, w b, w e ) with the ability to lift homomorphisms. Lemma 16. Let E 2 be a proper subalgebra of E that contains E 0 E ω, and let h : E 2 M be a homomorphism with h Eω 0. Then, for e E \ E 2 there is a homomorphism ĥ: E 2 e} M extending h if and only if there exists an m M with ψ e (h(a), h(b), m) holding in M. Proof. Set L = i 1,..., s}: f i (e) / E w }. Note that, since f i (e) C E 0 E ω, we have f i (e) E ω implies f i (e) E 0. Suppose that there exists an m M such that ψ e (h(a), h(b), m) holds in M, that is, ψ 0 (h(a), h(b)) & & f i(e)=f j(a) f i (m) = f j (h(a)). Define ζ 0, 1} s by ζ(i) = 1 if and only if f i (m) = 1 and i L. Apply Lemma 9 (with E = M, e = m, c = 1) to conclude that ζ Rows(M). Let m witness ζ. Then m satisfies f i (m f i (m) when i L, ) = ζ(i) = 0 when i / L. Since C E 0 E ω and for any i we have f i (e) C 0 E }, it follows that E 2 e} is a subuniverse of E. Extend h to E 2 e} by ĥ(e) = m. To see that ĥ is a homomorphism consider ĥ(f i(e)). For i L we have f i (e) / E ω so f i (e) E 0 and f i (e) = f j (a) for some j. Thus ĥ(f i(e)) = h(f i (e)) = h(f j (a)) = f j (h(a)). But f j (h(a)) = f i (m) because f j (a) = f i (e) and ψ e (h(a), h(b), m) holds. Thus ĥ(f i (e)) = f j (h(a)) = f i (m) = f i (m ) = f i (ĥ(e)). For i / L, we have f i(e) E ω and ζ(i) = 0; whence ĥ(f i(e)) = h(f i (e)) = 0 = ζ(i) = f i (m ) = f i (ĥ(e)). Thus ĥ is a homomorphism.
12 12 CASPERSON, HYNDMAN, MASON, NATION, AND SCHAAN On the other hand if ĥ(e) = m is a homomorphism extending h, use the fact that ψ e (a, b, e) holds and apply ĥ to the identities in ψ e(a, b, e) to get that ψ e (h(a), h(b), m) holds. We now have the machinery to prove that we can separate elements of a pair in J 1 = (a, b) D 2 : a b and f(a) = f(b)}. Lemma 17. If (a, b) in J 1, then there is a homomorphism h a,b : E M such that h a,b (a) h a,b (b). Proof. Fix (a, b) in J 1. With notation as above, define Q, a subset of M 2 \ M, as Q = (u, v): h H 1 with (u, v) = (h(a), h(b)) and u v and h does not lift to E}. If Q is empty then every h H 1 lifts to E, and since Lemma 15 says H 1 is nonempty, we are done. Thus we assume Q is non-empty. Again by Lemma 15, for each pair q in Q there is a unique h q : E 0 E ω M in H 1 such that (h q (a), h q (b)) = q. As q Q there must be at least one element e q E such that some maximal extension of h q does not contain e q. Indeed, by Lemma 16, e q is not in the domain of any extension of h q. We proceed to demonstrate that there must be an h 1 H 1 that has an extension whose domain includes each e q. This will imply that (h 1 (a), h 1 (b)) is not in Q, that is, h 1 extends to E. Consider now the formula ψ(w a, w b, w eq : q Q}) given by ψ(w a, w b, w eq : q Q}): ψ 0 (w a, w b ) & & q Q ψ eq (w a, w b, w eq ). The formula ψ essentially describes the relationships in E between the elements a, b, and e q q Q}. Notice that this formula has at most Q +2 M ( M 1)+2 = N variables. The quasi-equation ψ(w a, w b, w eq : q Q}) w a w b fails on E as a b and we can satisfy the left hand side with w a = a, w b = b, and w eq = e q. Thus it must also fail on M, as E satisfies all the N-variable quasi-equations satisfied by M. That means that there are elements m q in M for q Q, and distinct m a, m b, such that ψ(m a, m b, m q }) holds in M. In particular ψ 0 (m a, m b ) holds in M and (by Lemma 15) there is a homomorphism h 1 H 1 such that h 1 (a) = m a and h 1 (b) = m b. As each ψ eq (m a, m b, m q ) holds in M, each formula w ψ eq (m a, m b, w) holds in M, and by Lemma 16 any maximal extension of h 1 contains e q q Q} ( in its domain. Thus h 1 h q for all q Q, which implies that (m a, m b ) = h1 (a), h 1 (b) ) / Q, so h 1 lifts to E. Set h a,b to be an extension of h 1 to E. We now turn our attention to J 2. Recall that J 2 = (c, d) C D : f(c) = f(d)}. In addition I 0 = i 0, 1, 2,..., s}: f i (1) = 0} and I 1 = i 1, 2,..., s}: f i (1) = 1}. For i 0 I 0 and i 1 I 1 and any j, we have f i0 f j = f 0 and f i1 f j = f j. Lemma 18. Assume (c, d) J 2. Then I 1 and for 1 i s, c if i I 1, f i (d) = 0 E if i I 0.
13 13 Proof. By Lemma 6, f i (c) = c if i I 1, 0 E if i I 0. Since f(d) = f(c), the same holds for each f i (d). Again by Lemma 6, there is some i with f i (d) C, whence I 1 is non-empty. Note that the proofs of the next few lemmas use at most 2-variable quasiequations, whereas the proof of Lemma 17 used an N-variable quasi-equation. Lemma 19. If J 2 is non-empty, then either row(0) has two distinct witnesses or row(1) has two distinct witnesses. Proof. Assume that (c, d) J 2 with f(c) = f(d). By Lemma 18, I 1. For each i 1 I 1 we have f i1 (d) = f i1 (c) = c, and for i 0 I 0 we have f i0 (d) = f i0 (c) = 0 E. Consider the 2-variable quasi-equation Υ(x, y) [ ] f(x) f(y) & i1 I & f i1 (x) x & & f i0 (x) 0 x y. 1 i0 I 0 In E the pair (c, d) satisfies the hypothesis of Υ(x, y) but c d. Thus Υ does not hold in E, and consequently it must not hold in M either. In M, if the hypothesis of Υ(x, y) holds then x 0, 1}, so the failure of Υ(x, y) in M implies that either row(0) or row(1) is not uniquely witnessed. Lemma 20. Suppose that (c, d) J 2 and that 0 and m are distinct witnesses of row(0). Then h: E M defined by m if x = d h(x) = 0 otherwise is a homomorphism with h(c) h(d). Proof. For 1 j s, we have f j (h(d)) = f j (m) = 0 as m witnesses row(0); and h(f j (d)) = h(f j (c)) = 0 as f j (c) d. For u d we have f j (h(u)) = f j (0) = 0 and h(f j (u)) = 0. Thus h is a homomorphism with the desired property. Lemma 21. Suppose that (c, d) J 2 and that 1 and m are distinct witnesses of row(1). Then there is a homomorphism h: E M defined so that h(c) = 1 and h(d) = m. Proof. For u D define ρ u 0, 1} s by ρ u (j) = 1 if and only if f j (u) = c. By Corollary 10, ρ u is in Rows(M) so has a witness y u in M. Define h : D M by h m if u = d (u) = otherwise. y u By Lemma 18, f j (d) = c if and only if j I 1. Thus, by Equation (3), ρ d = row(1) which is also row(m). The map h satisfies the hypothesis of Lemma 13 so extends to a homomorphism h: E M with h(c) = 1. Lemma 22. If (c, d) J 2, then there is a homomorphism h c,d : E M such that h c,d (c) h c,d (d).
