MINIMAL POLYNOMIALS AND RADII OF ELEMENTS IN FINITEDIMENSIONAL POWERASSOCIATIVE ALGEBRAS


 Gavin Mitchell Burke
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1 TRANSACTIONS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 359, Number 8, August 2007, Pages S (06) Article electronically published on August 16, 2006 MINIMAL POLYNOMIALS AND RADII OF ELEMENTS IN FINITEDIMENSIONAL POWERASSOCIATIVE ALGEBRAS MOSHE GOLDBERG Abstract. In the first section of this paper we revisit the definition and some of the properties of the minimal polynomial of an element of a finitedimensional powerassociative algebra A over an arbitrary field F. Our main observation is that p a, the minimal polynomial of a A, may depend not only on a, but also on the underlying algebra. More precisely, if A is a subalgebra of B, andifq a is the minimal polynomial of a in B, thenp a may differ from q a, in which case we have q a (t) =tp a (t). In the second section we restrict attention to the case where F is either the real or the complex numbers, and define r(a), the radius of an element a in A, to be the largest root in absolute value of the minimal polynomial of a. We show that r possesses some of the familiar properties of the classical spectral radius. In particular, we prove that r is a continuous function on A. In the third and last section, we deal with stability of subnorms acting on subsets of finitedimensional powerassociative algebras. Following a brief survey, we enhance our understanding of the subject with the help of our findings of the previous section. Our main new result states that if S, a subset of an algebra A, satisfies certain assumptions, and f is a continuous subnorm on S, thenf is stable on S if and only if f majorizes the radius r defined above. 1. The minimal polynomial revisited Let A be a nontrivial, finitedimensional algebra over a field F. Throughout this paper we assume that A is powerassociative; i.e., that the subalgebra generated by any one element of A is associative, thus ensuring that powers of each element in A are uniquely defined. Let a be an element of A. As usual, by a minimal polynomial of a we mean a monic polynomial of lowest positive degree with coefficients in F that annihilates a. Surely, if A does not have a unit, and p is a nonzero member of F[t], the ring of polynomials over F, thenanelementa of A can be substituted in p only if p does not have a constant term; so in this case, a minimal polynomial of a must be void of a constant term. With this in mind, we record the following familiar assertion. Received by the editors December 18, 2005 and, in revised form, April 17, Mathematics Subject Classification. Primary 15A60, 16B99, 17A05, 17A15. Key words and phrases. Finitedimensional powerassociative algebras, minimal polynomial, radius of an element in a finitedimensional powerassociative algebra, norms, subnorms, submoduli, stable subnorms c 2006 American Mathematical Society Reverts to public domain 28 years from publication
2 4056 MOSHE GOLDBERG Theorem 1.1. Let A be a finitedimensional powerassociative algebra over F. Then: (a) Every element a Apossesses a unique minimal polynomial. (b) The minimal polynomial of a divides every other polynomial in F[t] that annihilates a. Proof. For completeness, we first address the often encountered case where A has a unit, say e. LetdimA = m, and select a A. Then the elements e,a,...,a m are linearly dependent in A. SoK, the set of polynomials in F[t] that annihilates a, contains a nonzero polynomial. Further, it is easily verified that K is an ideal (and hence a principal ideal) in F[t]. Thus, (e.g., [BM, Section 3.8, Theorem 11], [R, Theorem 3.54]), K is generated by a unique monic polynomial of positive degree, and the proof follows. If A does not have a unit, the above argument must be slightly modified: Selecting a A, we note that the elements a,...,a m+1 are linearly dependent. So K, the set of polynomials in F[t] that have no constant term and annihilate a, contains a nonzero member. Now, take p in K and let q(t) =α n t n + + α 1 t + α 0 be an arbitrary polynomial in F[t]. Then, (pq)(t) = p(t)(α n t n + + α 1 t + α 0 ) = α n p(t)t n + + α 1 p(t)t + α 0 p(t). Hence it is permissible to substitute a in pq, whichgives (pq)(a) =α n p(a)a n + + α 1 p(a)a + α 0 p(a). Since p(a) = 0, it follows that (pq)(a) =0. So K is an ideal in F[t], and the rest of the proof follows just as in the previous case. As we shall see in the following two examples, p a, the minimal polynomial of an element a A, may depend not only on a, but also on the underlying algebra. Thus, from now on we shall often refer to p a as the minimal polynomial of a in A. Example 1.1. Consider the matrix algebras (1.1) A = α 22 α 23 : α jk F 0 α 32 α 33 and B = α 21 α 22 α 23 : α jk F, α 31 α 32 α 33 with the usual matrix operations. Evidently, A is a subalgebra of B. Further, A has a unit, I A = , 0 0 1
3 MINIMAL POLYNOMIALS AND RADII OF ELEMENTS 4057 whose minimal polynomial in A is p IA (t) =t 1. The algebra B, on the other hand, does not have a unit; for if it did, say I B, then multiplying I B from the left by E 31,the3 3 matrix all of whose entries are zero except for the lowerleft entry which is 1, we would get E 31 = E 31 I B = 0, a contradiction. It thus follows that p IA, having a constant term, is not the minimal polynomial of I A in B. Indeed, it is not hard to verify that q IA, the minimal polynomial of I A in B, is q IA (t) =tp IA (t) =t 2 t. We observe that for other elements of A, the minimal polynomials in A and in B may coincide; for instance, the minimal polynomials of A = in A and in B are both equal to t 2 2t. Example 1.2. Let A be the algebra in (1.1), and take B to be F 3 3, the algebra of 3 3 matrices over F with the usual operations. Then, in contrast to Example 1.1, both A and B have (different) units. Yet, just as in Example 1.1, the minimal polynomials of I A in A and in B are p IA (t) =t 1andq IA (t) =tp IA (t) =t 2 t respectively, and those of A satisfy p A (t) =q A (t) =t 2 2t. As we shall now see, Examples 1.1 and 1.2 are but special cases of the following more general phenomenon. Theorem 1.2. Let A and B be finitedimensional powerassociative algebras over F, such that A is a subalgebra of B. Let a be an element of A, andletp a and q a denote the minimal polynomials of a in A and in B, respectively. Then either p a = q a or q a (t) =tp a (t). More specifically: (a) If either A and B do not have units, or A and B have the same unit, or A does not have a unit but B does, then p a = q a. (b) If either A has a unit but B does not, or A and B have different units, then p a = q a or q a (t) =tp a (t). Proof. If either A and B do not have units, or A and B have the same unit, then the proof that p a = q a is straightforward and is left to the reader. To complete the proof of (a), assume that A does not have a unit but B does. Note that if q a has no constant term, then again, p a = q a.otherwise,q a is of the form q a (t) =β n t n + + β 1 t + β 0, β 0 0; hence, q a (a) =β n a n + + β 1 a + β 0 e B =0, where e B is the unit in B. Consequently, (1.2) e B = β0 1 (β na n + + β 1 a) A; so e B is a unit in A, a contradiction. As for the proof of (b), we attend first to the case where A has a unit but B does not. Let p a (t) =α m t m + + α 1 t + α 0 be the minimal polynomial of a in A. If α 0 = 0, then once again, p a = q a. So suppose α 0 0,inwhichcaseq a,havingnoconstantterm,mustdifferfromp a.
