POLYNOMIAL IDENTITY RINGS AS RINGS OF FUNCTIONS


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1 POLYNOMIAL IDENTITY RINGS AS RINGS OF FUNCTIONS Z. REICHSTEIN AND N. VONESSEN Abstract. We generalize the usual relationship between irreducible Zariski closed subsets of the affine space, their defining ideals, coordinate rings, and function fields, to a noncommutative setting, where varieties carry a PGL naction, regular and rational functions on them are matrixvalued, coordinate rings are prime polynomial identity algebras, and function fields are central simple algebras of degree n. In particular, a prime polynomial identity algebra of degree n is finitely generated if and only if it arises as the coordinate ring of a variety in this setting. For n = 1 our definitions and results reduce to those of classical affine algebraic geometry. Contents 1. Introduction 2 2. Preliminaries Matrix Invariants The ring of generic matrices and its trace ring Central polynomials 6 3. Definition and first properties of nvarieties 7 4. Irreducible nvarieties 9 5. The Nullstellensatz for prime ideals Regular maps of nvarieties Rational maps of nvarieties Generically free PGL n varieties BrauerSeveri Varieties 20 References 23 Date: August 5, Mathematics Subject Classification. Primary: 16R30, 16R20; Secondary 14L30, 14A10. Key words and phrases. Polynomial identity ring, central simple algebra, trace ring, coordinate ring, the Nullstellensatz. Z. Reichstein was supported in part by an NSERC research grant. N. Vonessen gratefully acknowledges the support of the University of Montana and the hospitality of the University of British Columbia during his sabbatical in 2002/2003, when part of this research was done. 1
2 2 Z. REICHSTEIN AND N. VONESSEN 1. Introduction Polynomial identity rings (or PIrings, for short) are often viewed as being close to commutative ; they have large centers, and their structure (and in particular, their maximal spectra) have been successfully studied by geometric means (see the references at the end of the section). In this paper we revisit this subject from the point of view of classical affine algebraic geometry. We will show that the usual relationship between irreducible Zariski closed subsets of the affine space, their defining ideals, coordinate rings, and function fields, can be extended to the setting of PIrings. Before proceeding with the statements of our main results, we will briefly introduce the objects that will play the roles of varieties, defining ideals, coordinate rings, etc. Throughout this paper we will work over an algebraically closed base field k of characteristic zero. We also fix an integer n 1, which will be the PIdegree of most of the rings we will consider. We will write M n for the matrix algebra M n (k). The vector space of mtuples of n nmatrices will be denoted by (M n ) m ; we will always assume that m 2. The group PGL n acts on (M n ) m by simultaneous conjugation. The PGL n invariant dense open subset U m,n = {(a 1,..., a m ) (M n ) m a 1,..., a m generate M n as kalgebra} of (M n ) m will play the role of the affine space A m in the sequel. (Note that U m,1 = A m.) The role of affine algebraic varieties will be played by PGL n invariant closed subvarieties of U m,n ; for lack of a better term, we shall call such objects nvarieties; see Section 3. (Note that, in general, nvarieties are not affine in the usual sense.) The role of the polynomial ring k[x 1,..., x m ] will be played by the algebra G m,n = k{x 1,..., X m } of m generic n n matrices, see 2.4. Elements of G m,n may be thought of as PGL n  equivariant maps (M n ) m M n ; if n = 1 these are simply the polynomial maps k m k. Using these maps, we define, in a manner analogous to the commutative case, the associated ideal I(X) G m,n, the PIcoordinate ring k n [X] = G m,n /I(X), and the central simple algebra k n (X) of rational functions on an irreducible nvariety X U m,n ; see Definitions 3.1 and 7.1. We show that Hilbert s Nullstellensatz continues to hold in this context; see Section 5. We also define the notions of a regular map (and, in particular, an isomorphism) X Y and a rational map (and, in particular, a birational isomorphism) X Y between nvarieties X U m,n and Y U l,n ; see Definitions 6.1 and 7.5. In categorical language our main results can be summarized as follows. Let Var n be the category of irreducible nvarieties, with regular maps of n varieties as morphisms (see Definition 6.1), and PI n be the category of finitely generated prime kalgebras of PIdegree n (here the morphisms are the usual kalgebra homomorphisms).
3 1.1. Theorem. The functor defined by POLYNOMIAL IDENTITY RINGS 3 X k n [X] (f : X Y ) (f : k n [Y ] k n [X]) is a contravariant equivalence of categories between Var n and PI n. In particular, every finitely generated prime PIalgebra is the coordinate ring of a uniquely determined nvariety; see Theorem 6.4. For a proof of Theorem 1.1, see Section 6. Every nvariety is, by definition, an algebraic variety with a generically free PGL n action. It turns out that, up to birational isomorphism, the converse holds as well; see Lemma 8.1. To summarize our results in the birational context, let Bir n be the category of irreducible generically free PGL n varieties, with dominant rational PGL n equivariant maps as morphisms, and CS n be the category of central simple algebras A of degree n, such that the center of A is a finitely generated field extension of k. Morphisms in CS n are kalgebra homomorphisms (these are necessarily injective) Theorem. The functor defined by X k n (X) (g : X Y ) (g : k n (Y ) k n (X)) is a contravariant equivalence of categories between Bir n and CS n. Here for any PGL n variety X, k n (X) denotes the kalgebra of PGL n  equivariant rational maps X M n (with addition and multiplication induced from M n ), see Definition 8.2. If X is an irreducible nvariety then k n (X) is the total ring of fractions of k n [X], as in the commutative case; see Definition 7.1 and Proposition 7.3. Note also that g (α) stands for α g (again, as in the commutative case). For a proof of Theorem 1.2, see Section 8. Note that for n = 1, Theorems 1.1 and 1.2 are classical results of affine algebraic geometry; cf., e.g., [13, Corollary 3.8 and Theorem 4.4]. It is well known that central simple algebras A/K of degree n are in a natural bijection with n 1dimensional BrauerSeveri varieties over K and (if K/k is a finitely generated field extension) with generically free PGL n  varieties X/k such that k(x) PGLn K. Indeed, all three are parametrized by the Galois cohomology set H 1 (K, PGL n ); for details, see Section 9. Theorem 1.2 may thus be viewed as a way of explicitly identifying generically free PGL n varieties X with central simple algebras A, without going through H 1 (K, PGL n ). The fact that the map X k n (X) is bijective was proved in [24, Proposition 8.6 and Lemma 9.1]; here we give a more conceptual proof and show that this map is, in fact, a contravariant functor. In Section 9 we show how to construct the BrauerSeveri variety of A directly from X.
