# The Geometry-Algebra Dictionary

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1 Chapter 1 The Geometry-Algebra Dictionary This chapter is an introduction to affine algebraic geometry. Working over a field k, we will write A n (k) for the affine n-space over k and k[x 1,..., x n ] for the ring of polynomials in n variables over k. Our geometric objects of study will be subsets of A n (k), called (affine) algebraic sets, which arise as sets of solutions of polynomial equations. Although we will not require this in our definition, every algebraic set A A n (k) can be described using finitely many polynomials. The proof of this fact is a first example of how algebra enters the study of solution sets: We will write A as the (common) vanishing locus V(I) of the elements in an ideal I k[x 1,..., x n ], and apply Hilbert s basis theorem which states that every such ideal is finitely generated. As a kind of inverse to the correspondence I V(I), we will define a correspondence I by associating to each algebraic set A A n (k) the ideal I(A) of all polynomials vanishing on A: {algebraic subsets of A n (k)} I {ideals of k[x 1,..., x n ]}. These correspondences behave particularly well if k is algebraically closed: Under this assumption, Hilbert s celebrated Nullstellensatz 1 identifies the ideals of type I(A). Based on the Nullstellensatz, we will develop a dictionary between geometric and algebraic statements. As part of the dictionary, we will study a number of natural geometric operations on algebraic sets together with their algebraic counterparts. Furthermore, we will give examples of how properties of algebraic sets can be expressed in terms of ideals I k[x 1,..., x n ] and, in turn, of quotient rings k[x 1,..., x n ]/I. The notion of modules will allow us to treat ideals and quotient rings on equal footing (modules other than ideals and quotient rings will arise naturally in subsequent chapters). In the final section of this chapter, we will see that each algebraic set A A n (k) comes equipped with a ring of functions, the ring of polynomial functions, which is naturally isomorphic to the quotient ring k[x 1,..., x n ]/I(A). 1 theorem of zeroes V

2 4 1 The Geometry-Algebra Dictionary We will use the polynomial functions to define the natural maps, or morphisms, between affine algebraic sets, and to relate these maps to ring homomorphisms on the algebraic side. In presenting explicit examples, we will occasionally use a piece of terminology whose meaning should be intuitively clear, but whose formal definition will be given later in the book. 1.1 Polynomials In this section, we will fix our terminology for dealing with polynomials. If R is a ring, and x 1,..., x n is a collection of variables, R[x 1,..., x n ] denotes the set of polynomials in x 1,..., x n with coefficients in R. To write the elements of R[x 1,..., x n ], we use multiindices. First, a monomial in x 1,..., x n is a product x α = x α1 1 xαn n, where α = (α 1,..., α n ) N n. Second, a term in R[x 1,..., x n ] is an element of R times a monomial. Then, each polynomial 0 f R[x 1,..., x n ] can be uniquely expressed as the sum of finitely many nonzero terms involving distinct monomials. These terms (monomials) are called the terms (monomials) of f. Given a monomial x α = x α1 1 xαn n, the degree of x α is α = α α n. Given any polynomial 0 f R[x 1,..., x n ], the degree of f, written deg f, is the maximum degree of its monomials. For convenience, we write deg 0 =. With the usual algebraic operations, the set R[x 1,..., x n ] becomes a ring which contains R as the subring of constant polynomials, and which is characterized by the following universal property: Given any homomorphism φ from R to a ring S, and s 1,..., s n S, there exists a unique homomorphism Φ : R[x 1,..., x n ] S extending φ, and such that Φ(x i ) = s i for all i. In fact, Φ is the map f f(s 1,..., s n ), where the value f(s 1,..., s n ) is obtained by substituting the s i for the x i in f and evaluating the corresponding expression in S. We refer to Φ as a substitution homomorphism, and write R[s 1,..., s n ] for its image in S. A polynomial in R[x 1,..., x n ] is homogeneous (of degree d) if all its monomials have degree d or if the polynomial is zero. We usually write R[x 1,..., x n ] d = {f R[x 1,..., x n ] f is homogenous of degree d}. Subsets of polynomials such as R[x 1,..., x n ] d and R[x 1,..., x n ] <d are defined similarly. Note that if R = k is a field, then k[x 0,..., x n ] d is a k-vector space of dimension ( ) d+n n. Indeed, the monomials of degree d form a k-basis. Every nonzero polynomial f R[x 1,..., x n ] can be uniquely written as a sum f = f 0 + f 1 + f , where the f i are homogenenous of degree i. The f i are called the homogeneous components of f. Given an extra variable x 0, the polynomial f h := x deg(f) 0 f(x 1 /x 0,..., x n /x 0 ) R[x 0, x 1,..., x n ]

3 1.1 Polynomials 5 is homogeneous of degree deg(f), and is called the homogenization of f with respect to x 0. Conversely, the dehomogenization of a homogeneous polynomial F R[x 0, x 1,..., x n ] with respect to x 0 is defined to be the polynomial F (1, x 1,..., x n ) R[x 1,..., x n ]. We have f h (1, x 1,..., x n ) = f and F = x s 0 F (1, x 1,..., x n ) h, where s is the highest power of x 0 dividing F. If u x = {x 1,..., x n } is a subset of variables, then R[x 1,..., x n ] is canonically isomorphic to R[u][x \ u]. In particular, R[x 1,..., x n ] = R[x 1,..., x n 1 ][x n ]. (1.1) Explicitly, every polynomial in R[x 1,..., x n ] can be uniquely expressed as a polynomial in x n with coefficients in R[x 1,..., x n 1 ]. The isomorphism (1.1) is often used to prove a result on polynomials in several variables by induction on the number of variables. We briefly recall a typical example of how this works (for details, see, for instance, Dummit and Foote (2003), Sections 8.3 and 9.3): The polynomial ring k[x] in one variable x over a field k is an Euclidean domain and, hence, a principal ideal domain (PID for short). It is, then, also a unique factorization domain (UFD for short). In particular, if R is an integral domain, and Q(R) is its quotient field, then Q(R)[x] is a UFD. Using this and Gauss lemma, one shows that if R is a UFD, then R[x] is a UFD as well. Inductively, R[x 1,..., x n ] is a UFD. We will return to some of this later in the book: Euclidean division with remainder will be a topic of Section 2.2, the definition of a PID will be recalled in Section 1.4 below, and quotient fields will be discussed in Section 2.6. As ususal, k(x 1,..., x n ) = Q(k[x 1,..., x n ]) will denote the field of rational functions in x 1,..., x n with coefficients in k.. Partial derivatives of polynomials are defined for polynomials with coefficients in any ring R by formally writing the formula familiar from calculus: Definition If f = α c αx α R[x 1,..., x n ] is a polynomial, its ith formal partial derivative is defined by the formula f x i = α c α α i x α1 1 xαi 1 The usual rules of differentiation apply: i x αn n. Exercise Show that x i is R-linear. 2. (Product Rule) Given f, g R[x 1,..., x n ], show that x i (fg) = f x i g + f g x i.