4 4058 MOSHE GOLDBERG Consider the monic polynomial q(t) = tp a (t) which has no constant term and annihilates a. If q = q a, then we are done. Otherwise, q is not the minimal polynomial of a in B; sodegq a < deg q. Consequently,degq a deg p a, and since p a q a, it follows that p a is not the minimal polynomial of a in A, a contradiction. Finally, assume that A and B have different units. As before, let α 0 and β 0 denote the constant terms of p a and q a, respectively. If α 0 and β 0 are both zero, then once more we conclude that p a = q a.further,ifβ 0 0, then by (1.2), e B is a unit in A, a contradiction. Hence, it remains to examine the case where α 0 0and β 0 = 0. Consider again the monic polynomial q(t) =tp a (t). If q = q a, then there is nothing left to be proved. If not, then as in the previous case, deg q a < deg q; hence, deg q a deg p a and it follows that p a is not the minimal polynomial of a in A, a contradiction. The two cases in part (b) of Theorem 1.2 are illustrated by Examples 1.1 and 1.2 above. Regarding part (a) of the theorem, we give two additional examples of a more general flavor. Example 1.3. Let a be an element of a finitedimensional powerassociative algebra A with a unit, and denote by A a the associative (even commutative) subalgebra of A generated by a and by the unit. If b is an element of A a, then by the first case in part (a) of Theorem 1.2, the minimal polynomials of b in A and in A a coincide. Example 1.4. Suppose A does not have a unit, and let B be the algebra obtained by adding a unit to A in the familiar manner; that is, B = F A, where scalar multiplication and addition are defined componentwise, and multiplication is given by (α + a)(β + b) =αβ +(ab + βa + αb), a,b A, α,β F. Clearly, e = is a unit in B, anda is a subalgebra (even an ideal) in B. Thus, by the third case in part (a) of Theorem 1.2, if a A, then the minimal polynomials of a in A and in B are identical. Note that since A is powerassociative, so is B. Similarly, if A happens to be associative, then B is associative too. For our next example we shall need the following assertion that hardly requires aproof. Proposition 1.1. Let A and B be algebraically isomorphic, finitedimensional, powerassociative algebras over F, with an algebra isomorphism ϕ : A B. Leta be an element of A, andletp a and q ϕ(a) denote the minimal polynomials of a in A and of ϕ(a) in B, respectively. Then p a = q ϕ(a). Example 1.5. Let A be an mdimensional, associative algebra with a unit e. Consider the regular left representation which associates with each element a of A the linear transformation T a : A A, defined by (1.3) T a (x) =ax, x A. Let F m m denote the algebra of m m matrices over F with the usual operations. Select a basis {b 1,...,b m } of A, andforeachelementa Alet A a be the matrix in F m m that represents T a with respect to this basis. Then obviously, the familiar mapping ϕ : A F m m, defined by (1.4) ϕ(a) =A a, a A,
5 MINIMAL POLYNOMIALS AND RADII OF ELEMENTS 4059 satisfies (1.5) ϕ(αa + βb) =αϕ(a)+βϕ(b), ϕ(ab) =ϕ(a)ϕ(b), a,b A, α,β F. Further, Ker ϕ = {0} since a Ker ϕ implies a = ae = 0. Hence A is algebraically isomorphic to A ϕ = {A a : a A}, the image of A under ϕ; so by Proposition 1.1, the minimal polynomials of a in A and of A a in A ϕ coincide. Moreover, since T e is the identity transformation on A, we observe that regardless of the choice of our basis, ϕ(e), the unit in A ϕ,isi m, the identity in F m m. So, by Theorem 1.2(a), the minimal polynomial of A a in A ϕ is that of A a in F m m. Thus, the minimal polynomials of a in A and of A a in F m m are one and the same; so in particular, the roots of the minimal polynomial of a in A are the eigenvalues of A a. If A is an mdimensional associative algebra without a unit, we modify the above discussion by adjoining a unit to A, obtaining, as in Example 1.4, the (m + 1)dimensional associative algebra B = F A. Selecting a basis {b 1,...,b m+1 } for B, wenowletψ : B F (m+1) (m+1) be the mapping that associates with each element a in B the matrix A a which represents the regular left representation