4 4 Z. REICHSTEIN AND N. VONESSEN Many of the main themes of this paper (such as the use of PGL n actions, generic matrices, trace rings, and affine geometry in the study of polynomial identity algebras) were first systematically explored in the pioneering work of Amitsur, Artin and Procesi [2, 4, 19, 20, 21] in the 1960s and 70s. Our approach here was influenced by these, as well as other papers in this area, such as [3, 5, 7, 23]. In particular, Proposition 5.3 is similar in spirit to Amitsur s Nullstellensatz [1] (cf. Remark 5.6), and Theorem 1.1 to Procesi s functorial description of algebras satisfying the nth CayleyHamilton identity [22, Theorem 2.6]. (For a more geometric statement of Procesi s theorem, along the lines of our Theorem 1.1, see [15, Theorem 2.3].) We thank L. Small and the referee for bringing some of these connections to our attention. 2. Preliminaries In this section we review some known results about matrix invariants and related PItheory Matrix invariants. Consider the diagonal action of PGL n on the space (M n ) m of mtuples of n nmatrices. We shall denote the ring of invariants for this action by C m,n = k[(m n ) m ] PGLn, and the affine variety Spec(C m,n ) by Q m,n. It is known that C m,n is generated as a kalgebra, by elements of the form (A 1,..., A m ) tr(m), where M is a monomial in A 1,..., A m (see [21]); however, we shall not need this fact in the sequel. The inclusion C m,n k[(m n ) m ] of kalgebras induces the categorical quotient map (2.2) π : (M n ) m Q m,n. We shall need the following facts about this map in the sequel. Recall from the introduction that we always assume m 2, the base field k is algebraically closed and of characteristic zero, and U m,n = {(a 1,..., a m ) (M n ) m a 1,..., a m generate M n as kalgebra } Proposition. (a) If x U m,n then π 1 (π(x)) is the PGL n orbit of x. (b) PGL n orbits in U m,n are closed in (M n ) m. (c) π maps closed PGL n invariant sets in (M n ) m to closed sets in Q m,n. (d) π(u m,n ) is Zariski open in Q m,n. (e) If Y is a closed irreducible subvariety of Q m,n then π 1 (Y ) U m,n is irreducible in (M n ) m. Proof. (a) is proved in [4, (12.6)]. (b) is an immediate consequence of (a). (c) is a special case of [18, Corollary to Theorem 4.6]. (d) It is easy to see that U m,n is Zariski open in (M n ) m. Let U c m,n be its complement in (M n ) m. By (c), π(u c n,m) is closed in Q m,n and by (a), π(u m,n ) = Q m,n \ π(u c m,n).
5 POLYNOMIAL IDENTITY RINGS 5 (e) Let V 1,..., V r be the irreducible components of π 1 (Y ) in (M n ) m. Since PGL n is connected, each V i is PGL n invariant. By part (c), π(v 1 ),..., π(v r ) are closed subvarieties of Q m,n covering Y. Since Y is irreducible, we may assume, after possibly renumbering V 1,..., V r, that Y = π(v 1 ). It suffices to show that π 1 (Y ) U m,n = V 1 U m,n ; since V 1 U m,n is irreducible, this will complete the proof of part (e). As V 1 is an irreducible component of π 1 (Y ), we clearly have V 1 U m,n π 1 (Y ) U m,n. To prove the opposite inclusion, let y π 1 (Y ) U m,n. We want to show y V 1. Since π(y) Y = π(v 1 ), there is a point v V 1 such that π(y) = π(v). That is, v lies in π 1 (π(y)), which, by part (a), is the PGL n orbit of y. In other words, y = g v for some g PGL n. Since V 1 is PGL n invariant, this shows that y V 1, as claimed The ring of generic matrices and its trace ring. Consider m generic matrices X 1 = (x (1) ij ) i,j=1,...,n,..., X m = (x (m) ij ) i,j=1,...,n, where x (h) ij are mn 2 independent variables over the base field k. The k subalgebra generated by X 1,..., X m inside M n (k[x (h) ij ]) is called the algebra of m generic n nmatrices and is denoted by G m,n. If the values of n and m are clear from the context, we will simply refer to G m,n as the algebra of generic matrices. The trace ring of G m,n is denoted by T m,n ; it is the kalgebra generated, inside M n (k[x (h) Elements of M n (k[x (h) ij ij ]) by elements of G m,n and their traces. ]) can be naturally viewed as regular (i.e., polynomial) maps (M n ) m M n. (Note that k[x (h) ij ] in the coordinate ring of (M n) m.) Here PGL n acts on both (M n ) m and M n by simultaneous conjugation; Procesi [21, Section 1.2] noticed that T m,n consists precisely of those maps (M n ) m M n that are equivariant with respect to this action. (In particular, the ith generic matrix X i is the projection to the ith component.) In this way the invariant ring C m,n = k[(m n ) m ] PGLn which we considered in 2.1, is naturally identified with the center of T m,n via f fi n n. We now recall the following definitions Definition. (a) A prime PIring is said to have PIdegree n if its total ring of fractions is a central simple algebra of degree n. (b) Given a ring R, Spec n (R) is defined as the set of prime ideals J of R such that R/J has PIdegree n; cf. e.g., [19, p. 58] or [26, p. 75]. The following lemma shows that Spec n (G m,n ) and Spec n (T m,n ) are closely related.