14 14 CASPERSON, HYNDMAN, MASON, NATION, AND SCHAAN Proof. Assume that (c, d) J 2. By Lemma 19 either row(0) has distinct witnesses or row(1) has distinct witnesses. Use Lemma 20 or Lemma 21, respectively, for the existence of the desired homomorphism. Finally we show that E is actually in ISP(M) by verifying that for all (e 1, e 2 ) E 2 \ E there is a homomorphism h separating e 1 and e 2. Lemma 23. For all (e 1, e 2 ) E 2 \ E there is a homomorphism h: E M such that h(e 1 ) h(e 2 ). The homomorphism may be chosen from h c c C} h a,b (a, b) J 1 } h c,d (c, d) J 2 } as defined in Corollary 14, Lemma 17, and Lemma 22 respectively. Proof. Let e 1 and e 2 be distinct elements in E. If both e 1 and e 2 are in E \D = C 0 E }, then without loss of generality e 1 = c C. By Corollary 14, h c (e 1 ) h c (e 2 ). For the rest of the proof we assume that e 2 D. If f(e 1 ) f(e 2 ), then there is a basic term operation f i with f i (e 1 ) f i (e 2 ). Pick c f i (e 1 ), f i (e 2 )} C. Let c denote the other element of f i (e 1 ), f i (e 2 )}. Using the homomorphism h c of Corollary 14 we have f i (h c (e 1 )), f i (h c (e 2 ))} = h c (f i (e 1 )), h c (f i (e 2 ))} = h c (c), h c (c )}, a two-element set. Thus f i (h c (e 1 )) f i (h c (e 2 )) and consequently h c (e 1 ) h c (e 2 ). The only cases that remain are when f(e 1 ) = f(e 2 ) while still having e 2 D. Then (e 1, e 2 ) J 1 J 2. (We cannot have e 1 = 0 E by Lemma 6.) Thus by either Lemma 17 or Lemma 22 we can construct a homomorphism h such that h(e 1 ) h(e 2 ). Corollary 24. For E in HSP(M) with E finite, quasicritical, and satisfying the N-variable quasi-equations of M, we have E in ISP(M). Proof. The map E M Hom(E,M) given by e h(e) : h Hom(E, M) is an embedding. The proof of Theorem 8 is now complete, as every finite quasicritical algebra that satisfies the N-variable quasi-equations is actually already in the quasivariety. Lemma 2 shows that the N-variable quasi-equations form a basis of the quasi-equations of M. An interesting observation is that the proof of Lemma 17 potentially uses all N variables for a pair in J 1, while the proofs for J 2 and unique witnesses only require 2-variable quasi-equations. 5. When the Rows Do Not Form an Order Ideal The content of this section is the proof of the converse of Theorem 8: that a finite 0, 1}-valued unary algebra with 0 whose rows do not form an order ideal does not have a finite basis for its quasi-equations. We first do this for algebras on a 4-element universe and then generalize to an arbitrary finite algebra. As before, throughout this section assume that M is a finite 0, 1}-valued unary algebra with 0 and that the clone is f 0, f 1, f 2,..., f s+1 where f 0 is the constant 0
15 15 function and f s+1 is the identity map. To prove the converse of Theorem 8 we need conditions for the non-existence of a finite basis for the quasi-equations of M. Results and definitions from [5] and [6] give these conditions. A positive primitive formula is an existentially quantified conjunction of atomic formulas. In the case of unary algebras the atomic formulas are of the form f(x) g(y) for notnecessarily-different variables x and y and terms (possibly the identity) f and g. A unary algebra M is pp-acyclic if there is a positive primitive formula φ(x, y) defining an acyclic binary relation such that there exist 0 and 1 in M with 0 1, φ(0, 0), φ(0, 1) and φ(1, 1). The simplest pp-acyclic relation is on the set 0, 1}. Theorem 25 ([6]). If M is a finite pp-acyclic unary algebra, then M does not have a finite basis for its quasi-equations. More generally, on an algebra M, an n-ary relation R is pp-defined if there is a positive primitive formula φ(x 1, x 2,..., x n ) such that R = (a 1, a 2,..., a n ) M n : φ(a 1, a 2,..., a n ) holds in M}. Theorem 26 ([5]). If M is a finite unary algebra that has a pp-defined relation that is the graph of a non-trivial group, then M does not have a finite basis for its quasi-equations. The example given in Figure 3 has the graph of addition modulo 2 defined via w [ x p(w) & y q(w) & z r(w)]. That is, the set of triples (p(m), q(m), r(m)) M 2 3 m M 2 } is (0, 0, 0), (0, 1, 1), (1, 0, 1), (1, 1, 0)}, which in turn is (x, y, z) 0, 1} 3 : x + y = z}. By Theorem 26, M 2 does not have a finite basis for its quasi-equations. However, for many examples, the positive primitive formulas are much more complex as will become evident in the upcoming proofs p q r p q r Figure 3. The graph of the two-element group is pp-defined in M 2. Theorem 27. If M is a 4-element 0, 1}-valued unary algebra with 0, then one of the following holds: (1) the relation on 0, 1} can be positive primitively defined via a formula of the form w x f(w) & y g(w); (2) the graph of addition modulo 2 on 0, 1} can be positive primitively defined via a formula of the form w x p(w) & y q(w) & z r(w); (3) the rows of M form an order ideal.