4 6 1 The Geometry-Algebra Dictionary 3. (Chain Rule) Given g R[y 1,..., y m ] and f j R[x 1,..., x n ], j = 1,..., m, show that x i (g(f 1,..., f m )) = m j=1 g y j (f 1,..., f m ) f j x i. 4. (Euler s rule) If f R[x 1,..., x n ] is homogeneous of degree d, show that n f d f = x i. x i A polynomial with coefficients in a field of characteristic zero is constant iff all its formal partial derivatives are zero. In characteristic p > 0, however, this is not true (for instance, xp i x i = px p 1 i = 0). Instead, we have: Exercise Show that if k is a field of characteristic p > 0, and f k[x 1,..., x n ], then f x i = 0 iff f k[x 1,..., x p i,..., x n]. Conclude that all the f x i are zero iff f k[x p 1,..., xp n ]. By allowing infinitely many terms instead of just finitely many, we pass from polynomials to formal power series: Remark-Definition Let k be a field. A formal power series in the variables x 1,..., x n with coefficients in k is an expression of type α N n a α x α, with all a α k. i=1 These expressions form a ring, with algebraic operations a α x α + b α x α = (a α + b α )x α α N n α N n α N n α N n a α x α α N n b α x α = ( γ N n α+β=γ and a α b β ) x α. This ring, denoted k[[x 1,..., x n ]], is called the ring of formal power series in n variables x 1,..., x n with coefficients in k. Note that k[x 1,..., x n ] is naturally contained in k[[x 1,..., x n ]] as a subring. 1.2 Algebraic Sets Let k be any field. The affine n-space over k, denoted A n (k), is defined to be the set A n (k) = { (a 1,..., a n ) a 1,..., a n k }. An element p = (a 1,..., a n ) A n (k) is called a point, and the a i are called the coordinates of p. We say that A 1 (k) and A 2 (k) are the affine line and the affine plane over k, respectively.

5 1.2 Algebraic Sets 7 Remark In writing A n (k) instead of k n, we emphasize that we consider the set of n-tupels with coordinates in k stripped from its vector space structure. In particular, there is no distinguished origin. As a matter of convenient terminology, we will nevertheless refer to the point o A n (k) with all coordinates zero as the origin of A n (k). If k[x 1,..., x n ] is the ring of polynomials in n variables with coefficients in k, then each element f k[x 1,..., x n ] defines a function f : A n (k) k, (a 1,..., a n ) f(a 1,..., a n ). We refer to such a function as a polynomial function on A n (k), with values in k. Particular examples are the coordinate functions x i : A n (k) k, (a 1,..., a n ) a i. Considering a polynomial f k[x 1,..., x n ] as a function on A n (k) allows us to talk about its vanishing locus (or locus of zeros) in A n (k), namely V(f) := {p A n (k) f(p) = 0}. Exercise Let k be an infinite field, and let f k[x 1,..., x n ] be a polynomial. If f is nonzero, show that the complement A n (k) \ V(f) is an infinite set. Conclude that f is the zero polynomial iff the polynomial function f : A n (k) k is zero. Hint. Proceed by induction on the number n of variables. To begin with, recall that a nonzero polynomial in one variable has at most finitely many roots. Exercise Let F 2 be the field with two elements. Find a polynomial in F 2 [x 1,..., x n ] involving all the x i and vanishing at each point of A n (F 2 ). Definition A subset A A n (k) is called a hypersurface in A n (k) if A = V(f) for some nonconstant polynomial f k[x 1,..., x n ]. A hypersurface defined by a linear (degree-1) polynomial f = a 1 x a n x n b k[x 1,..., x n ] is called a hyperplane. A hypersurface in A 2 (k) is called an affine plane curve. In showing first examples of such curves, we choose k = R as our ground field so that we can draw pictures: Example Subsets of A 2 (R) defined by a degree-2 polynomial ax 2 + bxy + cy 2 + dx + ey + f are classically known as conics. Of particular importance are the nondegenerate conics which were already studied by the ancient Greek mathematicians and which are ellipses, parabolas, and hyperbolas. For instance:

6 8 1 The Geometry-Algebra Dictionary x y2 = 1 y = x 2 4x 2 4y 2 = 1 ellipse parabola hyperbola In addition, there are peculiar examples of conics such as the pair of lines with equation xy = 0. Can you find other peculiar examples? 2. The vanishing locus of a degree-3 polynomial in A 2 (R) is classically called a cubic curve in A 2 (R). A first systematic treatment of these curves is due to Newton (1666). Here are some particular examples: y 2 = x 3 + x + 1 y 2 = x 3 + x 2 y 2 = x 3 y 2 = x 3 x The equation y 2 = xy+x 2 y x 3 defines the union of a parabola and a line: 3. If f R[x, y] is the degree-seven polynomial f = 11 y y 6 x + 8 y 5 x 2 3 y 4 x 3 10 y 3 x 4 10 y 2 x 5 x 7 33 y 6 29 y 5 x 13 y 4 x y 3 x y 2 x yx x y y 4 x 8 y 3 x 2 33 y 2 x 3 20 yx 4 3 x 5 11 y 4 15 y 3 x + 13 y 2 x yx 3 + x 4, the curve C = V(f) A 2 (R) has three triple points and one quadruple point:

7 1.2 Algebraic Sets y x 1 2 Exercise Let f R[x, y] and C = V(f) A 2 (R) be as in the preceeding example, and let R(t) = Q(R[t]) be the field of rational functions in one variable t with coefficients in R. If x(t), y(t) R(t) are the rational functions x (t) = 121 t t t t t 3 92 t t 121 t t t t t t 2 20 t + 1, y (t) = 77 t t t t t t 2 20 t t t t t t t 2 20 t + 1, compute that f(x(t), y(t)) = 0 R(t). Hence, there is a well-defined map ϕ : U C, a (x(a), y(a)), where U consists of all points of A 1 (R) except the real roots of the denominator of x(t) and y(t). Hint. The coefficients of f, x(t), and y(t) are rational numbers (in fact, integers). Thus, the actual computation takes place in Q(t). Rather than doing the computation bare-handed, use your favorite computer algebra system. Remark Rational parametrizations such as the map ϕ in the exercise above will be treated systematically in Section 2.6. In the second half of the book, we will discuss how to decide whether a given curve admits such a parametrization (actually, most curves don t). And, we will present a method for computing rational parametrizations of plane curves in cases where such parametrizations exist. Hypersurfaces in affine 3-space provide examples of surfaces: Example Let k = R. 1. The surface is a cone with vertex at the origin: V(x 2 + y 2 z 2 ) A 3 (R)

8 10 1 The Geometry-Algebra Dictionary Note that the ancient Greeks (most notably, Apollonius) realized the nondegenerate conics as sections of cones by planes (see Kline (1972) for some historical remarks). 2. Clebsch s diagonal cubic in A 3 (R) is a surface containing precisely 27 real lines (see Clebsch (1871), 16): It is defined by the equation where (p 3 + q 3 + r 3 s 3 ) (p + q + r s) 3 = 0, p = 1 z cx, q = 1 z + cx, r = 1 + z + cy, s = 1 + z cy, with c = Barth s sextic in A 3 (R) is a surface with 50 nodes (see Barth (1996)):

9 1.2 Algebraic Sets 11 It is defined by the equation (8c + 4)x 2 y 2 z 2 c 4 (x 4 y 2 + y 4 z 2 + x 2 z 4 ) + c 2 (x 2 y 4 + y 2 z 4 + x 4 z 2 ) 2c+1 4 (x 2 + y 2 + z 2 1) 2 = 0, where c = is the golden section. In general, we are not only concerned with hypersurfaces, but also with sets defined by more than one polynomial equation: If T k[x 1,..., x n ] is any subset, its vanishing locus (or locus of zeros) in A n (k) is the set V(T ) = {p A n (k) f(p) = 0 for all f T }. If T = {f 1,..., f r } is finite, we write V(f 1,..., f r ) = V(T ). Definition A subset A A n (k) is called an algebraic subset, or simply an algebraic set, if A = V(T ) for some subset T k[x 1,..., x n ]. An affine algebraic set is an algebraic subset of some A n (k). Since V(T ) = f T V(f), a subset of An (k) is algebraic iff it can be written as the intersection of hypersurfaces. Example The intersection of hyperplanes is the set of solutions of a system of linear equations as studied in linear algebra. We will refer to such a set as a linear subvariety of A n (k). Example Let k = R. 1. The intersection of the two hypersurfaces V(y x 2 ) and V(z x 3 ) in A 3 (R) is called the twisted cubic curve in A 3 (R):

10 12 1 The Geometry-Algebra Dictionary 2. Intersecting the hypersurfaces V(xz) and V(yz) in A 3 (R) gives the union of the xy-plane and the z-axis: Exercise Use your favorite system(s) for visualization to draw your own pictures of the algebraic sets in Examples 1.2.5, 1.2.8, and Remark-Definition Given a polynomial f k[x 1,..., x n ], we write D(f) := A n (k) \ V(f). We have D(1) = A n (k), and if f, g k[x 1,..., x n ], then D(f) D(g) = D(fg). Hence, the D(f) form the basis for a topology on A n (k) whose closed sets are the algebraic subsets of A n (k). This topology is called the Zariski topology on A n (k), and the D(f) are called the distinguished open sets. In this book, if not otherwise mentioned, the affine n-space A n (k) will be endowed with the Zariski topology. Subsets of A n (k) will carry the induced topology which is called the Zariski topology on the subset. Topological notions such as open, closed, dense, or neighborhood will refer to this topology. If A A n (k) is a subset, then A will denote its closure in the Zariski topology. Remark If k = R or k = C, every subset of A n (k) which is open in the Zariski topology is also open in the usual Euclidean topology. Indeed, polynomial functions on A n (k) are continuous in the Euclidean topology. As we know from the linear case, the equations describing an algebraic set are by no means unique. In fact, we usually solve a system of linear equations by transforming it to an equivalent system from which the solutions can be read off. Each new equation is obtained as a linear combination of the original ones,