T a : B Bwith respect to this basis. Hence, by our previous discussion, B is algebraically isomorphic to the (m + 1)dimensional subalgebra B ψ = {A a : a B} of F (m+1) (m+1) ; and consequently, our original algebra A is algebraically isomorphic to the mdimensional subalgebra A ψ = {A a : a A}of B ψ, the image of A under the mapping (1.6) ψ(a) =A a, a A. Now, by Proposition 1.1, the minimal polynomials of a in A and of A a in A ψ coincide. Also, since A has no unit, A ψ has no unit; so by Theorem 1.2(a), the minimal polynomial of A a in A ψ is that of A a in F (m+1) (m+1). Hence, by analogy with the previous case, the minimal polynomials of a in A and of A a in F (m+1) (m+1) are identical; so we observe again that the roots of the minimal polynomial of a in A are the eigenvalues of A a. Examining the first part of Example 1.5, we note that if A does not have a unit, then ϕ in (1.4) will still satisfy (1.5), hence A ϕ = {A a : a A}is a homomorphic image of A. However, since A lacks a unit, ϕ may fail to be an isomorphism; so contrary to our findings above, the minimal polynomials of an element a in A and of the corresponding matrix A a = ϕ(a) maydiffer. For instance, if A = {αa : α F} is a onedimensional algebra generated by a single element a 0suchthata 2 =0, then the minimal polynomial of a in A is p a (t) =t 2. On the other hand, T a is the zero transformation on A; soa a = 0, and the corresponding minimal polynomial is p Aa (t) =t. 2. The radius of an element and its properties DuringtheentirecourseofthissectionweshallassumethatthebasefieldF of our finitedimensional powerassociative algebra A is either R or C. Having established the existence and uniqueness of p a, the minimal polynomial of an element a in A, we proceed by defining the radius of a in A: (2.1) r(a) =max{ λ : λ C arootofp a }.
6 4060 MOSHE GOLDBERG We note that, unlike the minimal polynomial of an element a in A, the radius r(a) is independent of A in the sense that if B is another finitedimensional powerassociative algebra over F, such that A is contained in B, then the radii of a in A and in B coincide. Indeed, by Theorem 1.2, p a and q a, the minimal polynomials of a in A and in B, are either identical or satisfy q a (t) =tp a (t). Hence, the nonzero roots of p a and q a are equal; so max{ λ : λ C arootofp a } =max{ λ : λ C arootofq a }, and our assertion follows. In particular, we conclude that if A is a matrix in a subalgebra A of F m m,then the radius of A in A satisfies (2.2) r(a) =ρ(a), where ρ(a) =max{ λ : λ C an eigenvalue of A} is the spectral radius of A. As we shall see in this section, the radius r retains some of the familiar properties of the spectral radius not only for matrices but in the general case as well properties that will play a role in Section 3 where we discuss stability of subnorms on subsets of finitedimensional powerassociative algebras. We begin with the following observations that were noted already in [GL3]: Theorem 2.1 ([GL3, Section 2]). Let A be a finitedimensional powerassociative algebra over F, eitherr or C. Then: (a) The radius r in (2.1) is a nonnegative function on A. (b) For all a Aand α F, r(αa) = α r(a). (c) For all a Aand all positive integers k, r(a k )=r(a) k. (d) The radius r vanishes only on nilpotent elements of A. Proof. We begin with (b). If α = 0, then there is not much to prove; so suppose α 0. Letp a and p αa be the minimal polynomials of a and αa in A, respectively. Then obviously, p αa (t) =p a (α 1 t); so the roots of p αa are those of p a multiplied by α, and (b) follows. To prove (c), select a A,andletA a denote the subalgebra of A generated by a. By the remark following the definition of the radius in (2.1), if b is an element in A a, then the radii of b in A a and in A coincide. Further, since A a is associative, we may employ the appropriate isomorphism in Example 1.5 (either ϕ in (1.4) or ψ in (1.6)), by which A a is algebraically isomorphic to a matrix algebra, such that the roots of the minimal polynomial of b in A a are the eigenvalues of the corresponding matrix A b.thus, r(b) =ρ(a b ) for all b A a.