16 16 CASPERSON, HYNDMAN, MASON, NATION, AND SCHAAN In the first two cases there is no finite basis for the quasi-equations, and in the last case there is a finite basis for the quasi-equations. Proof. Let F be the non-constant, non-identity operations in the clone of M. On a four-element set the possible non-identity functions for a 0, 1}-valued unary algebra with 0 are given in Table 1. If F is empty or has one element then, by g 0 g 1 g 2 g 3 g 4 g 5 g 6 g Table 1. All possible operations of a four-element 0, 1}-valued unary algebra with 0. Lemma 7, there is a finite basis for the quasi-equations. Either Rows(M) is the empty ideal or there is one non-trivial function and the ideal formed by Rows(M) is 0, 1}. So we assume F 2. The formula w x g 3 (w) & y g 5 (w) & z g 6 (w) defines the graph of addition modulo 2, so we may assume not all of g 3, g 5, and g 6 are in F. If g 7 F then for any f F \ g 7 } the formula w x f(w) & y g 7 (w) defines on 0, 1}. Thus we may assume g 7 F. Similarly, any of the pairs g 1, g 3 }, g 1, g 5 }, g 2, g 3 }, g 2, g 6 }, g 4, g 5 }, or g 4, g 6 } can be used to define. Thus we may assume none of these pairs are in F. The remaining possible clones are g 1, g 2 }, g 1, g 4 }, g 1, g 6 }, g 2, g 4 }, g 2, g 5 }, g 3, g 4 }, g 3, g 5 }, g 3, g 6 }, g 5, g 6 }, and g 1, g 2, g 4 }. It is straightforward to check that all of these clones give order ideals. The remainder of this section is the proof that Theorem 27 can be generalized to an arbitrary finite 0, 1}-valued unary algebra with 0 if we do not specify the form of the positive primitive formulas. Lemma 28. If Rows(M) is not relatively complemented, then on 0, 1} can be positive primitively defined in M. Proof. As Rows(M) is not relatively complemented, there exists an interval of at least 3-elements, so we may assume a < c < b are in Rows(M) and the complement of c between a and b is not in Rows(M). Let S 000 = i: a(i) = c(i) = b(i) = 0} S 001 = i: a(i) = c(i) = 0 and b(i) = 1} S 011 = i: a(i) = 0 and c(i) = b(i) = 1} S 111 = i: a(i) = b(i) = c(i) = 1}. As a < c < b the disjoint union of these sets, S 000 S 001 S 011 S 111, is all of 1, 2,..., s}. In addition, as a, b, and c are distinct, the sets S 001 and S 011 are non-empty. Define Ĉ to be the formula that identifies rows which have a constant
17 17 value on each of these four sets and are 0 on S 000. That is, Ĉ(w) is Ĉ(w): & f i (w) 0 & & f i (w) f j (w) i S 000 i,j S001 & i,j S011 & f i (w) f j (w) & i,j S111 & f i (w) f j (w). When S 000 or S 111 is empty, then the conjunctions in Ĉ over S 000 or S 111 are omitted. Pick t 1 S 001, t 2 S 011. First assume S 111 is empty. This implies that a is row(0). The positive primitive formula ˆR(x, y) defined by ˆR(x, y): w Ĉ(w) & x f t 1 (w) & y f t2 (w) defines on 0, 1}. To see this, note that the witnesses of the rows a, c and b witness ˆR(0, 0), ˆR(0, 1), and ˆR(1, 1). If the witness of some row d were a witness to ˆR(1, 0) then d would be the relative complement of c in the interval between a and b. This contradicts the assumption that the relative complement of c between a and b is not in Rows(M). Now assume S 111 is non-empty and pick t 3 S 111. Note that a row(0) because a(t 3 ) = 1. Consider the relation E(x 1, x 2, x 3 ) defined by E(x 1, x 2, x 3 ): w Ĉ(w) & x 1 f t1 (w) & x 2 f t2 (w) & x 3 f t3 (w). There are two cases. First assume E(0, 1, 0) is witnessed in M. Define R(x, y) via R(x, y): w Ĉ(w) & x f t 1 (w) f t3 (w) & y f t2 (w). The zero element is witness to R(0, 0), and the witness of b gives R(1, 1) while the witness of E(0, 1, 0) gives R(0, 1). If R(1, 0) holds, then the witness to R(1, 0) is a witness to a complement of c between a and b, a contradiction. Thus is defined. Now assume E(0, 1, 0) is not witnessed in M. Define R (x, y) via R (x, y): w Ĉ(w) & 0 f t 1 (w) & x f t2 (w) & y f t3 (w). The witnesses of the zero row, a, and c witness R (0, 0), R (0, 1), and R (1, 1) respectively. R (1, 0) does not hold, as otherwise E(0, 1, 0) has a witness. Thus is again defined. Lemma 29. Assume M is a 0, 1}-valued unary algebra with 0 such that Rows(M) is relatively complemented but is not an order ideal. Then there is a pair of elements a, b Rows(M) with (1) a < b; (2) there exists c 0, 1} s \ Rows(M) with a < c < b; (3) for d < b with d Rows(M), the set of elements below d in Rows(M) form an order ideal; and (4) the interval from a to b in Rows(M) is a, b}. Proof. We start by choosing an interval of Rows(M) that is, in a sense, minimal with respect to not being an order ideal. As Rows(M) is not an order ideal and row(0) is in Rows(M), there exists a minimal element b in Rows(M) and c in 0, 1} s \ Rows(M) with row(0) < c < b. Fix c minimal with respect to this property. Thus for all d < b, if d Rows(M) then the interval below d in 0, 1} s is in Rows(M). By the minimality of c, each element strictly below c in 0, 1} s is also in Rows(M). Fix a a maximal element with row(0) a < c.
18 18 CASPERSON, HYNDMAN, MASON, NATION, AND SCHAAN We now show that the interval from a to b in Rows(M) consists exactly of a and b. Suppose d in Rows(M) with a d b. If d c = c then d c and in fact d > c and by the minimality of b, we have d = b. Otherwise, using a d c < c and the minimality of c we get that d c is in Rows(M). In fact d c = a by maximality of a. Let ˆd be the complement of d in the interval from a to b. As Rows(M) is relatively complemented, ˆd is in Rows(M). Because d c = a, by distributivity of 0, 1} s we have ˆd c. In fact, ˆd > c as c Rows(M). By the minimality of b, the element ˆd is actually b. By uniqueness of complements d = a, showing that the interval from a to b in Rows(M) consists exactly of a and b. Lemma 30. Assume M is a 0, 1}-valued unary algebra with 0 such that Rows(M) is relatively complemented but is not an order ideal. Then either there is a positive primitive formula that defines on 0, 1}, or there is a positive primitive formula that defines the graph of addition modulo 2. Proof. By Lemma 29, there are elements a, b in Rows(M) such that a < b, the interval from a to b in Rows(M) is a, b}, and there is an element c 0, 1} s \ Rows(M) with a < c < b. Set T 00 = i: b(i) = 0} T 11 = i: a(i) = 1} T 01 = 1, 2,..., s} \ (T 00 T 11 ). We show that when T 11 is non-empty we can define with a positive primitive formula. When T 11 is empty we may be able to define but, if we cannot, then addition modulo 2 is definable. Notice that T 00 and T 11 are disjoint and that a is row(0) if and only if T 11 is empty. Note that T 01 is the set of co-ordinates on which a and b differ. As a < c < b we have that T Case 1: Assume T 11 is non-empty. Let C 1 (w) be the formula that identifies rows which are constantly 0 on T 00, and are constant on T 11 and on T 01. That is, C 1 (w) is C 1 (w): & i T00 f i (w) 0 & & i,j T11 f i (w) f j (w) & Pick l 1 in T 01 and j 1 T 11. Define the formula R 1 (x, y) as & i,j T01 f i (w) f j (w). R 1 (x, y): w C 1 (w) & x f l1 (w) & y f j1 (w). From the rows a, b, and row(0) we obtain R 1 (0, 1), R 1 (1, 1), and R 1 (0, 0) respectively. When R 1 defines on 0, 1} we are done with this case. Otherwise R 1 (1, 0) holds for some witness. This means there is a row d in Rows(M) with 0 if i T 00 T 11, d(i) = 1 if i T 01. Thus d < b. By Lemma 29, we must have that Rows(M) is an order ideal below d. Thus the atom e which is 1 exactly on the co-ordinate l 1 is in Rows(M).