6 6 Z. REICHSTEIN AND N. VONESSEN 2.6. Lemma. The assignment J J G m,n defines a bijective correspondence between Spec n (T m,n ) and Spec n (G m,n ). In addition, for any prime ideal J Spec n (T m,n ), we have the following: (a) The natural projection φ: T m,n T m,n /J is tracepreserving, and T m,n /J is the trace ring of G m,n /(J G m,n ). (b) tr(p) J for every p J. Proof. The first assertion and part (a) are special cases of results proved in [3, 2]. Part (b) follows from (a), since for any p J, φ(tr(p)) = tr(φ(p)) = tr(0) = 0. In other words, tr(p) Ker(φ) = J, as claimed Central polynomials. We need to construct central polynomials with certain nonvanishing properties. We begin by recalling two wellknown facts from the theory of rings satisfying polynomial identities Proposition. (a) Let k{x 1,..., x m } be the free associative algebra. Consider the natural homomorphism k{x 1,..., x m } G m,n, taking x i to the ith generic matrix X i. The kernel of this homomorphism is precisely the ideal of polynomial identities of n nmatrices in m variables. (b) Since k is an infinite field, all prime kalgebras of the same PIdegree satisfy the same polynomial identities (with coefficients in k). Proof. See [19, pp ] or [26, p. 16] for a proof of part (a) and [27, pp ] for a proof of part (b). For the convenience of the reader and lack of a suitable reference, we include the following definition Definition. An (mvariable) central polynomial for n n matrices is an element p = p(x 1,..., x m ) k{x 1,..., x m } satisfying one of the following equivalent conditions: (a) p is a polynomial identity of M n 1, and the evaluations of p in M n are central (i.e., scalar matrices) but not identically zero. (b) p is a polynomial identity for all prime kalgebras of PIdegree n 1, and the evaluations of p in every prime kalgebra of PIdegree n are central but not identically zero. (c) p is a polynomial identity for all prime kalgebras of PIdegree n 1, and the canonical image of p in G m,n is a nonzero central element. (d) The constant coefficient of p is zero, and the canonical image of p in G m,n is a nonzero central element. That the evaluations of p in an algebra A are central is equivalent to saying that x m+1 p px m+1 is a polynomial identity for A, where x m+1 is another free variable. Thus the equivalence of (a) (c) easily follows from Proposition 2.8. The equivalence of (c) and (d) follows from [19, p. 172]. The existence of central polynomials for n nmatrices was established independently by Formanek and Razmyslov; see [11]. Because of Proposition 2.8(a),
7 POLYNOMIAL IDENTITY RINGS 7 one can think of mvariable central polynomials of n n matrices as nonzero central elements of G m,n (with zero constant coefficient). The following lemma, establishing the existence of central polynomials with certain nonvanishing properties, will be repeatedly used in the sequel Lemma. Let A 1,..., A r U m,n. Then there exists a central polynomial s = s(x 1,..., X m ) G m,n for n nmatrices such that s(a i ) 0 for i = 1,..., r. In other words, each s(a i ) is a nonzero scalar matrix in M n. Proof. First note that if A i and A j are in the same PGL n orbit then s(a i ) = s(a j ). Hence, we may remove A j from the set {A 1,..., A r }. After repeating this process finitely many times, we may assume that no two of the points A 1,..., A r lie in the same PGL n orbit. By the abovementioned theorem of Formanek and Razmyslov, there exists a central polynomial c = c(x 1,..., X N ) G N,n for n nmatrices. Choose b 1,..., b N M n such that c(b 1,..., b N ) 0. We now define s by modifying c as follows: s(x 1,..., X m ) = c ( p 1 (X 1,..., X m ),..., p N (X 1,..., X m ) ), where the elements p j = p j (X 1,..., X m ) G m,n will be chosen below so that for every j = 1,..., N, (2.11) p j (A 1 ) = p j (A 2 ) = = p j (A r ) = b j M n. We first check that this polynomial has the desired properties. Being an evaluation of a central polynomial for n n matrices, s is a central element in G m,n and a polynomial identity for all prime kalgebras of PIdegree n 1. Moreover, s(a i ) = c(b 1,..., b N ) 0 for every i = 1,..., r. Thus s itself is a central polynomial for n n matrices. Consequently, s(a i ) is a central element in M n, i.e., a scalar matrix. It remains to show that p 1,..., p N G m,n can be chosen so that (2.11) holds. Consider the representation φ i : G m,n M n given by p p(a i ). Since each A i lies in U m,n, each φ i is surjective. Moreover, by our assumption on A 1,..., A r, no two of them are conjugate under PGL n, i.e., no two of the representations φ i are equivalent. The kernels of the φ i are thus pairwise distinct by [4, Theorem (9.2)]. Hence the Chinese Remainder Theorem tells us that φ 1... φ r : G m,n (M n ) r is surjective; p j can now be chosen to be any preimage of (b j,..., b j ) (M n ) r. This completes the proof of Lemma Definition and first properties of nvarieties 3.1. Definition. (a) An nvariety X is a closed PGL n invariant subvariety of U m,n for some m 2. In other words, X = X U m,n, where X is the Zariski closure of X in (M n ) m. Note that X is a generically free PGL n variety (in fact, for every x X, the stabilizer of x in PGL n is trivial). (b) Given a subset S G m,n (or S T m,n ), we define its zero locus as Z(S) = {a = (a 1,..., a m ) U m,n p(a) = 0, p S}.
8 8 Z. REICHSTEIN AND N. VONESSEN Of course, Z(S) = Z(J), where J is the 2sided ideal of G m,n (or T m,n ) generated by S. Conversely, given an nvariety X U m,n we define its ideal as I(X) = {p G m,n p(a) = 0, a X}. Similarly we define the ideal of X in T m,n, as I T (X) = {p T m,n p(a) = 0, a X}. Note that I(X) = I T (X) G m,n. (c) The polynomial identity coordinate ring (or PIcoordinate ring) of an nvariety X is defined as G m,n /I(X). We denote this ring by k n [X] Remark. Elements of k n [X] may be viewed as PGL n equivariant morphisms X M n. The example below shows that not every PGL n equivariant morphism X M n (k) is of this form. On the other hand, if X is irreducible, we will later prove that every PGL n equivariant rational map X M n (k) lies in the total ring of fractions of k n [X]; see Proposition Example. Recall that U 2,2 is the open subset of M 2,2 defined by the inequality c(x 1, X 2 ) 0, where c(x 1, X 2 ) = ( 2 tr(x 2 1) tr(x 1 ) 2)( 2 tr(x 2 2) tr(x 2 ) 2) ( 2 tr(x1 X 2 ) tr(x 1 ) det(x 2 ) ) 2 ; see, e.g., [12, p. 191]. Thus for X = U 2,2, the PGL n equivariant morphism f : X M 2 given by (X 1, X 2 ) 1 c(x 1,X 2 ) I 2 2 is not in k 2 [X] = G 2,2 (and not even in T 2,2 ) Remark. (a) Let J be an ideal of G m,n. Then the points of Z(J) are in bijective correspondence with the surjective kalgebra homomorphisms φ: G m,n M n such that J Ker(φ) (or equivalently, with the surjective kalgebra homomorphisms G m,n /J M n ). Indeed, given a Z(J), we associate to it the homomorphism φ a given by φ a : p p(a). Conversely, a surjective homomorphism φ: G m,n M n such that J Ker(φ) gives rise to the point a φ = (φ(x 1 ),..., φ(x m )) Z(J), where X i G m,n is the ith generic matrix in G m,n. One easily checks that the assignments a φ a and φ a φ are inverse to each other. (b) The claim in part (a) is also true for an ideal J of T m,n. That is, the points of Z(J) are in bijective correspondence with the surjective kalgebra homomorphisms φ: T m,n M n such that J Ker(φ) (or equivalently, with the surjective kalgebra homomorphisms T m,n /J M n ). The proof goes through without changes Lemma. Let a = (a 1,..., a m ) U m,n and let J be an ideal of G m,n (or of T mn ). Let J(a) = {j(a) j J} M n.