7 MINIMAL POLYNOMIALS AND RADII OF ELEMENTS 4061 Since A a k = A k a,wenowget r(a k )=ρ(a a k)=ρ(a k a)=ρ(a a ) k = r(a) k, k =1, 2, 3,..., and (c) is in the bag. As for (d), we note that r(a) = 0 if and only if p a, the minimal polynomial of a in A, isoftheformp a (t) =t n for some positive integer n. Hence, r(a) =0ifand only if there exists an integer n for which a n = 0, and the proof is complete. The rest of this section will be devoted to proving a much deeper property, namely the continuity of r on A. If A is associative, then our goal is almost at hand. We appeal to Example 1.5, and recall that A is algebraically isomorphic to a subalgebra of F n n,where n =dima if A has a unit and n =dima + 1 if not, and where the isomorphism associates with each a AamatrixA a F n n whose eigenvalues are the roots of the minimal polynomial of a in A. Whence, r(a) coincideswithρ(a a ), the spectral radius of A a ; and since ρ is a continuous function on F n n,weget: Theorem 2.2 ([GL3, Section 2]). Let A be a finitedimensional associative algebra over F, eitherr or C. Then the radius r is a continuous function on A. A similar, if more tedious argument holds when we weaken the assumption of associativity and assume that A is merely alternative, namely, that the subalgebra generated by any two elements in A is associative. We begin this argument with the case where A, a finitedimensional alternative algebra over F, has a unit e. Following Example 1.5, let dim A = m, and consider the regular left representation T a : A Ain (1.3). Select a basis for A, andfor each element a Alet A a be the m m matrix that represents T a with respect to this basis. It follows that the mapping ϕ in (1.4) satisfies (2.3) ϕ(αa + βb) =αϕ(a)+βϕ(b), a,b A,α,β F; Kerϕ = {0}; hence A, regarded as a linear space over F, is isomorphic to A ϕ = {A a : a A}. Further, since A is alternative, the product a k x is unambiguous for all a, x A and all positive integers k. Thus, the linear transformation T a k is well defined and we have T a k = Ta k ; i.e., A a k = A k a;orinotherwords, (2.4) ϕ(a k )=ϕ(a) k, a A, k =1, 2, 3,... Moreover, since T e is the identity transformation on A, wehave (2.5) ϕ(e) =I m where I m is the identity matrix in F m m. Using (2.3) (2.5), it is not hard to see that p a,theminimalpolynomialofa in A, annihilates A a,andthatinfact,p a is the only monic polynomial of the lowest positive degree that does so. It follows that p a is the minimal polynomial of A a in F m m ;sotherootsofp a are the eigenvalues of A a. Consequently, r(a) =ρ(a a ) for all a A, and since ρ is continuous on A ϕ,soisr on A. If A is an mdimensional alternative algebra over F without a unit, we invoke Example 1.4, and extend A to the (m+1)dimensional algebra B = F A. Selecting abasis{b 1,...,b m+1 } for B, weconsiderψ : B F (m+1) (m+1), the mapping in (1.6) that associates with each a in B the matrix A a which represents the regular left representation T a : B Bwith respect to this basis. As in the previous case where A had a unit, it follows that B, as a linear space, is isomorphic to B ψ = {A b : b B};
8 4062 MOSHE GOLDBERG so A is isomorphic to the mdimensional subspace {A ψ : a A}of B ψ, the image of A under ψ. Moreover, we employ the alternativity of A and, again as in the previous case, we find that the linear isomorphism ψ satisfies ψ(a k )=ψ(a) k, a A, k =1, 2, 3,... Thus p a, the minimal polynomial of a in A, annihilates A a. We claim that p a, which is now void of a constant term, coincides with q Aa,the minimal polynomial of A a in F (m+1) (m+1). Indeed, if q Aa has no constant term, then the assertion is easy. On the other hand, if q Aa has a constant term, then it is of the form q Aa (t) =β n t n + + β 1 t + β 0, β 0 0. So (compare (1.2)), I m+1 = β0 1 (β na n + + β 1 a) A ψ, and it follows that there exists an element ẽ in A such that the matrix representing Tẽ with respect to our basis is I m+1. Therefore, Tẽ is the identity transformation on B. Hence ẽ is a unit in B; soa has a unit, a contradiction. Having established the claim stated in the previous paragraph, we conclude, as before, that r(a) =ρ(a a ) for all a A; so the continuity of ρ on A ψ implies that r is continuous on A. Collecting our findings for the above two cases where A is alternative (with or without a unit), we may thus register: Theorem 2.3 (compare [GL3, Section 2]). Let A be a finitedimensional, alternative algebra over F, eitherr or C. Then the radius r is a continuous function on A. In passing, we recall that, unlike the radius r, the coefficients of the minimal polynomial may fail to behave continuously, even when A is associative. For example, the minimal polynomial of the matrix ( ) 0 ε A ε =, ε > 0, ε 0 is p Aε (t) =t 2 ε 2,whereasforA 0,the2 2 zero matrix, we get p A0 (t) =t. To demonstrate the computation of the radius r we offer the following example. Example 2.1. Viewing the complex numbers, C = {z = α + iβ : α, β R}, as a 2dimensional algebra over the reals, it has been noticed in [GL3] that the minimal polynomial of z = α + iβ C is p z (t) =t 2 2αt + α 2 + β 2. So since the roots of p z are z and z, it follows that (2.6) r(z) = z. A similar observation (compare, [GL1]) can be made by regarding the quaternions, H = {q = α + iβ + jγ + kδ : α, β, γ, δ R}, i 2 = j 2 = k 2 = ijk = 1,
9 MINIMAL POLYNOMIALS AND RADII OF ELEMENTS 4063 as a 4dimensional algebra over R, and recalling the wellknown mapping α β γ δ q A q β α δ γ γ δ α β, q = α + iβ + jγ + kδ H, δ γ β α which implies that H is algebraically isomorphic to the real matrix algebra A 4 (R) ={A q : q H}. Since for each q = α + iβ + jγ+kδ H the eigenvalues of the corresponding matrix A q are α ± i β 2 + γ 2 + δ 2 (each with multiplicity 2), we appeal to Proposition 1.1 and to (2.2), obtaining r(q) =r(a q )=ρ(a q )= α 2 + β 2 + γ 2 + δ 2 = q, q = α + iβ + jγ + kδ H, an analogue of (2.6). The algebraic techniques that led to Theorems 2.2 and 2.3 do not seem to lend themselves to the case where A is merely powerassociative. In this case, our proof that r is continuous is essentially analytic, and we begin with several preliminary remarks. As usual, a realvalued function N : A R is a norm on A if for all a, b Aand α F, N(a) > 0, a 0, (2.7) N(αa) = α N(a), N(a + b) N(a)+N(b). We recall that since A is finitedimensional, all norms on A are equivalent, implying a unique topology on A. Thus, selecting a norm N, the continuity of r means, of course, that for each a Aand ε>0, there exists δ>0, such that if x Asatisfies N(x a) <δ,then r(x) r(a) <ε. Having selected a norm N on A, weset (2.8) γ =max{n(xy) : x, y A,N(x) =N(y) =1}. Since A is finitedimensional, a simple compactness argument implies that γ is a welldefined positive constant satisfying N(xy) γn(x)n(y), x,y A. Consequently, using induction, we find that (2.9) N(x k ) γ k 1 N(x) k for all x A,k=2, 3, 4,... The finitedimensionality of A also implies that the operations in A (addition, multiplication, and scalar multiplication) are continuous; so if {x k } k=1 and {y k} k=1 are sequences in A that converge to x and ỹ respectively, and if {α k } k=1 is a sequence in F whose limit is α, then (2.10) lim (x k + y k )= x +ỹ, lim x ky k = xỹ, lim α kx k = α x. k k k A quick way to verify (2.10) is to note that in terms of any linear basis of our finitedimensional algebra, the convergence of {x k } k=1 and {y k} k=1 to x and ỹ holds coordinatewise, and that the coordinates of x k + y k, x k y k and α k x k are polynomials (i.e., continuous functions) in the coordinates of x k and y k.