19 19 Let C 2 (w) be the formula that identifies rows which are constantly 0 on T 00, and are constant on T 11 T 01 \ l 1 }. That is, C 2 (w) is C 2 (w): i T00 & f i (w) 0 & & f i (w) f j (w). i,j T 11 T 01\l 1} With the same j 1 as above, define the formula R 2 (x, y) as R 2 (x, y): w C 2 (w) & x f j1 (w) & y f l1 (w). From the rows e, b, and row(0) we obtain R 2 (0, 1), R 2 (1, 1), and R 2 (0, 0) respectively. Suppose R 2 (1, 0) holds for some witness, then there is a row r with 0 if i T 00 l 1 }, r(i) = 1 if i T 11 T 01 \ l 1 }. Thus a < r < b, which is a contradiction as the interval in Rows(M) from a to b contains just a and b. We have shown that when T 11 is non-empty we can define on 0, 1}. Case 2: Now assume T 11 is empty. This means a is row(0). Pick l 1 and l 2 distinct in T 01. As f l1 and f l2 are distinct functions, there must be a row e in Rows(M) with e(l 1 ) e(l 2 ). Pick such an e minimal and, without loss of generality, assume e(l 1 ) = 0 and e(l 2 ) = 1. As the interval from a to b in Rows(M) contains exactly a and b we have e b. For β, γ 0, 1} define the subsets of 1, 2,..., s} S βγ = i: b(i) = β and e(i) = γ}. The co-ordinate l 1 is in S 10 and l 2 is in S 11. The set S 01 is non-empty as otherwise a = row(0) < e < b which cannot hold. Define C 3 (w) be the formula that identifies rows which are constantly 0 on S 00, and are constant on each of S 01, S 10, and S 11. That is, C 3 (w) is C 3 (w): & i S00 f i (w) 0 & Let R(x, y) be the formula R(x, y): i,j S01 & f i (w) f j (w) & & f i (w) f j (w) & i,j S 10 w C 3 (w) & x f l1 (w) & y f l2 (w). & i,j S11 f i (w) f j (w). R(0, 0), R(0, 1), and R(1, 1) hold via row(0), e, and b respectively. If R(1, 0) does not occur then we have defined. We shall see that when R(1, 0) does occur we can define addition modulo 2. Assume that there is an m 0, 1} and a row d with 0 if i S 00 S 11, d(i) = m if i S 01, 1 if i S 10. That is, a witness to row d is a witness to R(1, 0). If m = 0 then a < d < b, contradicting the non-existence in Rows(M) of rows between a and b, so m = 1. Pick l 3 in S 01. Let P (x, y, z) be the formula P (x, y, z): w C 3 (w) & x f l1 (w) & y f l2 (w) & z f l3 (w).
20 20 CASPERSON, HYNDMAN, MASON, NATION, AND SCHAAN P (0, 0, 0), P (1, 1, 0), P (1, 0, 1), and P (0, 1, 1) hold via witnesses of row(0), b, d, and e respectively. Any witness of P (0, 1, 0) or P (1, 0, 0) would be a witness for a non-zero row below b which cannot happen. If P (0, 0, 1) holds then a non-zero row is witnessed that is below e. Take the relative complement to this row between row(0) and e. The witness to this new row is a witness to P (0, 1, 0), a contradiction. Finally if P (1, 1, 1) has a witness then use the relative complement of e to obtain a witness to P (1, 0, 0) and a corresponding row below b, another contradiction. Thus the formula P defines the graph of addition modulo Classifying 0, 1}-Valued Unary Algebras with 0 Theorem 31. If M is a 0, 1}-valued unary algebra with 0 then one of the following holds: (1) the relation on 0, 1} can be positive primitively defined; (2) the graph of addition modulo 2 on 0, 1} can be positive primitively defined; or (3) the rows of M form an order ideal. In the first two cases there is no finite basis for the quasi-equations, and in the last case there is a finite basis for the quasi-equations. Proof. If M has one or fewer non-identity, non-zero operations then Rows(M) forms a (possibly empty) order ideal. By Lemma 7, there is a finite basis for the quasi-equations. If Rows(M) has an interval that is not relatively complemented then by Lemma 28 we can positive primitively define on 0, 1}. By Theorem 25 there is no finite basis for the quasi-equations. If Rows(M) is relatively complemented but is not an order ideal then, by Lemma 30, there is either a positive primitive formula that defines on 0, 1} or there is a positive primitive formula that defines the graph of addition modulo 2. By Theorem 25 or Theorem 26 there is no finite basis for the quasi-equations. If none of the above situations holds, then Rows(M) forms a non-empty order ideal. By Theorem 8 M has a finite basis for its quasi-equations. Corollary 32. Let M be a finite 0, 1}-valued unary algebra with 0. Then M has a finite basis for its quasi-equations if and only if the rows of M form an order ideal. References [1] I. P. Bestsennyi, Quasi-identities of finite unary algebras, Algebra and Logic, 28 (1989), [2] G. Birkhoff, On the structure of abstract algebras, Proceedings of the Cambridge Philosophical Society, 31 (1935), part 4, [3] V.A. Gorbunov, Algebraic theory of quasivarieties, translated from the Russian, Siberian School of Algebra and Logic, Consultants Bureau, New York, [4] V. A. Gorbunov, Quasi-identities of two-element algebras, Algebra and Logic, 22 (1983), no. 2, [5] D. Casperson and J. Hyndman, Primitive positive formulas preventing a finite basis of quasiequations, International Journal of Algebra and Computation, 19 (2009), no. 7, [6] J. Hyndman, Positive primitive formulas preventing enough algebraic operations, Algebra Universalis, 52 (2004), no. 2-3, [7] J. Hyndman and J. Pitkethly, How finite is a three-element unary algebra?, International Journal of Algebra and Computation, 15 (2005),
EXISTENCE OF FINITE BASES FOR QUASI-EQUATIONS OF UNARY ALGEBRAS WITH 0
EXISTENCE OF FINITE BASES FOR QUASI-EQUATIONS OF UNARY ALGEBRAS WITH 0 D. CASPERSON, J. HYNDMAN, J. MASON, J.B. NATION, AND B. SCHAAN Abstract. A finite unary algebra of finite type with a constant function
More informationSUBLATTICES OF LATTICES OF ORDER-CONVEX SETS, III. THE CASE OF TOTALLY ORDERED SETS
SUBLATTICES OF LATTICES OF ORDER-CONVEX SETS, III. THE CASE OF TOTALLY ORDERED SETS MARINA SEMENOVA AND FRIEDRICH WEHRUNG Abstract. For a partially ordered set P, let Co(P) denote the lattice of all order-convex
More informationSemilattice Modes II: the amalgamation property
Semilattice Modes II: the amalgamation property Keith A. Kearnes Abstract Let V be a variety of semilattice modes with associated semiring R. We prove that if R is a bounded distributive lattice, then
More informationA strongly rigid binary relation
A strongly rigid binary relation Anne Fearnley 8 November 1994 Abstract A binary relation ρ on a set U is strongly rigid if every universal algebra on U such that ρ is a subuniverse of its square is trivial.