11 1.3 Ideals 13 using scalars as coefficients. In the more general situation here, we consider linear combinations of polynomial equations with polynomials instead of just scalars as coefficients. For instance, considering 1 (z x 3 ) x (y x 2 ) = z xy, we see that the twisted cubic curve V(y x 2, z x 3 ) may also be described as the intersection of the hypersurfaces V(y x 2 ) and V(z xy): In general, if T k[x 1,..., x n ] is any set of polynomials, and T is the set of all k[x 1,..., x n ]-linear combinations g 1 f g r f r, where f 1,..., f r T, then V(T ) = V( T ). In the language of ideals, which we quickly recall in the following section, this means that in definining the algebraic set V(T ), we may replace T by the ideal generated by T. 1.3 Ideals Let R be a ring. Definition An ideal of R is an additive subgroup I of R such that if f R and g I, then fg I. If T is any nonempty subset of R, the set of all R-linear combinations of elements of T, written T, is an ideal of R. In fact, it is the smallest ideal of R containing T. We refer to it as the ideal generated by T. If T = {f 1,..., f r } is finite, we write f 1,..., f r for T. By convention, the ideal generated by the empty subset of R is 0. If I R is an ideal, any subset T of I satisfying I = T is called a set of generators for I. We say that I is finitely generated if it admits a finite set of generators. It is principal if it can be generated by a single element. Exercise If {I λ } is a family of ideals of R, show that the intersection λ I λ is also an ideal of R. 2. If I 1,..., I s are ideals of R, their product I 1 I s is the ideal generated by all elements of type f 1 f s, where f k I k for all k. Prove that I 1 I s s k=1 I k, and show by example that the inclusion may well be strict.

12 14 1 The Geometry-Algebra Dictionary The union of a family {I λ } of ideals of R is not necessarily an ideal. The sum of the I λ, written λ I λ, is the ideal generated by the union λ I λ. If I, J are two ideals of R, the set I : J = {f R fg I for all g J} is an ideal of R containing I. It is called the ideal quotient of I by J. If g is a single element of R, we usually write I : g instead of I : g. Exercise Let I, I k, J, J k, K be ideals of R, for 1 k s, and let g R. Show: 1. I : J = R J I. ( s ) s 2. I k : J = (I k : J). k=1 ( s ) 3. I : J k = k=1 k=1 s (I : J k ). k=1 4. (I : J) : K = I : JK. 5. I : g m = I : g m+1 = I = (I : g m ) I, g m. We say that an ideal I of R is a proper ideal if I R. A proper ideal p of R is a prime ideal if f, g R and fg p implies f p or g p. A proper ideal m of R is a maximal ideal if there is no ideal I of R such that m I R. Exercise Show: 1. Every maximal ideal of R is a prime ideal of R. 2. If I 1,..., I s R are ideals, and p R is a prime ideal containing the product I 1 I s, then p contains one of the I k. 3. (Prime Avoidance) If p 1,..., p s R are prime ideals, and I R is an ideal contained in the union s k=1 p k, then I is contained in one of the p k. In other words, if I p k for all k, then there is an element in I which avoids to be in the union s k=1 p k. Conditions on an ideal I of R may also be expressed as conditions on the quotient ring R/I. We briefly recall the definition of the quotient ring: Remark-Definition Let I R be an ideal. Two elements f, g of R are said to be congruent modulo I, written f g mod I, if f g I. The relation on R defined by congruence modulo I is an equivalence relation. We usually write f = f + I for the equivalence class of f R,

13 1.3 Ideals 15 and call it the residue class of f modulo I. The set of all residue classes becomes a ring, with algebraic operations f + g = f + g and f g = f g. We refer to this ring as the quotient ring R/I, and to the map R R/I, f f, as the canonical projection onto R/I. Any homomorpism of rings φ : R S gives rise to a monomorphism R/ ker φ S, f φ(f), which is an isomorphism iff φ is an epimorphism. A proof of this fact, which is known as the homomorphy theorem, will be given in Exercise , where the theorem will be treated in a more general setting. Exercise Let I be an ideal of R. Show: 1. I is prime R/I is an integral domain. 2. I is maximal R/I is a field. The relationship between ideals of R/I and ideals of R will be discussed in Exercise Definition A ring R is called a local ring if it has exactly one maximal ideal. If m is this ideal, we also say that (R, m) is a local ring, and refer to R/m as the residue field of R. Remark The name local comes from geometry (see Section 4.2). Note that a ring R is local iff its nonunits form a (maximal) ideal. Two ideals I, J R are called coprime if I + J = 1. Exercise (Chinese Remainder Theorem). Let I 1,..., I s be ideals of R. Consider the natural ring homomorphism Show: φ : R s R/I k, f (f + I 1,..., f + I s ). k=1 1. The kernel of φ is ker φ = s k=1 I k. 2. If the I k are pairwise coprime, then I 1 I s = s k=1 I k. 3. The I k are pairwise coprime iff φ is surjective. If the I k are pairwise coprime, conclude from the homomorphy theorem that R/( s I k ) = k=1 s R/I k. k=1

14 16 1 The Geometry-Algebra Dictionary Remark Let φ : R S be a homomorphism of rings. If J is an ideal of S, then φ 1 (J) is an ideal of R. In contrast, if I is an ideal of R, then φ(i) is not necessarily an ideal of S (consider, for instance, the inclusion Z Q and any nonzero ideal I of Z). We usually write IS = φ(i)s for the ideal generated by φ(i) in S. 1.4 Hilbert s Basis Theorem Representing algebraic sets by ideals will allow us to use algebraic results in the study of solution sets. Corollary below is an example of how this works. Theorem (Hilbert s Basis Theorem). Every ideal of k[x 1,..., x n ] has a finite set of generators. Corollary Every algebraic subset of A n (k) can be expressed as the vanishing locus of finitely many polynomials. Proof (of the corollary). Given V(T ), apply the basis theorem to the ideal T k[x 1,..., x n ]. Remark In terms of the Zariski topology, this means that every open subset of A n (k) is the finite union of distinguished open sets. There are several proofs of the basis theorem which all proceed by induction on the number of variables. The first step in the induction is the univariate case which is rather special: Remark The polynomial ring k[x] in one variable x is a principal ideal domain (PID for short). That is, every ideal I of k[x] is principal. Indeed, if f I is a nonzero polynomial of minimal degree, use Euclidean division with remainder to show that I = f. Hilbert s original proof of the basis theorem was published in the first of his two milestone papers on invariant theory (1890, 1893). These papers contain further fundamental results some of which will play a prominent role in this book: the Nullstellensatz 1.6.2, the Syzygy Theorem , and Theorem on the polynomial nature of what is nowadays called the Hilbert function. Note that Hilbert and his contemporaries used the word basis as another name for a (finite) set of generators. In Chapter 2, we will encounter bases of a special type, nowadays called Gröbner bases, which are well-suited for computational purposes. Historically, bases of this type were first considered by Gordan (1899) who used them to give his own proof of the basis theorem. We refer to Exercise and Corollary for this proof. The general theory of rings in which every ideal is finitely generated was developed by Emmy Noether (1921), a student of Gordan. In particular, Noether