9 POLYNOMIAL IDENTITY RINGS 9 Then either J(a) = (0) (i.e, a Z(J)) or J(a) = M n. Proof. Since a 1,..., a m generate M n, φ a (J) is a (2sided) ideal of M n. Since M n is simple, the lemma follows Lemma. (a) Z(J) = Z(J G m,n ) for every ideal J T m,n. (b) If X U m,n is an nvariety, then X = Z(I(X)) = Z(I T (X)). Proof. (a) Clearly, Z(J) Z(J G m,n ). To prove the opposite inclusion, assume the contrary: there exists a y U m,n such that p(y) = 0 for every p J G m,n but f(y) 0 for some f J. By Lemma 2.10 there exists a central polynomial s G m,n for n nmatrices such that s(y) 0. By [28, Theorem 1], p = s i f lies in G m,n (and hence, in J G m,n ) for some i 0. Our choice of y now implies 0 = p(y) = s i (y)f(y). Since s(y) is a nonzero element of k, we conclude that f(y) = 0, a contradiction. (b) Clearly X Z(I T (X)) Z(I(X)). Part (a) (with J = I T (X)) tells us that Z(I(X)) = Z(I T (X)). It thus remains to be shown that Z(I T (X)) X. Assume the contrary: there exists a z U m,n such that p(z) = 0 for every p I T (X) but z X. Since X = X U m,n, where X is the closure of X in (M n ) m, we conclude that z X. Let C = PGL n z be the orbit of z in (M n ) m. Since z U m,n, C is closed in (M n ) m ; see Proposition 2.3(b). Thus C and X are disjoint closed PGL n invariant subsets of (M n ) m. By [16, Corollary 1.2], there exists a PGL n invariant regular function f : (M n ) m k such that f 0 on X but f 0 on C. The latter condition is equivalent to f(z) 0. Identifying elements of k with scalar matrices in M n, we may view f as a central element of T m,n. So f I T (X) but f(z) 0, contradicting our assumption. 4. Irreducible nvarieties Of particular interest to us will be irreducible nvarieties. Here irreducible is understood with respect to the nzariski topology on U m,n, where the closed subsets are the nvarieties. However, since PGL n is a connected group, each irreducible component of X in the usual Zariski topology is PGL n invariant. Consequently, X is irreducible in the nzariski topology if and only if it is irreducible in the usual Zariski topology Lemma. Let X U m,n be an nvariety. The following are equivalent: (a) X is irreducible. (b) I T (X) is a prime ideal of T m,n. (c) I(X) is a prime ideal of G m,n. (d) k n [X] is a prime ring. Proof. (a) (b): Suppose that I T (X) is not prime, i.e., there are ideals J 1 and J 2 such that J 1 J 2 I T (X) but J 1, J 2 I T (X). We claim that X is not irreducible. Indeed, by Lemma 3.5, X Z(J 1 ) Z(J 2 ). In other words, we can write X = X 1 X 2, as a union of two nvarieties, where
10 10 Z. REICHSTEIN AND N. VONESSEN X 1 = Z(J 1 ) X and X 2 = Z(J 2 ) X. It remains to be shown that X i X for i = 1, 2. Indeed, if say, X 1 = X then every element of J 1 vanishes on all of X, so that J 1 I T (X), contradicting our assumption. (b) (c): Clear, since I(X) = I T (X) G m,n. (c) (d): I(X) G m,n is, by definition, a prime ideal if and only if k n [X] = G m,n /I(X) is a prime ring. (c) (a): Assume I(X) is prime and X = X 1 X 2 is a union of two nvarieties in U m,n. Our goal is to show that X = X 1 or X = X 2. Indeed, I(X 1 ) I(X 2 ) I(X) implies I(X i ) I(X) for i = 1 or 2. Taking the zero loci and using Lemma 3.6(b), we obtain as desired. X i = Z(I(X i )) Z(I(X)) = X, 4.2. Proposition. Let J Spec n (T m,n ). Then (a) Z(J) = Z(J C m,n ) (b) Z(J) is irreducible. Proof. (a) Clearly Z(J C m,n ) Z(J). To prove the opposite inclusion, suppose a Z(J C m,n ) and consider the evaluation map φ a : T m,n M n given by φ a (p) = p(a). Recall that φ a is tracepreserving (see, e.g., [3, Theorem 2.2]). Since tr(j) J C m,n for every j J, we see that tr(j(a)) = 0 for every j J. By Lemma 3.5 this implies that a Z(J), as claimed. (b) Consider the categorical quotient map π : (M n ) m Q m,n for the PGL n action on (M n ) m. Recall that C m,n = k[q m,n ] is the coordinate ring of Q m,n. Note that elements of C m,n may be viewed in two ways: as regular functions on Q m,n or (after composing with π) as a PGL n invariant regular function on (M n ) m. Let Y Q m,n be the zero locus of J C m,n in Q m,n. Then by part (a), Z(J) = Z(J C m,n ) = π 1 (Y ) U m,n. Since J is a prime ideal of T m,n, J C m,n is a prime ideal of C m,n ; see, e.g., [19, Theorem II.6.5(1)]. Hence, Y is irreducible. Now by Proposition 2.3(e), we conclude that Z(J) = π 1 (Y ) U m,n is also irreducible, as claimed Corollary. If J 0 Spec n (G m,n ), then Z(J 0 ) is irreducible. Proof. By Lemma 2.6, J 0 = J G m,n for some J Spec n (T m,n ). By Lemma 3.6(a), Z(J 0 ) = Z(J), and by Proposition 4.2(b), Z(J) is irreducible. 5. The Nullstellensatz for prime ideals 5.1. Proposition. (Weak form of the Nullstellensatz) Let A denote the algebra G m,n or T m,n, and let J be a prime ideal of A. Then Z(J) if and only if A/J has PIdegree n.