10 4064 MOSHE GOLDBERG For brevity, we shall refer to the roots of the minimal polynomial of an element a Aas the roots of a. Finally, given ε>0andapointζ 0 C, we follow standard terminology and refer to the open set {ζ C : ζ ζ 0 <ε} as the εneighborhood of ζ 0. With these preliminaries, we are now ready to prove the following lemma. Lemma 2.1. Let A be a finitedimensional powerassociative algebra over F, either R or C, andletn be a norm on A. Given a Aand ε>0, thereexistsδ>0, such that if x is an element of A with N(x a) <δ,then: (a) Every root λ x of x belongs to the εneighborhood of some root λ a of a. (b) Every root λ a of a belongs to the εneighborhood of some root λ x of x. Proof. 1 Select x A,andletA x be the subalgebra of A generated by x and by the unit in A (if A has one). Then surely, each y in A x is a polynomial in x with coefficients in F. Letλ x be a root of p x, the minimal polynomial of x in A, andlet be the mapping defined by Γ λx : A x C (2.11) Γ λx (y) =q y (λ x ), y A x, where q y is any polynomial over F such that y = q y (x). We note that while q y is not unique (y = q y (x) implies, for example, y = q y (x)+p x (x)), the value of Γ λx (y) is uniquely defined. This is so because if q 1 and q 2 are two polynomials such that q 1 (x) =q 2 (x), then (q 1 q 2 )(x) = 0; hence p x divides q 1 q 2, and it follows that (q 1 q 2 )(λ x ) = 0, i.e., q 1 (λ x )=q 2 (λ x ). Take y 1,y 2 A x and α 1,α 2 F, andletq 1, q 2,q 3,andq 4 be polynomials over F such that y 1 = q 1 (x), y 2 = q 2 (x), α 1 y 1 + α 2 y 2 = q 3 (x), y 1 y 2 = q 4 (x). Then, (q 3 α 1 q 1 α 2 q 2 )(x) =0, (q 4 q 1 q 2 )(x) =0; so p x divides q 3 α 1 q 1 α 2 q 2 and q 4 q 1 q 2, and consequently, (q 3 α 1 q 1 α 2 q 2 )(λ x )=0, (q 4 q 1 q 2 )(λ x )=0. Hence, q 3 (λ x )=α 1 q 1 (λ x )+α 2 q 2 (λ x ), q 4 (λ x )=q 1 (λ x )q 2 (λ x ); thus, Γ λx is both linear and multiplicative, i.e., for all y 1,y 2 A x and α 1,α 2 F, Γ λx (α 1 y 1 + α 2 y 2 )=α 1 Γ λx (y 1 )+α 2 Γ λx (y 2 ), Γ λx (y 1 y 2 )=Γ λx (y 1 )Γ λx (y 2 ). We claim that (2.12) Γ λx (y) γn(y) for all y A x, where N is the given norm on A and γ is the constant defined in (2.8). To prove (2.12), we fix a linear basis {b 1,...,b m } for A x,andlet y = α 1 b α m b m 1 The author is grateful to Eliahu Levi for important discussions that greatly facilitated this proof.