15 1.5 The Correspondences V and I 17 realized the importance of the ascending chain condition which we will highlight below (in Section 1.8, following Noether, we will use the condition to derive the existence of primary decompositions). Exercise Show that the following conditions on a ring R are equivalent: 1. (Finiteness condition) Every ideal of R is finitely generated. 2. (Ascending chain condition) Every chain I 1 I 2 I 3... of ideals of R is eventually stationary. That is, I m = I m+1 = I m+2 =... for some m (Maximal condition) Every nonempty set of ideals of R has a maximal element with respect to inclusion. Definition A ring satisfying the equivalent conditions above is called a Noetherian ring. In the following exercise, we give a proof of the basis theorem which makes use of the ascending chain condition: Exercise (Hilbert s Basis Theorem, General Version). If R is a Noetherian ring, show that R[x] is Noetherian. Conclude that the polynomial rings Z[x 1,..., x n ] and k[x 1,..., x n ] are Noetherian. Hint. Suppose that there is an ideal I R[x] which is not finitely generated. Let f 1 I be a nonzero polynomial of minimal degree, and let a 1 R be its leading coefficient (that is, the coefficient of the term of highest degree). Construct an ascending chain of ideals a 1 a 1, a 2 R. 1.5 The Correspondences V and I By taking vanishing loci, we get a map V which sends subsets of k[x 1,..., x n ] to algebraic subsets of A n (k). We summarize some properties of this map: Proposition Let R = k[x 1,..., x n ]. Then: 1. V(0) = A n (k). V(1) =. 2. If I J are subsets of R, then V(I) V(J). 3. If I, J are ideals of R, then V(I) V (J) = V(I J) = V(I J).

16 18 1 The Geometry-Algebra Dictionary 4. If {I λ } is a family of ideals of R, then ( ) V(I λ ) = V I λ. λ λ 5. If a 1,..., a n k, then V(x 1 a 1,..., x n a n ) = {(a 1,..., a n )}. Exercise Prove Proposition Now, proceeding in the other direction, we define a map I which sends subsets of A n (k) to ideals in k[x 1,..., x n ]: Remark-Definition If A A n (k) is any subset, the set I(A) := {f k[x 1,..., x n ] f(p) = 0 for all p A} is an ideal of k[x 1,..., x n ]. It is called the vanishing ideal of A. Exercise Show that every polynomial f k[x, y, z] has a representation of type f = g 1 (y x 2 ) + g 2 (z x 3 ) + h, where g 1, g 2 k[x, y, z] and h k[x]. 2. Let k be infinite, and let C = V(y x 2, z x 3 ) A 3 (k) be the twisted cubic curve in A 3 (k). Show that I(C) = y x 2, z x 3. Hint. To obtain the representation in part 1, first suppose that f is a monomial. For part 2, use that C can be parametrized: C = {(a, a 2, a 3 ) a k}. The expression for f in terms of y x 2 and z x 3 in the exercise above can be computed in a more systematic way, using a general algorithm for division with remainder. We will come back to this in Exercise Exercise Let k = R, and let C = {(a 2 + 1, a 3 + a) a R} A 2 (R). Show that I(C) = y 2 x 3 + x 2, and conclude that C = C {(0, 0)}.

17 1.5 The Correspondences V and I 19 Hint. For the second statement, consider lines through o = (0, 0). Remark The computations in both exercises above make use of a parametrization of the given curve. In general, no such parametrization exists, and it can be a difficult task to compute I(A) (see Remark for hints on algorithms). A method which often allows one to decide whether a given set of polynomials defining A actually generates I(A) can be deduced from the Jacobian Criterion (see Corollaries and ). In the following proposition, we summarize some properties of I, and start examining how V and I are related: Proposition Let R = k[x 1,..., x n ]. Then: 1. I( ) = R. If k is infinite, then I(A n (k)) = If A B are subsets of A n (k), then I(A) I(B). 3. If A, B are subsets of A n (k), then I(A B) = I(A) I(B). 4. If (a 1,..., a n ) A n (k) is a point, then I({(a 1,..., a n )}) = x 1 a 1,..., x n a n. 5. For any subset A A n (k), we have V(I(A)) A, with equality occuring iff A is algebraic. In any case, V(I(A)) = A. 6. For any subset I R, we have I(V(I)) I, with equality occuring iff I is the vanishing ideal of a subset of A n (k). Exercise Prove Proposition Not every ideal I k[x 1,..., x n ] can occur as a vanishing ideal I(A). That is, the inclusion I(V(I)) I may well be strict. To put it yet in another way, the map V is not injective. In fact, there are two different ways in which distinct ideals can represent the same algebraic set. The following example indicates one possibility:

20 22 1 The Geometry-Algebra Dictionary For the opposite inclusion, let f I(V(I)), and let f 1,..., f r be polynomials generating I. Then f vanishes on V(I), and we have to show that f m = g 1 f g r f r for some m 1 and some g 1,..., g r k[x 1,..., x n ]. For this, we use the trick of Rabinowitch: Consider the ideal J := f 1,..., f r, yf 1 k[x 1,..., x n, y], where y is an extra variable. Proceeding in two steps, we will show in Step 1 that V(J) A n+1 (k) is empty. Then, in Step 2, we will apply the weak version of the Nullstellensatz to conclude that 1 J. The result will follow from a representation of 1 as a k[x 1,..., x n, y]-linear combination of f 1,..., f r, yf 1. Step 1. Consider a point p = (a 1,..., a n+1 ) A n+1 (k). To show that p / V(J), we distinguish two cases. If (a 1,..., a n ) V(I), then f(a 1,..., a n ) = 0 since f I(V(I)). Evaluating yf 1 in (a 1,..., a n+1 ) gives a n+1 f(a 1,..., a n ) 1 = 1 0, so that p = (a 1,..., a n+1 ) / V(J). If (a 1,..., a n ) / V(I), then f k (a 1,..., a n ) is nonzero for some k. Hence, again, p / V(J). We conclude that V(J) =. Step 2. By Step 1 and the weak version of the Nullstellensatz, we have 1 J. Hence, there are polynomials h 1,..., h r, h k[x 1,..., x n, y] such that 1 = r h i (x 1,..., x n, y)f i + h(x 1,..., x n, y)(yf 1). i=1 Let y m be the highest power of y appearing in any of the h i. Multiplying by f m and reducing modulo yf 1, we get polynomials g i k[x 1,..., x n ] such that r f m g i f i mod yf 1. i=1 Since the natural homomorphism k[x 1,..., x n ] k[x 1,..., x n, y]/ yf 1, x i x i, is injective, we actually have r f m = g i f i k[x 1,..., x n ]. i=1 Corollary If k = k is algebraically closed, then I and V define a oneto-one correspondence {algebraic subsets of A n (k)} I {radical ideals of k[x 1,..., x n ]}. V

21 1.6 Hilbert s Nullstellensatz 23 The weak version of the Nullstellensatz adresses the basic problem of solvability: Given f 1,..., f r k[x 1,..., x n ], the system f 1 (x 1,..., x n ) = 0,..., f r (x 1,..., x n ) = 0 fails to have a solution over the algebraic closure k iff 1 f 1,..., f r. The trick of Rabinowitch allows us to discuss a related problem: Corollary (Radical Membership). Let k be an arbitrary field, let I k[x 1,..., x n ] be an ideal, and let f k[x 1,..., x n ] be a polynomial. Then: f rad I 1 J = I, yf 1 k[x 1,..., x n, y], where y is an extra variable. Proof. The implication from right to left is clear from Step 2 of the proof of Theorem For the converse implication, let f rad I. Then f m I J for some m 1. Since yf 1 J as well, we get, as desired: m 1 1 = y m f m (y m f m 1) = y m f m (yf 1) y i f i J. i=0 Hilbert s Nullstellensatz is fundamental to the geometry-algebra dictionary. We will apply it to translate geometric statements into statements on ideals I k[x 1,..., x n ] and, in turn, statements on quotient rings k[x 1,..., x n ]/I. Here is, for instance, a result which extends the weak version of the Nullststellensatz in that it characterizes systems of polynomial equations with at most finitely many solutions: Exercise Let I k[x 1,..., x n ] be an ideal, and let k be the algebraic closure of k. Show that the following are equivalent: 1. The locus of zeros of I in A n (k) is a finite set of points (or empty). 2. For each i, 1 i n, there is a nonzero polynomial in I k[x i ]. 3. The k-vector space k[x 1,..., x n ]/I has finite dimension. How to decide algorithmically whether an ideal I k[x 1,..., x n ] contains 1 or whether it satisfies conditions 2 and 3 above will be explained in Sections 2.3 and 2.4. Exercise Show that every algebraic subset of A n (R) can be defined by a single polynomial equation. Give examples of ideals I R[x 1,..., x n ] whose locus of zeros in A n (R) is finite though dim R R[x 1,..., x n ]/I =. See also Exercise

22 24 1 The Geometry-Algebra Dictionary 1.7 Irreducible Components The algebraic set V(xz, yz) A 3 (R) in Example decomposes as the union of the xy-plane V(z) and the z-axis V(x, y) which are, again, algebraic sets. In this section, we will show that every algebraic set is the union of finitely many algebraic sets which cannot be decomposed any further. Definition A nonempty algebraic set A A n (k) is called irreducible, or a subvariety of A n (k), if it cannot be expressed as the union A = A 1 A 2 of algebraic sets A 1, A 2 properly contained in A. Otherwise, A is called reducible. The empty set is not considered to be irreducible. An affine variety is a subvariety of some A n (k). Proposition let A A n (k) be an algebraic set. Then the following conditions are equivalent: 1. A is irreducible. 2. I(A) is a prime ideal. 3. k[x 1,..., x n ]/I(A) is an integral domain. Proof. 1 = 2: Suppose that A is irreducible. Then A, so that I(A) is a proper ideal. Let f, g k[x 1,..., x n ] such that fg I(A). Then A = (A V(f)) (A V(g)). Since A is irreducible, we have either A = A V(f) or A = A V(g). Hence, either f I(A) or g I(A). 2 = 1: Now, suppose that I(A) is a prime ideal. Then I(A) is a proper ideal, so that A = V(I(A)). Let A 1, A 2 A n (k) be algebraic sets such that A = A 1 A 2. Then I(A) = I(A 1 ) I(A 2 ). Since I(A) is a prime ideal, we have either I(A) = I(A 1 ) or I(A) = I(A 2 ) (apply part 2 of Exercise 1.3.4). Hence, either A = A 1 or A = A : This is a special case of part 2 of Exercise Clearly, every prime ideal is a radical ideal. Corollary If k = k is algebraically closed, then I and V define a oneto-one correspondence {subvarieties of A n (k)} I {prime ideals of k[x 1,..., x n ]}. V Example If k is infinite, then I(A n (k)) = 0 by Exercise In particular, I(A n (k)) is a prime ideal, so that A n (k) is irreducible. In contrast, if F q is the finite field with q elements, then A n (F q ) is reducible since it consists of finitely many points. Accordingly, the ideal I(A n (F q )) is not prime. In fact, as we will show in Exercise 2.9.1, I(A n (F q )) = x q 1 x 1,..., x q n x n.

23 1.7 Irreducible Components 25 Example If k is infinite, every linear subvariety A of A n (k) is irreducible. Indeed, in this case, k[x 1,..., x n ]/I(A) = k[x i1,..., x id ] for some d and some i 1,..., i d. Example Let k be infinite. Using its parametrization, we show that the twisted cubic curve C = V(y x 2, z x 3 ) = {(a, a 2, a 3 ) a k} A 3 (k), is irreducible. In fact, we show that the vanishing ideal of C is prime. For this, if f, g k[x, y, z] such that f g I(C), set F (t) = f(t, t 2, t 3 ) and G(t) = g(t, t 2, t 3 ) k[t]. Since k is infinite, we have F G = 0, so that either F = 0 or G = 0. Hence, either f I(C) or g I(C). Since a set consisting of a single point is irreducible, its vanishing ideal is a prime ideal. In fact, even more is true: Remark If p = (a 1,..., a n ) A n (k) is a point, every polynomial f k[x 1,..., x n ] can be written as a polynomial in the x i a i : f = f(p) + terms of degree 1 in the x i a i. (1.2) Indeed, this is the Taylor expansion of f at p which is obtained by substituting the (x i a i )+a i for the x i in f and expanding the resulting expression. It is clear from (1.2) that the vanishing ideal is the kernel of the evaluation map I(p) := I({p}) = x 1 a 1,..., x n a n k[x 1,..., x n ] k, f f(p). Hence, k[x 1,..., x n ]/I(p) = k by the homomorphy theorem. In particular, k[x 1,..., x n ]/I(p) is a field, so that I(p) is a maximal ideal. Conversely, the following holds: Proposition If k = k is algebraically closed, every maximal ideal of k[x 1,..., x n ] is of the form x 1 a 1,..., x n a n for some a 1,..., a n k. Proof. Let m k[x 1,..., x n ] be a maximal ideal. Then V(m) by the weak version of the Nullstellensatz since m is proper. If p = (a 1,..., a n ) V(m) is a point, we have m I(p) = x 1 a 1,..., x n a n. Hence, m = I(p) since m is maximal.

24 26 1 The Geometry-Algebra Dictionary Corollary If k = k is algebraically closed, then I and V define a oneto-one correspondence {points of A n (k)} I {maximal ideals of k[x 1,..., x n ]}. V If k is not necessarily algebraically closed, the maximal ideals of k[x 1,..., x n ] can be described as follows: Exercise Let k be the algebraic closure of k, and let G be the Galois group of k over k. Show: 1. Let p = (a 1,..., a n ) A n (k) be a point, and let m p be the kernel of the evaluation map k[x 1,..., x n ] k, f f(p). Then m p is a maximal ideal of k[x 1,..., x n ]. Moreover, its locus of zeros in A n (k) is the orbit of p under the natural action of G on A n (k). We, then, say that the points of this locus are pairwise conjugate over k. 2. Every maximal ideal of k[x 1,..., x n ] is of type m p for some p A n (k). Example The principal ideal generated by x 2 +1 in R[x] is maximal, and its locus of zeros in A 1 (C) is {±i}. We, now, establish the main result of this section: Theorem-Definition Every nonempty algebraic set A A n (k) can be written as a finite union A = V 1 V s of irreducible algebraic sets V i. We may, in fact, achieve that this decomposition is minimal in the sense that V i V j for i j. The V i are, then, uniquely determined up to order and are called the irreducible components of A. Proof. The existence part of the proof is a typical example of Noetherian induction. Expressed in geometric terms, the maximal condition for ideals in the Noetherian ring k[x 1,..., x n ] reads that every nonempty collection of algebraic subsets of A n (k) has a minimal element with respect to inclusion. Using this, we show that the collection Γ of all algebraic subsets of A n (k) which cannot be written as a finite union of irreducible algebraic sets is empty. Suppose that Γ. Then Γ has a minimal element A which, by the very definition of Γ, must be reducible. That is, A = A 1 A 2 for some algebraic sets A 1, A 2 A. Due to the minimality of A, both A 1 and A 2 can be written as a finite union of irreducible algebraic sets. Then the same is true for A, a contradiction to A Γ.

25 1.8 Primary Decomposition 27 We conclude that every algebraic set A A n (k) can be written as a finite union of irreducible algebraic sets. Throwing away superfluous sets if necessary, we get a minimal decomposition, as required. To show uniqueness, let A = V 1 V s = V 1 V t be two minimal decompositions. Then, for each i, we have V i = V i A = V i (V 1 V t ) = (V i V 1 ) (V i V t ). Since V i is irreducible, we must have V i = V i V j for some j, so that V i V j. The same argument yields an inclusion V j V k for some k. By minimality, i = k and, thus, V i = V j. So every V i occurs as one of the V j which implies that s t. Similarly, we get t s. Uniqueness up to order follows. Exercise Show: 1. Every proper algebraic subset of A 1 (k) is a finite set of points (or empty). 2. If f, g k[x, y] are polynomials without a common factor, then V(f, g) = V(f) V(g) A 2 (k) is a finite set of points (or empty). Hint. Prove that f and g are coprime in the PID k(x)[y], and deduce that there exist a, b k(x)[y] such that af + bg = Every proper algebraic subset of A 2 (k) is a finite union of points and (irreducible) curves (or empty). 1.8 Primary Decomposition The Nullstellensatz allows us to rephrase Theorem in algebraic terms as follows: If k = k is algebraically closed, the radical of every ideal I k[x 1,..., x n ] has a unique minimal prime decomposition. That is, rad I can be uniquely written as the intersection of finitely many prime ideals: rad I = p 1 p s, where p i p j for i j. This is a purely algebraic result which, in fact, can be proved by purely algebraic means (there is no need to translate the Noetherian condition into geometry and apply the Nullstellensatz). In what follows, we present the original argument of Emmy Noether which works for any Noetherian ring R. In fact, the argument applies to arbitrary ideals of R and not just to radical ideals. The resulting decomposition has, then, to be of a more general type, however, since the intersection of prime ideals is necessarily a radical ideal.