More informationEquational Logic. Chapter Syntax Terms and Term Algebras
Chapter 2 Equational Logic 2.1 Syntax 2.1.1 Terms and Term Algebras The natural logic of algebra is equational logic, whose propositions are universally quantified identities between terms built up from
More informationMATH 8253 ALGEBRAIC GEOMETRY WEEK 12
MATH 8253 ALGEBRAIC GEOMETRY WEEK 2 CİHAN BAHRAN 3.2.. Let Y be a Noetherian scheme. Show that any Y -scheme X of finite type is Noetherian. Moreover, if Y is of finite dimension, then so is X. Write f
More informationA GUIDE FOR MORTALS TO TAME CONGRUENCE THEORY
A GUIDE FOR MORTALS TO TAME CONGRUENCE THEORY Tame congruence theory is not an easy subject and it takes a considerable amount of effort to understand it. When I started this project, I believed that this
More informationSubdirectly Irreducible Modes
Subdirectly Irreducible Modes Keith A. Kearnes Abstract We prove that subdirectly irreducible modes come in three very different types. From the description of the three types we derive the results that
More informationNotes on ordinals and cardinals
Notes on ordinals and cardinals Reed Solomon 1 Background Terminology We will use the following notation for the common number systems: N = {0, 1, 2,...} = the natural numbers Z = {..., 2, 1, 0, 1, 2,...}
More informationarxiv:math/ v1 [math.gm] 21 Jan 2005
arxiv:math/0501341v1 [math.gm] 21 Jan 2005 SUBLATTICES OF LATTICES OF ORDER-CONVEX SETS, I. THE MAIN REPRESENTATION THEOREM MARINA SEMENOVA AND FRIEDRICH WEHRUNG Abstract. For a partially ordered set P,
More informationA CHARACTERIZATION OF LOCALLY FINITE VARIETIES THAT SATISFY A NONTRIVIAL CONGRUENCE IDENTITY
A CHARACTERIZATION OF LOCALLY FINITE VARIETIES THAT SATISFY A NONTRIVIAL CONGRUENCE IDENTITY KEITH A. KEARNES Abstract. We show that a locally finite variety satisfies a nontrivial congruence identity
More informationCLASSIFYING THE COMPLEXITY OF CONSTRAINTS USING FINITE ALGEBRAS
CLASSIFYING THE COMPLEXITY OF CONSTRAINTS USING FINITE ALGEBRAS ANDREI BULATOV, PETER JEAVONS, AND ANDREI KROKHIN Abstract. Many natural combinatorial problems can be expressed as constraint satisfaction
More informationHomework Problems Some Even with Solutions
Homework Problems Some Even with Solutions Problem 0 Let A be a nonempty set and let Q be a finitary operation on A. Prove that the rank of Q is unique. To say that n is the rank of Q is to assert that
More informationFinite Simple Abelian Algebras are Strictly Simple
Finite Simple Abelian Algebras are Strictly Simple Matthew A. Valeriote Abstract A finite universal algebra is called strictly simple if it is simple and has no nontrivial subalgebras. An algebra is said
More informationThis is a closed subset of X Y, by Proposition 6.5(b), since it is equal to the inverse image of the diagonal under the regular map:
Math 6130 Notes. Fall 2002. 7. Basic Maps. Recall from 3 that a regular map of affine varieties is the same as a homomorphism of coordinate rings (going the other way). Here, we look at how algebraic properties
More informationTHE CLOSED-POINT ZARISKI TOPOLOGY FOR IRREDUCIBLE REPRESENTATIONS. K. R. Goodearl and E. S. Letzter
THE CLOSED-POINT ZARISKI TOPOLOGY FOR IRREDUCIBLE REPRESENTATIONS K. R. Goodearl and E. S. Letzter Abstract. In previous work, the second author introduced a topology, for spaces of irreducible representations,
More informationA MODEL-THEORETIC PROOF OF HILBERT S NULLSTELLENSATZ
A MODEL-THEORETIC PROOF OF HILBERT S NULLSTELLENSATZ NICOLAS FORD Abstract. The goal of this paper is to present a proof of the Nullstellensatz using tools from a branch of logic called model theory. In
More information3. Abstract Boolean Algebras
3. ABSTRACT BOOLEAN ALGEBRAS 123 3. Abstract Boolean Algebras 3.1. Abstract Boolean Algebra. Definition 3.1.1. An abstract Boolean algebra is defined as a set B containing two distinct elements 0 and 1,
More informationJónsson posets and unary Jónsson algebras
Jónsson posets and unary Jónsson algebras Keith A. Kearnes and Greg Oman Abstract. We show that if P is an infinite poset whose proper order ideals have cardinality strictly less than P, and κ is a cardinal
More informationMathematics Course 111: Algebra I Part I: Algebraic Structures, Sets and Permutations
Mathematics Course 111: Algebra I Part I: Algebraic Structures, Sets and Permutations D. R. Wilkins Academic Year 1996-7 1 Number Systems and Matrix Algebra Integers The whole numbers 0, ±1, ±2, ±3, ±4,...
More informationChapter 1. Sets and Mappings
Chapter 1. Sets and Mappings 1. Sets A set is considered to be a collection of objects (elements). If A is a set and x is an element of the set A, we say x is a member of A or x belongs to A, and we write
More informationPETER A. CHOLAK, PETER GERDES, AND KAREN LANGE
D-MAXIMAL SETS PETER A. CHOLAK, PETER GERDES, AND KAREN LANGE Abstract. Soare [20] proved that the maximal sets form an orbit in E. We consider here D-maximal sets, generalizations of maximal sets introduced
More information4.4 Noetherian Rings
4.4 Noetherian Rings Recall that a ring A is Noetherian if it satisfies the following three equivalent conditions: (1) Every nonempty set of ideals of A has a maximal element (the maximal condition); (2)
More informationUniversal Algebra for Logics
Universal Algebra for Logics Joanna GRYGIEL University of Czestochowa Poland j.grygiel@ajd.czest.pl 2005 These notes form Lecture Notes of a short course which I will give at 1st School on Universal Logic
More informationBounded width problems and algebras
Algebra univers. 56 (2007) 439 466 0002-5240/07/030439 28, published online February 21, 2007 DOI 10.1007/s00012-007-2012-6 c Birkhäuser Verlag, Basel, 2007 Algebra Universalis Bounded width problems and
More informationHouston Journal of Mathematics. c 2007 University of Houston Volume 33, No. 1, 2007
Houston Journal of Mathematics c 2007 University of Houston Volume 33, No. 1, 2007 WHEN IS A FULL DUALITY STRONG? BRIAN A. DAVEY, MIROSLAV HAVIAR, AND TODD NIVEN Communicated by Klaus Kaiser Abstract.