11 MINIMAL POLYNOMIALS AND RADII OF ELEMENTS 4065 be an arbitrary element in A x. Using the linearity of Γ λx,weobtain Γ λx (y) = Γ λx (α 1 b α m b m ) = α 1 Γ λx (b 1 )+ + α m Γ λx (b m ) α 1 Γ λx (b 1 ) + + α m Γ λx (b m ). So, (2.13) Γ λx (y) κ x N (y) for all y A x, where κ x = m max λ x (b j ), 1 j m and where N (y) = max j, 1 j m y = α 1 b α m b m A x, is the sup norm on A x with respect to our basis. Next, since all norms on A x are equivalent, we can exhibit a constant κ x,such that (2.14). N (y) κ xn(y), y A x, So by (2.13) and (2.14), (2.15) Γ λx (y) κ xn(y), y A x, where κ x = κ x κ x is a constant depending on x. Finally, we appeal to (2.9), and note that by the multiplicativity of Γ λx and by (2.15), Γ λx (y) k = Γ λx (y k ) κ xn(y k ) κ xγ k 1 N(y) k for all y A x and k =2, 3, 4,... Thus, ( ) κ 1/k Γ λx (y) γn(y) γn(y) as k, x γ and (2.12) follows. The inequality in (2.12) has two useful corollaries. First, by taking y = x and q y (t) =t, weobtainγ λx (x) =λ x. Hence, by (2.12), (2.16) λ x γn(x) for all x A, i.e., the roots of each x in A are bounded by γn(x). Second, if y = q y (x) isan element of A x, then by (2.11) and (2.12), the polynomial q y satisfies (2.17) q y (λ x ) γn(q y (x)). Now suppose part (a) of our lemma is false. Then, for some a Aand ε>0, we can find a sequence {x k } k=1 in A with x k a, such that each x k has a root λ k which does not belong to the εneighborhood of any root of a. Since {x k } k=1 is a bounded sequence, so is {λ k} k=1 by (2.16). Thus, passing to an appropriate subsequence if necessary, we may assume that there exists λ C, such that lim λ k = λ. k
12 4066 MOSHE GOLDBERG Further, as the λ k are not in the εneighborhood of any root of a, we conclude that λ is not arootofa. In the next paragraph we will show, however, that λ is aroot of a, a contradiction that will conclude the proof of (a). Indeed, let p a be the minimal polynomial of a in A. Then by (2.10), lim p a(x k )=p a (a). k Since p a (x k )isanelementofa xk, the subalgebra of A generated by x k and by the unit of A (if a unit exists), (2.17) implies p a ( λ) = lim p a(λ k ) γ lim N(p a(x k )) = γn(p a (a)) = 0, k k and the desired contradiction is at hand. To prove (b), let p 1 = α n + + α 0, p(t) =α n t n + + α 1 t + α 0 C[t], denote the l 1 norm on C[t]. Observe that 1 is submultiplicative, i.e., (2.18) pq 1 p 1 q 1, p,q C[t]. Given a scalar ζ C and a polynomial 0 q C[t], define the nonnegative function Ω ζ (q) = q(ζ), q 1 and note that for every 0 β R, (2.19) Ω ζ (βq) =Ω ζ (q). So, if Clearly, t α 1 =1+ α, α C. p(t) = n (t α j ) j=1 is a monic polynomial of positive degree, then by (2.18), n p 1 (1 + α j ). Thus, (2.20) Ω ζ (p) = p(ζ) p 1 j=1 p(ζ) n n j=1 (1 + α j ) = ζ α j 1+ α j. Now suppose (b) is false. Then, there exist an element a Awith a root λ a,a positive constant 0 <ε<1, and a sequence {x k } k=1 with x k a, such that λ a does not belong to the εneighborhood of any of the roots of x k, k =1, 2, 3,... Denote by p k the minimal polynomial of x k in A, and let the roots of x k be λ k,j, j =1,...,n k (n k =degp k 1), so that n k p k (t) = (t λ k,j ). j=1 j=1
13 MINIMAL POLYNOMIALS AND RADII OF ELEMENTS 4067 Then, by (2.20), (2.21) Ω λa (p k ) n k j=1 λ a λ k,j, k =1, 2, 3, λ k,j Since λ a does not belong to the εneighborhood of any of the roots of x k,we have (2.22) λ a λ k,j ε, k =1, 2, 3,..., j =1,...,n k. Further, since {x k } k=1 is bounded, we use (2.16) to infer that all the λ k,j are uniformly bounded; so we can exhibit a constant µ>0 such that (2.23) λ k,j µ, k =1, 2, 3,..., j =1,...,n k. Thus, by (2.21) (2.23), ( ) nk ε (2.24) Ω λa (p k ), k =1, 2, 3,... 1+µ Moreover, setting m =dima +1,wenotethatforeveryx A,thepowers x, x 2,...,x m are linearly dependent. Hence p x, the minimal polynomial of x in A, isofdegree at most m; so by (2.24), ( ) m ε (2.25) Ω λa (p k ), k =1, 2, 3,... 1+µ Let C m [t] denotethe(m + 1)dimensional subspace of C[t] consisting of all polynomials p with deg p m. Evidently, p x C m [t] for all x A, and 1 is a norm on C m [t]. Since p k 0, we put q k = p k p k 1, and observe that (2.26) q k (x k )=0, k =1, 2, 3,... As the polynomials {q k } k=1 lie on the unit sphere with respect to our l 1 norm on C m [t], and since this sphere is a compact set, we may assume (passing, if necessary, to a suitable subsequence) that (2.27) q k q, where q, amemberofc m [t], satisfies q 1 =1. Since C m [t] is finitedimensional, we note that the convergence in (2.27) holds coefficientwise; that is, for every j =0,...,m, the sequence of coefficients of t j in {q k } k=1 converges to the coefficient of tj in q. Thus, by (2.10), (2.26) and (2.27), we obtain q(a) = lim q k(x k )=0. k Consequently, p a, the minimal polynomial of a in A, divides q. Hence, λ a is a root of q, so (2.28) Ω λa ( q) = q(λ a) =0. q 1
14 4068 MOSHE GOLDBERG On the other hand, by (2.19), Ω λa (q k )=Ω λa (p k ), so (2.25) implies ( ) m ε Ω λa (q k ), k =1, 2, 3,... 1+µ Thus, passing to the limit, we get ( ) m ε Ω λa ( q), 1+µ which contradicts (2.28) and so completes the proof. Our main result in this section is now only a small step away. Theorem 2.4. Let A be a finitedimensional powerassociative algebra over F, either R or C. Then the radius r is a continuous function on A. Proof. By Lemma 2.1, given a Aand ε>0, there exists δ>0, such that if x is an element of A with N(x a) <δ,then r(x) r(a) +ε and r(a) r(x) +ε; hence, r(x) r(a) <εwhich forces the desired result. 3. Stable subnorms Throughout this last part of the paper we will follow Section 2 in assuming (often without further mention) that the base field F of our finitedimensional powerassociative algebra A is either R or C. Let S, a subset of A, be closed under scalar multiplication (so that a Sand α F imply αa S). Following [GL1], we call a realvalued function f : S R a subnorm on S if for all a S and α F, f(a) > 0, a 0, f(αa) = α f(a). If in addition, S is closed under raising to powers (i.e., a S implies a k S, k =1, 2, 3,...), then a subnorm f on S shall be called a submodulus if f(a k )=f(a) k for all a S and k =1, 2, 3,... Finally, if S is also closed under multiplication, we say that a submodulus f on S is a modulus if f is multiplicative, i.e., f(ab) =f(a)f(b) for all a, b S. We recall that if S, a subset of A, is closed under scalar multiplication and under addition, then a realvalued function N is a norm on S if (2.7) holds for all a, b S and α F; thus, in our finitedimensional context, a norm is a subadditive (hence continuous) subnorm on S. Examples of subnorms, submoduli and moduli were exhibited in [GL1, GL2, GGL, GL3, G]. A selection of these examples is listed below for the reader s convenience.