11 POLYNOMIAL IDENTITY RINGS 11 Note that for n = 1, Proposition 5.1 reduces to the usual (commutative) weak Nullstellensatz (which is used in the proof of Proposition 5.1). Indeed, a prime ring of PIdegree 1 is simply a nonzero commutative domain; in this case G m,1 /J = k[x 1,..., x m ]/J has PIdegree 1 if and only if J k[x 1,..., x m ]. Proof. First assume that Z(J). Since A has PIdegree n, its quotient A/J clearly has PIdegree n. To show PIdeg(A/J) n, recall that a point a = (a 1,..., a m ) Z(J) gives rise to a surjective kalgebra homomorphism A/J M n ; see Remark 3.4. Conversely, assume that R = A/J is a kalgebra of PIdegree n. Note that R is a Jacobson ring (i.e., the intersection of its maximal ideals is zero), and that it is a Hilbert kalgebra (i.e., every simple homomorphic image is finitedimensional over k and thus a matrix algebra over k), see [2, Corollary 1.2]. So if c is a nonzero evaluation in R of a central polynomial for n nmatrices, there is some maximal ideal M of R not containing c. Then R/M M n, and we are done in view of Remark Corollary. For any irreducible nvariety X, T m,n /I T (X) is the trace ring of the prime kalgebra k n [X]. Proof. By Lemma 4.1 and Proposition 5.1, I T (X) is a prime ideal of T m,n of PIdegree n. Consequently, I(X) = I T (X) G m,n is a prime ideal of G m,n, and the desired conclusion follows from Lemma 2.6(a) Proposition. (Strong form of the Nullstellensatz) (a) I(Z(J 0 )) = J 0 for every J 0 Spec n (G m,n ). (b) I T (Z(J)) = J for every J Spec n (T m,n ). For n = 1 both parts reduce to the usual (commutative) strong form of the Nullstellensatz for prime ideals (which is used in the proof of Proposition 5.3). Proof. We begin by reducing part (a) to part (b). Indeed, by Lemma 2.6, J 0 = J G m,n for some J Spec n (T m,n ). Now I(Z(J 0 )) (1) = I(Z(J)) = I T (Z(J)) G m,n (2) = J G m,n = J 0, where (1) follows from Lemma 3.6(a) and (2) follows from part (b). It thus remains to prove (b). Let X = Z(J). Then X (see Proposition 5.1), X is irreducible (see Proposition 4.2(b)) and hence I T (X) is a prime ideal of T m,n (see Lemma 4.1). Clearly J I T (X); our goal is to show that J = I T (X). In fact, we only need to check that (5.4) J C m,n = I T (X) C m,n. Indeed, suppose (5.4) is established. Choose p I T (X); we want to show that p J. For every q T m,n we have pq I T (X) and thus tr(p q) I T (X) C m,n = J C m,n,
12 12 Z. REICHSTEIN AND N. VONESSEN see Lemma 2.6(b). Hence, if we denote the images of p and q in T m,n /J by p and q respectively, Lemma 2.6(a) tells us that tr(p q) = 0 in T m,n /J for every q T m,n /J. Consequently, p = 0, i.e., p J, as desired. We now turn to proving (5.4). Consider the categorical quotient map π : (M n ) m Q m,n for the PGL n action on (M n ) m ; here C m,n = k[q m,n ] is the coordinate ring of Q m,n. Given an ideal H C m,n, denote its zero locus in Q m,n by Z 0 (H) = {a Q m,n h(a) = 0 h H}. Since Z(J C m,n ) = Z(J) = X in U m,n (see Proposition 4.2(a)), we conclude that Z 0 (J C m,n ) π(u m,n ) = π(x). On the other hand, since π(u m,n ) is Zariski open in Q m,n (see Lemma 2.3(d)), and J C m,n is a prime ideal of C m,n (see [19, Theorem II.6.5(1)]), we have (5.5) Z 0 (J C m,n ) = π(x), where π(x) is the Zariski closure of π(x) in Q m,n. Now suppose f I T (X) C m,n. Our goal is to show that f J C m,n. Viewing f as an element of C m,n, i.e., a regular function on Q m,n, we see that f 0 on π(x) and hence, on π(x). Now applying the usual (commutative) Nullstellensatz to the prime ideal J C m,n of C m,n, we see that (5.5) implies f J C m,n, as desired Remark. In this paper, we consider zeros of ideals of G m,n in U m,n. In contrast, Amitsur s Nullstellensatz [1] (see also [2]) deals with zeros in the larger space (M n ) m. Given an ideal J of G m,n, denote by Z(J; (M n ) m ) the set of zeroes of J in (M n ) m. Since Z(J; (M n ) m ) Z(J), it easily follows that J I ( Z(J; (M n ) m ) ) I ( Z(J) ). One particular consequence of Amitsur s Nullstellensatz is that the first inclusion is an equality if J is a prime ideal. Proposition 5.3 implies that both inclusions are equalities, provided J is a prime ideal of PIdegree n. Note that the second inclusion can be strict, e.g., if J is a prime ideal of PIdegree < n (since then Z(J) = by Proposition 5.1). The following theorem summarizes many of our results so far Theorem. Let n 1 and m 2 be integers. (a) Z( ) and I( ) are mutually inverse inclusionreversing bijections between Spec n (G m,n ) and the set of irreducible nvarieties X U m,n. (b) Z( ) and I T ( ) are mutually inverse inclusionreversing bijections between Spec n (T m,n ) and the set of irreducible nvarieties X U m,n. 6. Regular maps of nvarieties Recall that an element g of G m,n may be viewed as a regular PGL n  equivariant map g : (M n ) m M n. Now suppose X is an nvariety in U m,n. Then, restricting g to X, we see that g X = g X if and only if
13 POLYNOMIAL IDENTITY RINGS 13 g g I(X). Hence, elements of the PIcoordinate ring of X may be viewed as PGL n equivariant morphisms X M n. All of this is completely analogous to the commutative case, where n = 1, M 1 = k, k 1 [X] = k[x] is the usual coordinate ring of X k m, and elements of k[x] are the regular functions on X. It is thus natural to think of elements of k n [X] as regular functions on X, even though we shall not use this terminology. (In algebraic geometry, functions are usually assumed to take values in the base field k, while elements of k n [X] take values in M n.) We also remark that not every PGL n equivariant morphism X M n of algebraic varieties (in the usual sense) is induced by elements of k n [X], see Example Definition. Let X U m,n and Y U l,n be nvarieties. (a) A map f : X Y is called a regular map of nvarieties, if it is of the form f = (f 1,..., f l ) with each f i k n [X] (so that f sends a = (a 1,..., a m ) X (M n ) m to (f 1 (a),..., f l (a)) Y (M n ) l ). Note that a morphism of nvarieties X Y extends to a PGL n equivariant morphism (M n ) m (M n ) l. (b) The nvarieties X and Y are called isomorphic if there are mutually inverse regular maps X Y and Y X. (c) A regular map f = (f 1,..., f l ): X Y U l,n of nvarieties induces a kalgebra homomorphism f : k n [Y ] k n [X] given by X i f i for i = 1,..., l, where X 1,..., X l are the images of the generic matrices X 1,..., X l G l,n in k n [Y ] = G l,n /I(Y ). One easily verifies that for every g k n [Y ], f (g) = g f : X Y M n. (d) Conversely, a kalgebra homomorphism α: k n [Y ] k n [X] induces a regular map α = (f 1,..., f l ): X Y of nvarieties, where f i = α(x i ). It is easy to check that for every g k n [Y ], α(g) = g α : X Y M n Remark. It is immediate from these definitions that (f ) = f for any regular map f : X Y of nvarieties, and (α ) = α for any kalgebra homomorphism α: k n [Y ] k n [X]. Note also that (id X ) = id kn[x], and (id kn[x]) = id X Lemma. Let X U m,n and Y U l,n be nvarieties, and let k n [X] and k n [Y ] be their respective PIcoordinate rings. (a) If f : X Y and g : Y Z are regular maps of nvarieties, then (g f) = f g. (b) If α: k n [Y ] k n [X] and β : k n [Z] k n [Y ] are kalgebra homomorphisms, then (α β) = β α. (c) X and Y are isomorphic as nvarieties if and only if k n [X] and k n [Y ] are isomorphic as kalgebras. Proof. Parts (a) and (b) follow directly from Definition 6.1. The proofs are exactly the same as in the commutative case (where n = 1); we leave them as an exercise for the reader.