More informationPETER A. CHOLAK, PETER GERDES, AND KAREN LANGE
D-MAXIMAL SETS PETER A. CHOLAK, PETER GERDES, AND KAREN LANGE Abstract. Soare [23] proved that the maximal sets form an orbit in E. We consider here D-maximal sets, generalizations of maximal sets introduced
More informationLATTICES OF QUASI-EQUATIONAL THEORIES AS CONGRUENCE LATTICES OF SEMILATTICES WITH OPERATORS, PART II
International Journal of Algebra and Computation c World Scientific Publishing Company LATTICES OF QUASI-EQUATIONAL THEORIES AS CONGRUENCE LATTICES OF SEMILATTICES WITH OPERATORS, PART II KIRA ADARICHEVA
More informationKnowledge spaces from a topological point of view
Knowledge spaces from a topological point of view V.I.Danilov Central Economics and Mathematics Institute of RAS Abstract In this paper we consider the operations of restriction, extension and gluing of
More informationWalker Ray Econ 204 Problem Set 3 Suggested Solutions August 6, 2015
Problem 1. Take any mapping f from a metric space X into a metric space Y. Prove that f is continuous if and only if f(a) f(a). (Hint: use the closed set characterization of continuity). I make use of
More informationImplicational classes ofde Morgan Boolean algebras
Discrete Mathematics 232 (2001) 59 66 www.elsevier.com/locate/disc Implicational classes ofde Morgan Boolean algebras Alexej P. Pynko Department of Digital Automata Theory, V.M. Glushkov Institute of Cybernetics,
More informationLOCALLY SOLVABLE FACTORS OF VARIETIES
PROCEEDINGS OF HE AMERICAN MAHEMAICAL SOCIEY Volume 124, Number 12, December 1996, Pages 3619 3625 S 0002-9939(96)03501-0 LOCALLY SOLVABLE FACORS OF VARIEIES KEIH A. KEARNES (Communicated by Lance W. Small)
More informationA SUBALGEBRA INTERSECTION PROPERTY FOR CONGRUENCE DISTRIBUTIVE VARIETIES
A SUBALGEBRA INTERSECTION PROPERTY FOR CONGRUENCE DISTRIBUTIVE VARIETIES MATTHEW A. VALERIOTE Abstract. We prove that if a finite algebra A generates a congruence distributive variety then the subalgebras
More informationFINITELY RELATED CLONES AND ALGEBRAS WITH CUBE-TERMS
FINITELY RELATED CLONES AND ALGEBRAS WITH CUBE-TERMS PETAR MARKOVIĆ, MIKLÓS MARÓTI, AND RALPH MCKENZIE Abstract. E. Aichinger, R. McKenzie, P. Mayr [1] have proved that every finite algebra with a cube-term
More informationIsomorphisms between pattern classes
Journal of Combinatorics olume 0, Number 0, 1 8, 0000 Isomorphisms between pattern classes M. H. Albert, M. D. Atkinson and Anders Claesson Isomorphisms φ : A B between pattern classes are considered.
More informationTheorem 5.3. Let E/F, E = F (u), be a simple field extension. Then u is algebraic if and only if E/F is finite. In this case, [E : F ] = deg f u.
5. Fields 5.1. Field extensions. Let F E be a subfield of the field E. We also describe this situation by saying that E is an extension field of F, and we write E/F to express this fact. If E/F is a field
More informationMulti-coloring and Mycielski s construction
Multi-coloring and Mycielski s construction Tim Meagher Fall 2010 Abstract We consider a number of related results taken from two papers one by W. Lin [1], and the other D. C. Fisher[2]. These articles
More informationRings and Fields Theorems
Rings and Fields Theorems Rajesh Kumar PMATH 334 Intro to Rings and Fields Fall 2009 October 25, 2009 12 Rings and Fields 12.1 Definition Groups and Abelian Groups Let R be a non-empty set. Let + and (multiplication)
More informationClass Notes on Poset Theory Johan G. Belinfante Revised 1995 May 21
Class Notes on Poset Theory Johan G Belinfante Revised 1995 May 21 Introduction These notes were originally prepared in July 1972 as a handout for a class in modern algebra taught at the Carnegie-Mellon
More informationShort Introduction to Admissible Recursion Theory
Short Introduction to Admissible Recursion Theory Rachael Alvir November 2016 1 Axioms of KP and Admissible Sets An admissible set is a transitive set A satisfying the axioms of Kripke-Platek Set Theory
More information10. Finite Lattices and their Congruence Lattices. If memories are all I sing I d rather drive a truck. Ricky Nelson
10. Finite Lattices and their Congruence Lattices If memories are all I sing I d rather drive a truck. Ricky Nelson In this chapter we want to study the structure of finite lattices, and how it is reflected
More information0 Sets and Induction. Sets
0 Sets and Induction Sets A set is an unordered collection of objects, called elements or members of the set. A set is said to contain its elements. We write a A to denote that a is an element of the set
More informationA Proof of CSP Dichotomy Conjecture
58th Annual IEEE Symposium on Foundations of Computer Science A Proof of CSP Dichotomy Conjecture Dmitriy Zhuk Department of Mechanics and Mathematics Lomonosov Moscow State University Moscow, Russia Email:
More informationTHE EQUATIONAL THEORIES OF REPRESENTABLE RESIDUATED SEMIGROUPS
TH QUATIONAL THORIS OF RPRSNTABL RSIDUATD SMIGROUPS SZABOLCS MIKULÁS Abstract. We show that the equational theory of representable lower semilattice-ordered residuated semigroups is finitely based. We
More informationVC-DENSITY FOR TREES
VC-DENSITY FOR TREES ANTON BOBKOV Abstract. We show that for the theory of infinite trees we have vc(n) = n for all n. VC density was introduced in [1] by Aschenbrenner, Dolich, Haskell, MacPherson, and
More informationPart II. Logic and Set Theory. Year
Part II Year 2018 2017 2016 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2018 60 Paper 4, Section II 16G State and prove the ǫ-recursion Theorem. [You may assume the Principle of ǫ- Induction.]
More informationCOLLAPSING PERMUTATION GROUPS
COLLAPSING PERMUTATION GROUPS KEITH A. KEARNES AND ÁGNES SZENDREI Abstract. It is shown in [3] that any nonregular quasiprimitive permutation group is collapsing. In this paper we describe a wider class
More informationCONGRUENCE MODULAR VARIETIES WITH SMALL FREE SPECTRA
CONGRUENCE MODULAR VARIETIES WITH SMALL FREE SPECTRA KEITH A. KEARNES Abstract. Let A be a finite algebra that generates a congruence modular variety. We show that the free spectrum of V(A) fails to have
More informationFactorization in Polynomial Rings
Factorization in Polynomial Rings Throughout these notes, F denotes a field. 1 Long division with remainder We begin with some basic definitions. Definition 1.1. Let f, g F [x]. We say that f divides g,
More informationTESTING FOR A SEMILATTICE TERM
TESTING FOR A SEMILATTICE TERM RALPH FREESE, J.B. NATION, AND MATT VALERIOTE Abstract. This paper investigates the computational complexity of deciding if a given finite algebra is an expansion of a semilattice.