26 28 1 The Geometry-Algebra Dictionary Definition A proper ideal q of a ring R is a primary ideal if f, g R and fg q implies f q or g rad q. Clearly, every prime ideal is primary. Proposition Let R be a ring. 1. If q is a primary ideal of R, then p := rad q is the smallest prime ideal containing q. We refer to this fact by saying that q is p-primary. 2. A finite intersection of p-primary ideals is p-primary. Exercise Prove Proposition Definition Let I be an ideal of a ring R. A primary decomposition of I is an expression of I as a finite intersection of primary ideals, say I = q 1 q t. The decomposition is called minimal if the radicals rad q i are all distinct, and q i j i q j for all i. Theorem Every proper ideal of a Noetherian ring R has a minimal primary decomposition. Proof. We proceed in three steps. Step 1. In analogy to Definition 1.7.1, we say that a proper ideal of R is irreducible if it is not the intersection of two strictly larger ideals. The algebraic version of the proof of Theorem shows that every ideal of the Noetherian ring R can be written as a finite intersection of irreducible ideals. Step 2. Let I R be an irreducible ideal. We prove that I is primary. For this, let f, g R such that fg I and f I. To show that g rad I, observe that we have a chain of ideals I : g I : g 2. By the ascending chain condition, I : g m = I : g m+1 for some m 1. Then I = (I : g m ) I, g m by Exercise Since fg I, also fg m I, so that f I : g m. This implies that I I : g m since f I. Taking into account that I is irreducible, we must have I = I, g m, so that g m I. Hence, g rad I. Step 3. Let, now, I R be any proper ideal. By Steps 1 and2, there is a primary decomposition of I. If two of the primary ideals occuring in this decomposition have the same radical p, we replace them by their intersection which is p-primary by Proposition Continuing in this way, all primary ideals will eventually have distinct radicals. Throwing away superfluous primary ideals if necessary, we get a minimal primary decomposition of I.

27 1.8 Primary Decomposition 29 Not all the ideals occuring in a primary decomposition of an ideal I are uniquely determined by I: Example The ideal xy, y 2 k[x, y] admits, for instance, the following minimal primary decompositions: xy, y 2 = y x, y 2 = y x 2, xy, y 2. Note that both x, y 2 and x 2, xy, y 2 are x, y -primary. Furthermore, the prime ideal x, y contains the prime ideal y. Theorem (1st Uniqueness Theorem). Let I be a proper ideal of a Noetherian ring R, and let I = t i=1 q i be a minimal primary decomposition of I. Then the radicals p i = rad q i are precisely the prime ideals occuring in the set of ideals I : f, f R. Proof. See Exercise Remark-Definition In the situation of the 1st uniqueness theorem, we see, in particular, that the p i only depend on I (and not on the particular minimal primary decomposition). We call each p i an associated prime of I. We say that p i is a minimal associated prime of I if p i p j for all j i. Otherwise, p i is called an embedded associated prime of I. If, say, p 1..., p s are the mimimal associated primes of I, then rad I = p 1 p s is the uniquely determined minimal prime decompositionofrad I (defined as at the beginning of this section). Any primary ideal occurring in one of the minimal primary decompositions of I is called a primary component of I. It is called an isolated component of I if its radical is a minimal associated prime of I, and an embedded component of I, otherwise. The names isolated and embedded come from the geometric interpretation: If k = k is algebraically closed, the minimal associated primes of an ideal I k[x 1,..., x n ] correspond to the irreducible components of V(I), and the embedded associated primes to subvarieties of these. Theorem (2nd Uniqueness Theorem). Let I be a proper ideal of a Noetherian ring. Then the isolated primary components of I are uniquely determined by I. Proof. We will show this in Exercise Example If R is a UFD, and f R is a nonzero nonunit, then all the associated primes of f are minimal. Indeed, if f = u f µ1 1 f s µs

28 30 1 The Geometry-Algebra Dictionary is the decomposition of f into distinct irreducible factors, the minimal primary decomposition is f = f µ1 µs 1 f s. Note that historically, the concept of primary decomposition grew out from the search for some useful generalization of unique factorization. See Eisenbud (1995), Section 1.1. If I is a proper ideal of a ring R, and p R is a prime ideal containing I, we say that p is a minimal prime of I if there is no prime ideal p of R such that I p p. A minimal prime of the zero ideal of R is also called a minimal prime of R. Proposition Let I be a proper ideal of a Noetherian ring R. Then every prime ideal containing I contains a minimal associated prime of I. Thus, the minimal associated primes of I are precisely the minimal primes of I. Proof. Let p 1,..., p s be the minimal associated primes of I. If p I is a prime ideal, then p = rad p rad I = s i=1 p i. Hence, we must have p p i for some i by part 2 of Exercise Removing Algebraic Sets The ideal I = xz, yz R[x, y, z] in Example is the intersection of the prime ideals z and x, y. In particular, I is a radical ideal. Geometrically, I defines the union of the xy-plane and the z-axis. If we remove the xy-plane, the remaining set is a punctured line, which is not an algebraic subset of A 3 (k) (see Exercise ). In this section, we show how to describe the smallest algebraic set containing the difference of two algebraic sets. We need the following notation. If I, J are two ideals of a ring R, the set I : J := {f R fj m I for some m 1} = (I : J m ) m=1 is an ideal of R. It is called the saturation of I with respect to J. If g is a single element of R, we usually write I : g instead of I : g. In any case, we have an ascending chain of ideals I : J I : J 2 I : J 3 I : J. Thus, if R is Noetherian, we have I : J m = I : J m+1 = I : J for some m 1 by the ascending chain condition. Theorem Let k = k be algebraically closed, and let I, J be ideals of k[x 1,..., x n ]. Then V(I) \ V(J) = V(I : J ). If I is a radical ideal, then V(I) \ V(J) = V(I : J).

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