14 14 Z. REICHSTEIN AND N. VONESSEN To prove (c), suppose f : X Y and g : Y X are mutually inverse morphisms of nvarieties. Then by part (a), f : k n [Y ] k n [X] and g : k n [X] k n [Y ] are mutually inverse kalgebra homomorphisms, showing that k n [X] and k n [Y ] are isomorphic. Conversely, if α: k n [Y ] k n [X] and β : k n [X] k n [Y ] are mutually inverse homomorphisms of kalgebras then by part (b), α and β are mutually inverse morphisms between the nvarieties X and Y Theorem. Let R be a finitely generated prime kalgebra of PIdegree n. Then R is isomorphic (as a kalgebra) to k n [X] for some irreducible n variety X. Moreover, X is uniquely determined by R, up to isomorphism of nvarieties. Proof. By our assumptions on R there exists a surjective ring homomorphism ϕ: G m,n R. Then J 0 = Ker(ϕ) lies in Spec n (G m,n ). Set X = Z(J 0 ) U m,n. Then X is irreducible (see Corollary 4.3), and J 0 = I(X) (see Proposition 5.3(a)). Hence R is isomorphic to G m,n /I(X) = k n [X], as claimed. The uniqueness of X follows from Lemma 6.3(c). We are now ready to prove Theorem 1.1. Recall that for n = 1, Theorem 1.1 reduces to [13, Corollary 3.8]. Both are proved by the same argument. Since the proof of [13, Corollary 3.8] is omitted in [13], we reproduce this argument here for the sake of completeness. Proof of Theorem 1.1. By Lemma 6.3, the contravariant functor F in Theorem 1.1 is welldefined. It is full and faithful by Remark 6.2. Moreover, by Theorem 6.4, every object in PI n is isomorphic to the image of an object in Var n. Hence F is a covariant equivalence of categories between Var n and the dual category of PI n, see, e.g., [8, Theorem 7.6]. In other words, F is a contravariant equivalence of categories between Var n and PI n, as claimed. We conclude our discussion of regular maps of nvarieties with an observation which we will need in the next section Lemma. Let X be an irreducible nvariety, and c a central element of k n [X] or of its trace ring T m,n /I T (X). Then the image of c in M n consists of scalar matrices. Proof. Denote by k n (X) the common total ring of fractions of k n [X] and T m,n /I T (X). By Lemma 2.10, there exists a central polynomial s G m,n for n n matrices which does not identically vanish on X. Then R = G m,n [s 1 ] is an Azumaya algebra. Consider the natural map φ: R k n (X). Then the center of φ(r) is φ(center(r)); see, e.g., [10, Proposition 1.11]. Note that k n (X) is a central localization of φ(r). Hence the central element c of k n (X) is of the form c = φ(p)φ(q) 1 for central elements p, q G m,n with q 0 on X. All images of p and q in M n are central, i.e., scalar matrices. Thus c(x) is a scalar matrix for each x in the dense open subset
15 POLYNOMIAL IDENTITY RINGS 15 of X on which q is nonzero. Consequently, c(x) is a scalar matrix for every x X. 7. Rational maps of nvarieties 7.1. Definition. Let X be an irreducible nvariety. The total ring of fractions of the prime algebra k n [X] will be called the central simple algebra of rational functions on X and denoted by k n (X) Remark. One can also define k n (X) using the trace ring instead of the generic matrix ring. That is, k n (X) is also the total ring of fractions of T m,n /I T (X). Indeed, by Corollary 5.2, T m,n /I T (X) is the trace ring of k n [X], so the two have the same total ring of fractions. Recall that k n (X) is obtained from k n [X] by inverting all nonzero central elements; see, e.g., [26, Theorem 1.7.9]. In other words, every f k n (X) can be written as f = c 1 p, where p k n [X] and c is a nonzero central element of k n [X]. Recall from Lemma 6.5 that for each x X, c(x) is a scalar matrix in M n, and thus invertible if it is nonzero. Viewing p and c as PGL n equivariant morphisms X M n (in the usual sense of commutative algebraic geometry), we see that f can be identified with a rational map c 1 p: X M n. One easily checks that this map is independent of the choice of c and p, i.e., remains the same if we replace c and p by d and q, such that f = c 1 p = d 1 q. We will now see that every PGL n equivariant rational map X M n is of this form Proposition. Let X U m,n be an irreducible nvariety. Then the natural inclusion k n (X) RMaps PGLn (X, M n ) is an isomorphism. Here RMaps PGLn (X, M n ) denotes the kalgebra of PGL n equivariant rational maps X M n, with addition and multiplication induced from M n. Recall that a regular analogue of Proposition 7.3 (with rational maps replaced by regular maps, and k n (X) replaced by k n [X]) is false; see Remark 3.2 and Example 3.3. First proof (algebraic). Recall that k n (X) is, by definition, a central simple algebra of PIdegree n. By [24, Lemma 8.5] (see also [24, Definition 7.3 and Lemma 9.1]), RMaps PGLn (X, M n ) is a central simple algebra of PIdegree n as well. It is thus enough to show that the centers of k n (X) and RMaps PGLn (X, M n ) coincide. Let X be the closure of X in (M n ) m. By [24, Lemma 8.5], the center of RMaps PGLn (X, M n ) = RMaps PGLn (X, M n ) is the field k(x) PGLn of PGL n  invariant rational functions f : X k (or equivalently, the field k(x) PGLn ). Here, as usual, we identify f with f I n : X M n. It now suffices to show that (7.4) k(x) PGLn = Center(k n (X)). Recall from Lemma 2.6(a) that the natural algebra homomorphism G m,n k n (X) extends to a homomorphism T m,n k n (X). So the center of
16 16 Z. REICHSTEIN AND N. VONESSEN k n (X) contains all functions f X : X k, as f ranges over the ring C m,n = k[(m n ) m ] PGLn. Since X U m,n, these functions separate the PGL n  orbits in X; see Proposition 2.3(b). Equality (7.4) now follows a theorem of Rosenlicht; cf. [18, Lemma 2.1]. Alternative proof (geometric). By Remark 7.2, it suffices to show that for every PGL n equivariant rational map f : X M n there exists an h k[m n ] PGLn = C m,n such that h 0 on X and hf : x h(x)f(x) lifts to a regular map (M n ) m M n (and in particular, hf is a regular map X M n ). It is enough to show that the ideal I k[(m n ) m ] given by I = {h k[(m n ) m ] hf lifts to a regular map (M n ) m M n } contains a PGL n invariant element h such that h 0 on X. Indeed, regular PGL n equivariant morphisms (M n ) m M n are precisely elements of T m,n ; hence, hf : X M n would then lie in k n (X), and so would f = h 1 (hf), thus proving the lemma. Denote by Z the zero locus of I in (M n ) m (in the usual sense, not in the sense of Definition 3.1(b)). Then Z X is, by definition the indeterminacy locus of f; in particular, X Z. Choose a X \ Z and let C = PGL n a be the orbit of a in X. Since a X U m,n, Proposition 2.3(b) tells us that C is closed in (M n ) m. In summary, C and Z are disjoint PGL n invariant Zariski closed subsets of (M n ) m. Since PGL n is reductive, they can be separated by a regular invariant, i.e., there exists a 0 j k[(m n ) m ] PGLn such that j(a) 0 but j 0 on Z; see, e.g., [16, Corollary 1.2]. By Hilbert s Nullstellensatz, h = j r lies in I for some r 1. This h has the desired properties: it is a PGL n invariant element of I which is not identically zero on X Definition. Let X U m,n and Y U l,n be irreducible nvarieties. (a) A rational map f : X Y is called a rational map of nvarieties if f = (f 1,..., f l ) where each f i k n (X). Equivalently (in view of Proposition 7.3), a rational map X Y of nvarieties is simply a PGL n equivariant rational map (in the usual sense). (b) The nvarieties X and Y are called birationally isomorphic or birationally equivalent if there exist dominant rational maps of nvarieties f : X Y and g : Y X such that f g = id Y and g f = id X (as rational maps of varieties). (c) A dominant rational map f = (f 1,..., f l ): X Y of nvarieties induces a kalgebra homomorphism (i.e., an embedding) f : k n (Y ) k n (X) of central simple algebras defined by f (X i ) = f i, where X i is the image of the generic matrix X i G l,n in k n [Y ] k n (Y ). One easily verifies that for every g k n (Y ), f (g) = g f, if one views g as a PGL n equivariant rational map Y M n.
17 POLYNOMIAL IDENTITY RINGS 17 (d) Conversely, a kalgebra homomorphism (necessarily an embedding) of central simple algebras α: k n (Y ) k n (X) (over k) induces a dominant rational map f = α : X Y of nvarieties. This map is given by f = (f 1,..., f l ) with f i = α(x i ) k n (X), where X 1,..., X l are the images of the generic matrices X 1,..., X l G l,n. It is easy to check that for every g k n (Y ), α(g) = g α, if one views g as a PGL n equivariant rational map Y M n Remark. Once again, the identities (f ) = f and (α ) = α follow directly from these definitions. Similarly, (id X ) = id kn(x) and (id kn(x)) = id X. We also have the following analogue of Lemma 6.3 for dominant rational maps. The proofs are again the same as in the commutative case (where n = 1); we leave them as an exercise for the reader Lemma. Let X U m1,n, Y U m2,n and Z U m3,n be irreducible nvarieties. (a) If f : X Y and g : Y Z are dominant rational maps of n varieties then (g f) = f g. (b) If α: k n (Y ) k n (X) and β : k n (Z) k n (Y ) are homomorphisms (i.e., embeddings) of central simple algebras then (α β) = β α. (c) X and Y are birationally isomorphic as nvarieties if and only if the central simple algebras k n (X) and k n (Y ) are isomorphic as k algebras. We are now ready to prove the following birational analogue of Theorem Theorem. Let K/k be a finitely generated field extension and A be a central simple algebra of degree n with center K. Then A is isomorphic (as a kalgebra) to k n (X) for some irreducible nvariety X. Moreover, X is uniquely determined by A, up to birational isomorphism of nvarieties. Proof. Choose generators a 1,..., a N K for the field extension K/k and a Kvector space basis b 1,..., b n 2 for A. Let R be the ksubalgebra of A generated by all a i and b j. By our construction R is a prime kalgebra of PIdegree n, with total ring of fraction A. By Theorem 6.4 there exists an nvariety X such that k n [X] R and hence, k n (X) A. This proves the existence of X. Uniqueness follows from Lemma 7.7(c). 8. Generically free PGL n varieties An irreducible nvariety is, by definition, an irreducible generically free PGL n variety. The following lemma says that up to birational isomorphism, the converse is true as well Lemma. Every irreducible generically free PGL n variety X is birationally isomorphic (as PGL n variety) to an irreducible nvariety in U m,n for some m 2.
18 18 Z. REICHSTEIN AND N. VONESSEN Proof. Choose a U 2,n. By [24, Proposition 7.1] there exists a PGL n  equivariant rational map φ: X (M n ) 2 whose image contains a. Now choose PGL n invariant rational functions c 1,..., c r k(x) PGLn on X which separate PGL n orbits in general position (this can be done by a theorem of Rosenlicht; cf. e.g., [18, Theorem 2.3]). We now set m = r + 2 and define f : X (M n ) m by f(x) = ( c 1 (x)i n n,..., c r (x)i n n, φ(x) ) (M n ) r (M n ) 2 = (M n ) m. Let Y be the Zariski closure of f(x) in (M n ) m, and Y = Y U m,n. By our choice of φ, Y. It thus remains to be shown that f is a birational isomorphism between X and Y (or, equivalently, Y ). Since we are working over a base field k of characteristic zero, it is enough to show that X has a dense open subset S such that f(a) f(b) for every pair of distinct kpoints a, b S. Indeed, choose S X so that (i) the generators c 1,..., c r of k(x) PGLn separate PGL n orbits in S, (ii) f is welldefined in S and (iii) f(s) U m,n. Now let a, b S, and assume that f(a) = f(b). Then a and b must belong to the same PGL n orbit. Say b = h(a), for some h PGL n. Then f(a) = f(b) = hf(a)h 1. Since f(a) U m,n, h = 1, so that a = b, as claimed. Lemma 8.1 suggests that in the birational setting the natural objects to consider are arbitrary generically free PGL n varieties, rather than n varieties. The relationship between the two is analogous to the relationship between affine varieties and more general algebraic (say, quasiprojective) varieties in the usual setting of (commutative) algebraic geometry. In particular, in general one cannot assign a PIcoordinate ring k n [X] to an irreducible generically free PGL n variety in a meaningful way. On the other hand, we can extend the definition of k n (X) to this setting as follows Definition. Let X be an irreducible generically free PGL n variety. Then k n (X) is the kalgebra of PGL n equivariant rational maps f : X M n, with addition and multiplication induced from M n. Proposition 7.3 tells us that if X is an irreducible nvariety then this definition is consistent with Definition 7.1. In place of k n (X) we will sometimes write RMaps PGLn (X, M n ) Definition. A dominant rational map f : X Y of generically free PGL n varieties gives rise to a homomorphism (embedding) f : k n (Y ) k n (X) given by f (g) = g f for every g k n (Y ). If X and Y are nvarieties, this definition of f coincides with Definition 7.5(c). Note that (id X ) = id kn(x), and that (g f) = f g if g : Y Z is another PGL n equivariant dominant rational map. We will now show that Definition 7.5 and Remark 7.6 extend to this setting as well.
19 POLYNOMIAL IDENTITY RINGS Proposition. Let X and Y be generically free irreducible PGL n varieties and α: k n (X) k n (Y ) be a kalgebra homomorphism. Then there is a unique PGL n equivariant, dominant rational map α : Y X such that (α ) = α. Proof. If X and Y are nvarieties, i.e., closed PGL n invariant subvarieties of U m,n and U l,n respectively (for some m, l 2) then α : Y X is given by Definition 7.5(d), and uniqueness follows from Remark 7.6. In general, Lemma 8.1 tells us that there are birational isomorphisms X X and Y Y where X and Y are nvarieties. The proposition is now a consequence of the following lemma Lemma. Let f : X X and g : Y Y be birational isomorphisms of PGL n varieties. If Proposition 8.4 holds for X and Y then it holds for X and Y. Proof. Note that by our assumption, the algebra homomorphism β = (g ) 1 α f : k n (X ) k n (Y ) is induced by the PGL n equivariant, dominant rational map β : Y X : k n (X) f k n (X ) α k n (Y ) Y X g g f β k n (Y ) Y β X Now one easily checks that the dominant rational map α := f 1 β g : Y X has the desired property: (α ) = α. This shows that α exists. To prove uniqueness, let h: Y X be another PGL n equivariant dominant rational map such that h = α. Then (f h g 1 ) = (g 1 ) α f = β. By uniqueness of β, we have f h g 1 = β, i.e., h = f 1 β g = α. This completes the proof of Lemma 8.5 and thus of Proposition Corollary. Let X, Y and Z be generically free irreducible PGL n  varieties. (a) If f : X Y, g : Y Z are PGL n equivariant dominant rational maps then (g f) = f g. (b) If α: k n (Y ) k n (X) and β : k n (Z) k n (Y ) are homomorphisms (i.e., embeddings) of central simple algebras then (α β) = β α. (c) X and Y are birationally isomorphic as PGL n varieties if and only if k n (X) and k n (Y ) are isomorphic as kalgebras. Proof. (a) is immediate from Definition 8.3. (b) Let f = (α β) and g = β α. Part (a) tells us that f = g. The uniqueness assertion of Proposition 8.4 now implies f = g.
20 20 Z. REICHSTEIN AND N. VONESSEN (c) follows from (a) and (b) and the identities (id X ) (id kn(x)) = id X. = id kn(x), and Proof of Theorem 1.2. We use the same argument as in the proof of Theorem 1.1. The contravariant functor F is well defined by Corollary 8.6. Since (f ) = f and (α ) = α (see Proposition 8.4), F is full and faithful. By Theorem 7.8, every object in CS n is isomorphic to the image of an object in Bir n. The desired conclusion now follows from [8, Theorem 7.6]. 9. BrauerSeveri Varieties Let K/k be a finitely generated field extension. Recall that the following sets are in a natural (i.e., functorial in K) bijective correspondence: (1) the Galois cohomology set H 1 (K, PGL n ), (2) central simple algebras A of degree n with center K, (3) BrauerSeveri varieties over K of dimension n 1, (4) PGL n torsors over Spec(K), (5) pairs (X, φ), where is X is an irreducible generically free PGL n  variety and φ X : k(x) PGLn K is an isomorphism of fields (over k). Two such pairs (X, φ) and (Y, ψ) are equivalent, if there is a PGL n equivariant birational isomorphism f : Y X which is compatible with φ and ψ, i.e., there is a commutative diagram f k(x) PGLn φ K ψ k(y ) PGLn Bijective correspondences between (1), (2), (3) and (4) follow from the theory of descent; see [29, Sections I.5 and III.1], [30, Chapter X], [6, (1.4)] or [14, Sections 28, 29]. For a bijective correspondence between (1) and (5), see [17, (1.3)]. For notational simplicity we will talk of generically free PGL n varieties X instead of pairs (X, φ) in (5), and we will write k(x) PGLn = K instead of k(x) PGLn φ K, keeping φ in the background. Suppose we are given a generically free PGL n variety X (as in (5)). Then this variety defines a class α H 1 (K, PGL n ) (as in (1)) and using this class we can recover all the other associated objects (2)  (4). In the previous section we saw that the central simple algebra A can be constructed directly from X, as RMaps PGLn (X, M n ). The goal of this section is to describe a way to pass directly from (5) to (3), without going through (1); this is done in Proposition 9.2 below. In order to state Proposition 9.2, we introduce some notation. We will write points of the projective space P n 1 = P n 1 k as rows a = (a 1 :... : a n ).
21 POLYNOMIAL IDENTITY RINGS 21 The group PGL n acts on P n 1 by multiplication on the right: g : (a 1 :... : a n ) (a 1 :... : a n )g 1. Choose (and fix) a = (a 1 :... : a n ) P n 1 and define the maximal parabolic subgroup H of PGL n by (9.1) H = {h PGL n ah 1 = a}. If a = (1 : 0 : : 0) then H PGL n consists of of n nmatrices of the form Proposition. Let X be an irreducible generically free PGL n variety, A = k n (X) and σ : X/H X/PGL n. Then the BrauerSeveri variety BS(A) is the preimage of the generic point η of X/PGL n under σ. Before we proceed with the proof, three remarks are in order. First of all, by X/H we mean the rational quotient variety for the Haction on X. Recall that X/H is defined (up to birational isomorphism) by k(x/h) = k(x) H, and the dominant rational map σ : X/H X/PGL n by the inclusion of fields k(x) PGLn k(x) H. Secondly, recall that k(x/pgl n ) = k(x) PGLn = K, so that η Spec(K), and σ 1 (η) is, indeed, a Kvariety. Thirdly, while the construction of BS(A) in Proposition 9.2 does not use the Galois cohomology set H 1 (K, PGL n ), our proof below does. In fact, our argument is based on showing that σ 1 (η) and BS(A) are BrauerSeveri varieties defined by the same class in H 1 (K, PGL n ). Proof. Let X 0 /k be an algebraic variety with function field k(x 0 ) = K, i.e., a particular model for the rational quotient variety X/PGL n. The inclusion K φ k(x) PGLn X induces the rational quotient map π : X X 0. After replacing X 0 by a Zariski dense open subset, we may assume π is regular; after passing to another (smaller) dense open subset, we may assume π : X X 0 is, in fact, a torsor; cf., e.g., [18, Section 2.5]. We now trivialize this torsor over some etale cover U i X 0. Then for each i, j the transition map f ij : PGL n U ij PGL n U ij is an automorphism of the trivial PGL n torsor PGL n U ij on U ij. It is easy to see that f ij is given by the formula (9.3) f ij (u, g) = (u, g c ij (u)), for some morphism c ij : U ij PGL n. The morphisms c ij satisfy a cocycle condition (for Cech cohomology) which expresses the fact that the transition maps f ij are compatible on triple overlaps U hij. The cocycle c = (c ij ) gives rise to a cohomology class c H 1 (X 0, PGL n ), which maps to α under the natural restriction morphism H 1 (X 0, PGL n ) H 1 (K, PGL n ) from X 0 to
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