15 MINIMAL POLYNOMIALS AND RADII OF ELEMENTS 4069 Example 3.1. Revisit C, the 2dimensional algebra of the complex numbers over the reals, and note that for each fixed p, 0<p, (3.1) z p =( α p + β p ) 1/p, z = α + iβ C, is a continuous subnorm on C. Surely, p is a norm if and only if 1 p,and a submodulus in fact, a modulus only for p = 2whereweget (3.2) z z 2 = α 2 + β 2. Similarly, considering the quaternions, we observe that (3.3) q p =( α p + β p + γ p + δ p ) 1/p, q = α + iβ + jγ + kδ H, is a continuous subnorm for 0 <p, a norm precisely for 1 p,anda modulus, (3.4) q q 2 = α 2 + β 2 + γ 2 + δ 2, for p =2. In the same way, we may reflect on the real 8dimensional alternative algebra of the octonions, O = {c = γ 1 + γ 2 e γ 8 e 8 : γ j R}, with its intricate multiplication rule (e.g., [B]). In analogy with (3.1) and (3.3), we note that (3.5) c p =( γ 1 p + + γ 8 p ) 1/p is a continuous subnorm for 0 <p, a norm if and only if 1 p,anda modulus, (3.6) c c 2 = γ γ2 8, for p = 2 (a fact that stems from the Eight Square Theorem [D], which implies that cd = c d for all c, d O). Example 3.2. Since ρ, the spectral radius, vanishes on nonzero nilpotent matrices, it is not a subnorm on F m m. It is, however, a subnorm, in fact a continuous submodulus (but usually not a modulus), on any subset of F m m that is void of nonzero nilpotent matrices and closed under scalar multiplication and under raising to powers for instance, N m (F), the set of normal m m matrices over F. Example 3.3. Contrary to norms, subnorms and submoduli are often discontinuous. An example of such submoduli is given in [GGL], where the underlying algebra is F m m and the set is N m (F). Indeed, putting τ(a) =min{ λ : λ C an eigenvalue of A}, A N m (F), we observe that { ρ(a) (3.7) g κ (A) = κ+1 τ(a) κ, τ(a) > 0, ρ(a), τ(a) =0, is a submodulus on N m (F) for every real constant κ. For κ = 0 we obtain the (continuous) spectral radius. For κ 0, however, g κ is discontinuous, since for the normal matrix A ε =diag(1,...,1,ε), ε > 0,
16 4070 MOSHE GOLDBERG one gets whereas g κ (A 0 )=1. {, κ > 0, lim g κ(a ε )= ε 0 0, κ < 0, Example 3.4. We recall that the functional equation (3.8) h(s + t) =h(s)+h(t), s,t R, has discontinuous solutions (e.g., [HLP, Section 3.20]). It is well known that if h is such a solution, then it is discontinuous everywhere. Further, given a constant c>0, one can choose a discontinuous solution h with h(t + c) =h(t) for all t R, hence h can be made cperiodic. Aided by these facts, it was shown in [GL2, Theorem 2.1] that if f is a subnorm on C, the 2dimensional algebra of the complex numbers over the reals, and if h is a discontinuous πperiodic solution of (3.8), then: (a) The realvalued function g(z) =f(z)e h(arg z), z C, is a subnorm on C. (b) If f is a submodulus or a modulus on C, thensoisg. (c) If f is a continuous subnorm on C, theng is discontinuous everywhere in C. Similar pathological constructions, where the resulting subnorms and submoduli lack any shred of continuity, were obtained in [GL2] for the quaternions as well as for N m (F). Having recorded Examples , we proceed by recalling an elementary result that pertains to continuous subnorms. Proposition 3.1 ([GL1, Lemma 1.2]). Let S, a closed subset of a finitedimensional powerassociative algebra A over F, eitherr or C, be closed under scalar multiplication and under raising to powers. Let f be a continuous subnorm on S and let g be a continuous submodulus on S. Then, lim k f(ak ) 1/k = g(a) for all a S. Since the above limit is unique, we immediately get: Corollary 3.1 ([GL1, Corollary 1.1]). Let A, S and g be as in Proposition 3.1. Then g is the only continuous submodulus on S. The definition of submodulus gives rise to another simple, yet basic result: Theorem 3.1 ([GGL, Proposition 3]). Let S, a subset of a powerassociative algebra A over F, eitherr or C, be closed under scalar multiplication and under raising to powers. If S contains nonzero nilpotent elements, then S has no submodulus. This result, which holds for finite as well as for infinitedimensional algebras, implies, for instance, that F m m has no submodulus. Now, let S, a subset of A, be closed under scalar multiplication and under raising to powers. Following [GL1], we say that a subnorm f on S is stable if for some positive constant σ, (3.9) f(a k ) σf(a) k for all a S and k =1, 2, 3,...