More informationMath 588, Fall 2001 Problem Set 1 Due Sept. 18, 2001
Math 588, Fall 2001 Problem Set 1 Due Sept. 18, 2001 1. [Burris-Sanka. 1.1.9] Let A, be a be a finite poset. Show that there is a total (i.e., linear) order on A such that, i.e., a b implies a b. Hint:
More informationChapter 1. Sets and Numbers
Chapter 1. Sets and Numbers 1. Sets A set is considered to be a collection of objects (elements). If A is a set and x is an element of the set A, we say x is a member of A or x belongs to A, and we write
More informationFinite pseudocomplemented lattices: The spectra and the Glivenko congruence
Finite pseudocomplemented lattices: The spectra and the Glivenko congruence T. Katriňák and J. Guričan Abstract. Recently, Grätzer, Gunderson and Quackenbush have characterized the spectra of finite pseudocomplemented
More informationExistential Second-Order Logic and Modal Logic with Quantified Accessibility Relations
Existential Second-Order Logic and Modal Logic with Quantified Accessibility Relations preprint Lauri Hella University of Tampere Antti Kuusisto University of Bremen Abstract This article investigates
More informationVAUGHT S THEOREM: THE FINITE SPECTRUM OF COMPLETE THEORIES IN ℵ 0. Contents
VAUGHT S THEOREM: THE FINITE SPECTRUM OF COMPLETE THEORIES IN ℵ 0 BENJAMIN LEDEAUX Abstract. This expository paper introduces model theory with a focus on countable models of complete theories. Vaught
More informationγ γ γ γ(α) ). Then γ (a) γ (a ) ( γ 1
The Correspondence Theorem, which we next prove, shows that the congruence lattice of every homomorphic image of a Σ-algebra is isomorphically embeddable as a special kind of sublattice of the congruence
More information8. Distributive Lattices. Every dog must have his day.
8. Distributive Lattices Every dog must have his day. In this chapter and the next we will look at the two most important lattice varieties: distributive and modular lattices. Let us set the context for
More informationMULTIPLICITIES OF MONOMIAL IDEALS
MULTIPLICITIES OF MONOMIAL IDEALS JÜRGEN HERZOG AND HEMA SRINIVASAN Introduction Let S = K[x 1 x n ] be a polynomial ring over a field K with standard grading, I S a graded ideal. The multiplicity of S/I
More informationEnhanced Dynkin diagrams and Weyl orbits
Enhanced Dynkin diagrams and Weyl orbits E. B. Dynkin and A. N. Minchenko February 3, 010 Abstract The root system Σ of a complex semisimple Lie algebra is uniquely determined by its basis (also called
More informationAbelian Algebras and the Hamiltonian Property. Abstract. In this paper we show that a nite algebra A is Hamiltonian if the
Abelian Algebras and the Hamiltonian Property Emil W. Kiss Matthew A. Valeriote y Abstract In this paper we show that a nite algebra A is Hamiltonian if the class HS(A A ) consists of Abelian algebras.
More informationAn Algebraic View of the Relation between Largest Common Subtrees and Smallest Common Supertrees
An Algebraic View of the Relation between Largest Common Subtrees and Smallest Common Supertrees Francesc Rosselló 1, Gabriel Valiente 2 1 Department of Mathematics and Computer Science, Research Institute
More informationOn poset Boolean algebras
1 On poset Boolean algebras Uri Abraham Department of Mathematics, Ben Gurion University, Beer-Sheva, Israel Robert Bonnet Laboratoire de Mathématiques, Université de Savoie, Le Bourget-du-Lac, France
More informationNotes on Ordered Sets
Notes on Ordered Sets Mariusz Wodzicki September 10, 2013 1 Vocabulary 1.1 Definitions Definition 1.1 A binary relation on a set S is said to be a partial order if it is reflexive, x x, weakly antisymmetric,
More informationDISJOINT-UNION PARTIAL ALGEBRAS
Logical Methods in Computer Science Vol. 13(2:10)2017, pp. 1 31 https://lmcs.episciences.org/ Submitted Dec. 07, 2016 Published Jun. 22, 2017 DISJOINT-UNION PARTIAL ALGEBRAS ROBIN HIRSCH AND BRETT MCLEAN
More informationBoolean Algebras, Boolean Rings and Stone s Representation Theorem
Boolean Algebras, Boolean Rings and Stone s Representation Theorem Hongtaek Jung December 27, 2017 Abstract This is a part of a supplementary note for a Logic and Set Theory course. The main goal is to
More informationA GENERAL THEORY OF ZERO-DIVISOR GRAPHS OVER A COMMUTATIVE RING. David F. Anderson and Elizabeth F. Lewis
International Electronic Journal of Algebra Volume 20 (2016) 111-135 A GENERAL HEORY OF ZERO-DIVISOR GRAPHS OVER A COMMUAIVE RING David F. Anderson and Elizabeth F. Lewis Received: 28 April 2016 Communicated
More informationAN AXIOMATIC FORMATION THAT IS NOT A VARIETY
AN AXIOMATIC FORMATION THAT IS NOT A VARIETY KEITH A. KEARNES Abstract. We show that any variety of groups that contains a finite nonsolvable group contains an axiomatic formation that is not a subvariety.
More informationChapter 1 : The language of mathematics.
MAT 200, Logic, Language and Proof, Fall 2015 Summary Chapter 1 : The language of mathematics. Definition. A proposition is a sentence which is either true or false. Truth table for the connective or :
More informationSOLUTIONS TO EXERCISES FOR. MATHEMATICS 205A Part 1. I. Foundational material
SOLUTIONS TO EXERCISES FOR MATHEMATICS 205A Part 1 Fall 2014 I. Foundational material I.1 : Basic set theory Problems from Munkres, 9, p. 64 2. (a (c For each of the first three parts, choose a 1 1 correspondence
More informationarxiv:math/ v1 [math.gm] 21 Jan 2005
arxiv:math/0501340v1 [math.gm] 21 Jan 2005 SUBLATTICES OF LATTICES OF ORDER-CONVEX SETS, II. POSETS OF FINITE LENGTH MARINA SEMENOVA AND FRIEDRICH WEHRUNG Abstract. For a positive integer n, we denote
More informationK 4 -free graphs with no odd holes
K 4 -free graphs with no odd holes Maria Chudnovsky 1 Columbia University, New York NY 10027 Neil Robertson 2 Ohio State University, Columbus, Ohio 43210 Paul Seymour 3 Princeton University, Princeton
More informationOn the size of Heyting Semi-Lattices and Equationally Linear Heyting Algebras
On the size of Heyting Semi-Lattices and Equationally Linear Heyting Algebras Peter Freyd pjf@upenn.edu July 17, 2017 The theory of Heyting Semi-Lattices, hsls for short, is obtained by adding to the theory
More informationLecture 2: Connecting the Three Models
IAS/PCMI Summer Session 2000 Clay Mathematics Undergraduate Program Advanced Course on Computational Complexity Lecture 2: Connecting the Three Models David Mix Barrington and Alexis Maciel July 18, 2000
More informationEmbedding Differential Algebraic Groups in Algebraic Groups
Embedding Differential Algebraic Groups in Algebraic Groups David Marker marker@math.uic.edu April 8, 2009 Pillay proved that every differential algebraic group can be differentially embedded into an algebraic
More informationCHAPTER 1. AFFINE ALGEBRAIC VARIETIES
CHAPTER 1. AFFINE ALGEBRAIC VARIETIES During this first part of the course, we will establish a correspondence between various geometric notions and algebraic ones. Some references for this part of the
More informationA Discrete Duality Between Nonmonotonic Consequence Relations and Convex Geometries
A Discrete Duality Between Nonmonotonic Consequence Relations and Convex Geometries Johannes Marti and Riccardo Pinosio Draft from April 5, 2018 Abstract In this paper we present a duality between nonmonotonic
More informationReducts of Polyadic Equality Algebras without the Amalgamation Property
International Journal of Algebra, Vol. 2, 2008, no. 12, 603-614 Reducts of Polyadic Equality Algebras without the Amalgamation Property Tarek Sayed Ahmed Department of Mathematics, Faculty of Science Cairo
More informationIDEMPOTENT RESIDUATED STRUCTURES: SOME CATEGORY EQUIVALENCES AND THEIR APPLICATIONS
IDEMPOTENT RESIDUATED STRUCTURES: SOME CATEGORY EQUIVALENCES AND THEIR APPLICATIONS N. GALATOS AND J.G. RAFTERY Abstract. This paper concerns residuated lattice-ordered idempotent commutative monoids that
More informationA Do It Yourself Guide to Linear Algebra
A Do It Yourself Guide to Linear Algebra Lecture Notes based on REUs, 2001-2010 Instructor: László Babai Notes compiled by Howard Liu 6-30-2010 1 Vector Spaces 1.1 Basics Definition 1.1.1. A vector space
More informationLöwenheim-Skolem Theorems, Countable Approximations, and L ω. David W. Kueker (Lecture Notes, Fall 2007)
Löwenheim-Skolem Theorems, Countable Approximations, and L ω 0. Introduction David W. Kueker (Lecture Notes, Fall 2007) In its simplest form the Löwenheim-Skolem Theorem for L ω1 ω states that if σ L ω1
More information= ϕ r cos θ. 0 cos ξ sin ξ and sin ξ cos ξ. sin ξ 0 cos ξ
8. The Banach-Tarski paradox May, 2012 The Banach-Tarski paradox is that a unit ball in Euclidean -space can be decomposed into finitely many parts which can then be reassembled to form two unit balls
More informationFinitely generated free Heyting algebras; the well-founded initial segment
Finitely generated free Heyting algebras; the well-founded initial segment R. Elageili and J.K. Truss Department of Pure Mathematics, University of Leeds, Leeds LS2 9JT, UK e-mail yazid98rajab@yahoo.com,
More informationRESIDUAL SMALLNESS AND WEAK CENTRALITY
RESIDUAL SMALLNESS AND WEAK CENTRALITY KEITH A. KEARNES AND EMIL W. KISS Abstract. We develop a method of creating skew congruences on subpowers of finite algebras using groups of twin polynomials, and
More informationarxiv: v2 [math.lo] 25 Jul 2017
Luca Carai and Silvio Ghilardi arxiv:1702.08352v2 [math.lo] 25 Jul 2017 Università degli Studi di Milano, Milano, Italy luca.carai@studenti.unimi.it silvio.ghilardi@unimi.it July 26, 2017 Abstract The
More informationTopology. Xiaolong Han. Department of Mathematics, California State University, Northridge, CA 91330, USA address:
Topology Xiaolong Han Department of Mathematics, California State University, Northridge, CA 91330, USA E-mail address: Xiaolong.Han@csun.edu Remark. You are entitled to a reward of 1 point toward a homework
More informationOvergroups of Intersections of Maximal Subgroups of the. Symmetric Group. Jeffrey Kuan
Overgroups of Intersections of Maximal Subgroups of the Symmetric Group Jeffrey Kuan Abstract The O Nan-Scott theorem weakly classifies the maximal subgroups of the symmetric group S, providing some information
More informationNOTE ON A THEOREM OF PUTNAM S
NOTE ON A THEOREM OF PUTNAM S MICHAEL BARR DEPARTMENT OF MATHEMATICS AND STATISTICS MCGILL UNIVERSITY MONTREAL, QUEBEC, CANADA 1. Introduction In an appendix to his 1981 book, Putnam made the following
More informationA Class of Infinite Convex Geometries
A Class of Infinite Convex Geometries Kira Adaricheva Department of Mathematics School of Science and Technology Nazarbayev University Astana, Kazakhstan kira.adaricheva@nu.edu.kz J. B. Nation Department
More informationCHAPTER 2. FIRST ORDER LOGIC
CHAPTER 2. FIRST ORDER LOGIC 1. Introduction First order logic is a much richer system than sentential logic. Its interpretations include the usual structures of mathematics, and its sentences enable us
More informationMargulis Superrigidity I & II
Margulis Superrigidity I & II Alastair Litterick 1,2 and Yuri Santos Rego 1 Universität Bielefeld 1 and Ruhr-Universität Bochum 2 Block seminar on arithmetic groups and rigidity Universität Bielefeld 22nd
More informationPreliminaries. Introduction to EF-games. Inexpressivity results for first-order logic. Normal forms for first-order logic
Introduction to EF-games Inexpressivity results for first-order logic Normal forms for first-order logic Algorithms and complexity for specific classes of structures General complexity bounds Preliminaries
More information5 Set Operations, Functions, and Counting
5 Set Operations, Functions, and Counting Let N denote the positive integers, N 0 := N {0} be the non-negative integers and Z = N 0 ( N) the positive and negative integers including 0, Q the rational numbers,
More information9/19/2018. Cartesian Product. Cartesian Product. Partitions
Cartesian Product The ordered n-tuple (a 1, a 2, a 3,, a n ) is an ordered collection of objects. Two ordered n-tuples (a 1, a 2, a 3,, a n ) and (b 1, b 2, b 3,, b n ) are equal if and only if they contain
More informationThe complexity of recursive constraint satisfaction problems.
The complexity of recursive constraint satisfaction problems. Victor W. Marek Department of Computer Science University of Kentucky Lexington, KY 40506, USA marek@cs.uky.edu Jeffrey B. Remmel Department
More informationThis is logically equivalent to the conjunction of the positive assertion Minimal Arithmetic and Representability
16.2. MINIMAL ARITHMETIC AND REPRESENTABILITY 207 If T is a consistent theory in the language of arithmetic, we say a set S is defined in T by D(x) if for all n, if n is in S, then D(n) is a theorem of
More informationAxioms of separation
Axioms of separation These notes discuss the same topic as Sections 31, 32, 33, 34, 35, and also 7, 10 of Munkres book. Some notions (hereditarily normal, perfectly normal, collectionwise normal, monotonically
More informationGeneralized Pigeonhole Properties of Graphs and Oriented Graphs
Europ. J. Combinatorics (2002) 23, 257 274 doi:10.1006/eujc.2002.0574 Available online at http://www.idealibrary.com on Generalized Pigeonhole Properties of Graphs and Oriented Graphs ANTHONY BONATO, PETER
More informationCSE 20 DISCRETE MATH WINTER
CSE 20 DISCRETE MATH WINTER 2016 http://cseweb.ucsd.edu/classes/wi16/cse20-ab/ Today's learning goals Evaluate which proof technique(s) is appropriate for a given proposition Direct proof Proofs by contraposition
More informationERRATA. Abstract Algebra, Third Edition by D. Dummit and R. Foote (most recently revised on March 4, 2009)
ERRATA Abstract Algebra, Third Edition by D. Dummit and R. Foote (most recently revised on March 4, 2009) These are errata for the Third Edition of the book. Errata from previous editions have been fixed
More informationSergey Norin Department of Mathematics and Statistics McGill University Montreal, Quebec H3A 2K6, Canada. and
NON-PLANAR EXTENSIONS OF SUBDIVISIONS OF PLANAR GRAPHS Sergey Norin Department of Mathematics and Statistics McGill University Montreal, Quebec H3A 2K6, Canada and Robin Thomas 1 School of Mathematics
More information