17 MINIMAL POLYNOMIALS AND RADII OF ELEMENTS 4071 We say that f is strongly stable if (3.9) holds for σ = 1. Hence, for example, all submoduli on S are strongly stable. With the above definitions, we can now quote the main result in [GL1]. Theorem 3.2 ([GL1, Theorem 1.1(a)]). Let S, a closed subset of a finitedimensional powerassociative algebra A over F, eitherr or C, be closed under scalar multiplication and under raising to powers. Let f be a continuous subnorm on S, and let g be a continuous submodulus on S. Then f is stable if and only if f g on S. Appealing to Corollary 3.1, we find, for example, that the modulus functions in (3.2), (3.4) and (3.6) are the only continuous submoduli on C, H and O, respectively. Further, by Theorem 3.2, a continuous subnorm f is stable on C, H or O if and only if f majorizes the corresponding modulus function; so, in particular, the subnorms in (3.1), (3.3) and (3.5) are stable precisely when 0 <p 2. Another illustration of Corollary 3.1 and Theorem 3.2 is obtained by recalling that ρ, the spectral radius, is a continuous submodulus on N m (F), the set of m m normal matrices over F. Since N m (F) is a closed subset of F m m, Corollary 3.1 implies that ρ is the only continuous submodulus on N m (F), and by Theorem 3.2, a continuous subnorm f is stable on N m (F) if and only if f ρ there. We emphasize that the assumption in Corollary 3.1 and in Theorems 3.2 that S is closed, cannot be dropped. This was established in [G] by noting that GL m (F) {0}, the union of the general linear group of m m invertible matrices over F and the zero matrix, is not a closed subset of F m m, and that both ρ and τ(a) =min{ λ : λ C an eigenvalue of A} are continuous submoduli on GL m (F) {0}. Hence, ρ is not the only continuous submodulus on GL m (F){0}, andτ is stable on GL m (F) {0} without majorizing ρ. We also mention that while a closed set S has at most one continuous submodulus, S may have infinitely many discontinuous submoduli, as demonstrated by the action of g κ in (3.7) on N m (F). With the above survey completed, we are ready to conclude this paper by presenting two new results that hinge on our findings in Section 2 and enhance Corollary 3.1 and Theorem 3.2. First, we consult Theorems 2.1 and 2.4, which together with Corollary 3.1, yield: Theorem 3.3. Let A be a finitedimensional powerassociative algebra over F, either R or C. LetS, a subset of A, be void of nonzero nilpotents and closed under scalar multiplication and under raising to powers. Then: (a) (Compare [GL3, Theorem 2.2(b)].) The radius r is a continuous submodulus on S. (b) If S is closed, then r is the only continuous submodulus on S. Now, combining Theorems 3.3(b) and 3.2, we finally get: Theorem 3.4. Let S, a closed subset of a finitedimensional powerassociative algebra A over F, eitherr or C, be closed under scalar multiplication and under raising to powers. If S is void of nonzero nilpotents and f is a continuous subnorm on S, thenf is stable on S if and only if f majorizes the radius r on S. In view of the fundamental role of the radius r in Theorem 3.4, it seems useful to characterize this radius on given algebras and, in fact, we have done so in several
18 4072 MOSHE GOLDBERG familiar cases. For instance, by formula (2.2) and by Example 2.1, we already know that the radius on F m m is the classical spectral radius, and the radii on C and on H are the modulus functions in (3.2) and (3.4), respectively. Similarly, since the modulus function in (3.6) is the only continuous submodulus on the octonions O, it follows from Theorem 3.3(b) that this same modulus is the radius on O. We note that if A is associative, one can obtain the assertions in Theorems 3.3 and 3.4 by employing Theorem 2.2 instead of Theorem 2.4 (see [GL3, Theorem 2.1(b,c)] and [G, Theorem 3.8]). Similarly, if A is alternative, Theorems 3.3 and 3.4 can be obtained by using Theorem 2.3 in place of Theorem 2.4 (see [GL3, Theorem 2.3(b,c)] and [G, Theorems 3.11]). References [B] J.C.Baez,The octonions, Bull. Amer. Math. Soc. (N. S.) 39 (2002), MR (2003f:17003) [BM] G. Birkhoff and S. Mac Lane, A Survey of Modern Algebra, Macmillan Publishing Co., New York, MR (3:99h) (review of original 1941 edition) [D] L.E.Dickson,On quaternions and their generalization and the history of the Eight Square Theorem, Ann. of Math. 20 ( ), MR [G] M. Goldberg, Stable norms from theory to applications and back, Linear Algebra Appl. 404 (2005), MR (2006d:15045) [GGL] M. Goldberg, R. Guralnick and W. A. J. Luxemburg, Stable subnorms II, Linearand Multilinear Algebra 51 (2003), MR (2004c:17001) [GL1] M. Goldberg and W. A. J. Luxemburg, Stable subnorms, Linear Algebra Appl. 307 (2000), MR (2001m:15065) [GL2] M. Goldberg and W. A. J. Luxemburg, Discontinuous subnorms, Linear and Multilinear Algebra 49 (2001), MR (2003a:15026) [GL3] M. Goldberg and W. A. J. Luxemburg, Stable norms, Pac. J. Math. 215 (2004), MR (2005e:17001) [HLP] G.H.Hardy,J.E.LittlewoodandG.Pólya, Inequalities, Cambridge Univ. Press, Cambridge, [R] J. J. Rotman, Advanced Modern Algebra, Pearson Education, Upper Saddle River, New Jersey, MR (2005b:00002) Department of Mathematics, Technion Israel Institute of Technology, Haifa 32000, Israel address: