NOTES FOR BEILINSON-DRINFELD SEMINAR - EVEN SHORTER VERSION. Contents 1. Overview Steenrod s construction
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1 NOTES FOR BEILINSON-DRINFELD SEMINAR - EVEN SHORTER VERSION GUS LONERGAN Abstract. We use Steenrod s construction to prove that the quantum Coulomb branch is a Frobenius-constant quantization. Contents 1. Overview 1 2. Steenrod s construction 1 3. Frobenius-constant quantizations 3 4. End of first part - time for a break 4 5. Coulomb branch Gr, T, R Borel-Moore homology Frobenius-constancy 5 1. Overview This talk is essentially about power operations. In the first part I will introduce some power operations in homological algebra and in quantization theory. In the second part I will explain how these are related in the context of quantum Coulomb branch of Braverman-Finkelberg-Nakajima. Notation Steenrod s construction G - alg. gp./c X - alg. var./c, G X p - odd prime D G pxq - G-equivariant bounded constructible derived category of sheaves of F p -modules on X an Σ - suspension functor F p - equivariant constant sheaf ω - equivariant dualizing complex µ p - subgroup of C of order p X p Mappµ p, Xq G p µ p ^ b p : D G pxq Ñ D ý G ppx p q functor of p th external tensor power Biadjoint functors Res : D Gp µ p px p q Õ D G ppx p q : Ind ý 1
2 2 GUS LONERGAN Theorem-Definition 2.1 (Steenrod). There is a functor St : D G pxq Ñ D Gp µ p px p q satisfying Res St ^ b p. In a diagram: commutes. Facts 2.2. D G pxq D G pxq ^bp ÝÝÝÑ D G ppx p q Ò Res St ÝÑ D Gp µ p px p q. (1) StΣ Σ p St. (2) Given f, g : A Ñ B in D G pxq, can find a morphism h : A bp Ñ B bp in D Gp µpx p q s.t. Stpf ` gq Stpfq Stpgq equals composition Avphq : StpAq adjunction ÝÝÝÝÝÝÝÑ IndpA bp q Indphq ÝÝÝÝÑ IndpB bp q adjunction ÝÝÝÝÝÝÝÑ StpBq. (3) St commutes with multiplication by F p. (4) StpF p q F p. (5) Stpωq ω. (6) St is monoidal. Consequence 2.3. (1) Identify G-equivariant F p -cohomology H n G pxq with Hom D G pxqpf p, Σ n F p q. So St induces non-linear maps Pull back along diagonal : St : H n GpXq Ñ H pn G p µ p px p q. St : H n GpXq Ñ H pn Gˆµ p pxq Kunneth ÝÝÝÝÝÝÝÑ ph GpXqbH µ p p qq pn ph GpXqra, sq pn. Here we used: H µ p p q F p ra, s, degpaq 1, degp q 2. These are linear, because commutes with Av and Av : H G pxq Ñ H Gˆµ p pxq equals 0. Thus have a map of graded vector spaces St 1 : H GpXq p1q Ñ H GpXqra, s Here H G pxqp1q is the Frobenius twist of H G pxq. It is the algebra obtained from H G pxq by multiplying degrees by p and introducing signs: x p1q y p1q p 1q degpxq degpyqpp 2q pxyq p1q. Fact: St 1 is a map of algebras. Coefficients of monomials m, a m are the Steenrod operations, up to some correction factors. (2) Equivariant Borel-Moore homology of X is defined as n and we have maps of sets : Hom DG pxqpf p, Σ n ωq St : Hn BM,G pxq Ñ p µ p pn px p q. These are not linear, and unlike for cohomology there is no way in general to get something linear out of them. But we will see later that we can do something like that if X has a suitably symmetric multiplication.
3 NOTES FOR BEILINSON-DRINFELD SEMINAR - EVEN SHORTER VERSION 3 Fact 2.4. The graded vector space H BM,G pxq is a module over H G pxq, and in particular over H G p q. The family of maps St : n is compatible with the multiplication by in the natural way. pxq Ñ p µ p pn px p q St : H n GpXq Ñ H pn G p µ p px p q Example 2.5. Take X, G C. We have H F C p rcs with degpcq 2. The map of algebras St 1 : F p rcs p1q Ñ F p rc, a, s factors through the inclusion of F p rc, s for degree reasons, and sends c to c p p 1 c. Identifying F p rcs with Opg m q and taking Spec we get a G m -equivariant map AS : g m ˆ G a Ñ g p1q m. AS 0 is the Frobenius map, while AS 1 is the Artin-Schreier map. This observation generalizes in the natural way to X, G T complex torus. 3. Frobenius-constant quantizations AS plays a central role in the following theory. Let A be a commutative F p - algebra. Definition 3.1. (1) A quantization of A is a flat (i.e. torsion-free) associative F p r s-algebra A satisfying A { A. (2) A Frobenius-constant quantization of A is a quantization A of A together with a map of algebras F : A p1q Ñ ZpA q such that F mod is the Frobenius map. Here ZpA q is the center of A. The Frobenius twist A p1q has meaning if we are in an equivariant setting, see example. Example 3.2. Consider G m Spec F p rx, x 1 s. Set A OpT G m q F p rx, x 1, ys. Its canonical quantization is the Weyl algebra A D pg m q F p r sxx, x 1, By{prB, xs q. These are both G m ˆ G m -equivariant (regular action and homotheties) with x in bidegree p1, 0q and y, B in bidegree p 1, 2q. Note that x p, B p P ZpA q. So we can set F : A p1q Ñ ZpA q x i ÞÑ x p i y i ÞÑ B p i. F is G m ˆ G m -equivariant. Taking invariants for the regular action we get F p rxys Ñ F p r, xbs xy ÞÑ x p B p. It is an exercise to show that x n B n ś n 1 i 0 pxb i q. Thus we have p 1 ź x p B p pxb i q pxbq p p 1 xb. i 0
4 4 GUS LONERGAN Therefore if we identify F p rxys with F p rcs and F p rxb, s with F p rc, s we recover AS. Likewise if we replace G m with the F p -split Langlands dual torus T _ to complex torus T. 4. End of first part - time for a break 5. Coulomb branch 5.1. Gr, T, R. G - complex reductive algebraic group T - maximal torus of G N - G-module of dimension d ă 8 K Cpptqq, O Crrtss. For simplicity assume that p is large. We consider three reasonable ind-schemes of interest. (1) The affine Grassmannian Gr is an ind-projective ind-scheme whose C- points are GpKq{GpOq. Informally, Gr GpKq{GpOq. Its GpOq-orbits are parameterized by dominant cocharacters λ of T. Corresponding orbit closures, Gr λ, exhaust Gr. Gr parameterizes pairs pe, fq of a principal G-bundle E on the formal disc and a trivialization f of E on punctured formal disc o, up to isomorphism. (2) NpOq is a GpOq-module, and we consider the associated NpOq-bundle ˆ T : GpKq GpOq NpOq. It is an infinite-dimensional vector bundle over Gr. Its ind-scheme structure is given by pieces T λ T Gr λ. We write T λ n for finite-dimensional quotient bundle T λ {t n. T λ. The rank of this bundle is dim Gr λpt λ nq nd. Moduli: T parameterizes triples up to isomorphism pe, f, vq where E, f are as in Gr and v is an N-section of E. (3) We have T Ñ NpKq by multiplication. R is defined to be the preimage of NpOq. It is a closed sub-ind-scheme of T, given by pieces R λ R Gr λ Ă T λ. The fibers over Gr are vector subspaces of NpOq of finite codimension. Each R λ contains the bundle t n. T λ for some n ąą 0, and we will consider the fiberwise quotient R λ n R λ {t n. T λ. (Draw the picture). The possible n go to 8 as λ Ñ 8. R parameterizes triples up to isomorpihsm pe, f, vq as in T with the additional condition that fpvq extends over. Each of Gr, T, R and approximations Gr λ, T λ n, R λ n have natural actions of GpOq C. In the moduli interpretation, GpOq acts simply by changing the trivialization f, while C acts by automorphisms of. z.pe, f, vq pz E, z f, z vq. Example 5.1. G C, N C r, r ě 0. Then on C-points we identify: (1) Gr Z
5 NOTES FOR BEILINSON-DRINFELD SEMINAR - EVEN SHORTER VERSION 5 (2) T Z ˆ C r rrtss (3) R Z ď0 ˆ C r rrtss Y t1u ˆ t r C r rrtss Y t2u ˆ t 2r C r rrtss Y Borel-Moore homology. Fix λ, n such that R λ n exists. Let G be one of the groups GpOq, GpOq C. Since R λ n`1 Ñ R λ n is an N-bundle, the pullback induces isomorphisms i Therefore the shifted Borel-Moore homology pr λ nq i 2d prλ n`1q. i 2nd prλ nq i 2 dim Gr λ pt λ n qprλ nq is independent of n. We can therefore formally drop the n and write it as i 2 dim Gr λ pt λ q prλ q. We can then take a colimit as λ Ñ 8 (pushing forward along closed embeddings) to obtain the formally shifted Borel-Moore homology: i 2 dim Gr pt q prq. Definition 5.2. (1) The Coulomb branch is the graded H G p q-module A poq 2 dim Gr pt q prq. (2) The quantum Coulomb branch is the graded H GˆC p q-module A poq C 2 dim Gr pt q prq. A is a flat graded deformation of A over F p r s H C p q. Fact 5.3. (1) A, A are evenly graded. (2) A, A have algebra structures via convolution. (3) A is commutative, and A is its quantization. If I have time at the end I will explain how this works. Remark 5.4. We can replace GpOq-equivariance by G-equivariance in definition of A, A. But to define the convolution we need to use GpOq-equivariance. Example 5.5. (1) G C, N 0. Then A is the Weyl algebra F p r sxx, By{prB, xs q. Equivariant BM homology of point n P Z is identified with F p r, xbs.x n. (2) G C, N C r, r ě 0. Then A is the subalgebra of the Weyl algebra with basis over F p r, xbs.... x 2, x 1, 1, 5.3. Frobenius-constancy. rź ź2r prxb i qx, prxb i qx 2,... Main Theorem 5.1. A is a Frobenius-constant quantization.
6 6 GUS LONERGAN I will construct the map F : A p1q Ñ A. There is an ind-scheme R over G a parameterizing: $, px, E, f, vq : x P C & E a principal G-bundle over π 1 x /. f a trivialization of E over o π 1 x {. v an N-section of E such that fpvq extends over % π 1 x /- Here by definition: π : G a Ñ G a is the map x ÞÑ x p. π 1 x is the formal neighborhood in G a of π 1 x. In a formula, π 1 x Specplim ÐÝn Crts{pt p xq n q. o π 1 x is the punctured formal neighborhood, o π 1 x Specpplim ÐÝ Crts{pt p xq n qrpt p xq 1 sq. n There is a global version of GpOq acting on R, but for simplicity we just consider the actions of G by g.px, E, f, vq px, E, g f, vq. and of C by z.px, E, f, vq pz p x, z E, z f, z vq. Also R is embedded in T, defined the same way as R but without the condition on v. R Ă T. The version of R with N 0 is called Gr. T is an infinite-dimensional vector bundle over Gr, with a fiberwise shift map pt p xq. We may define 2 dim Gr T pr q for G G or G ˆ C, as in the case of R. Away from 0: fibers of R Ă T are identified with R p Ă T p. The action of G is identified with the diagonal action. C does not act, but µ p Ă C does act an its action is identified with the cyclic action. Therefore we have: ˆC 2 dim Gr o pt o qpro q HBM,Gˆµp `2 2 dim Gr p pt p q prp q. So by using Steenrod s construction we can define a non-linear graded map A p1q Ñ p µ p 2 dim Gr p pt p q prp q Ñ ˆµp 2 dim Gr p pt p q prp q ˆC pt o qpro q. We will compose this with the restriction ˆC pt o qpro q Ñ HBM,Gˆµp pt o qpro q. Since G ˆ µ p acts trivially on the base G a, we are able to apply specialization in Borel-Moore homology to get a map ˆµp pt o qpro q Ñ HBM,Gˆµp 2 dim pgr q 0 ppt q 0q ppr q 0 q.
7 NOTES FOR BEILINSON-DRINFELD SEMINAR - EVEN SHORTER VERSION 7 Over 0: R Ă T is identified with R Ă T. The action of G ˆ C is its usual action. Thus we have produced a map of graded sets: A p1q Ñ ˆµp 2 dim pgr q 0 ppt q 0q ppr q 0 q ˆµp 2 dim Gr pt q prq. But we have ˆµp 2 dim Gr pt q prq A ras, so for degree reasons we produced a graded map of sets F : A p1q Ñ A. This map is linear, because the averaging map A Ñ A ras equals 0. Fact 5.6. F gives the structure of Frobenius-constant quantization. Example 5.7. G C, N C r, r ě 0. Then F is the map induced by usual Frobenius-constant structure on the Weyl algebra It must be therefore that x ÞÑ x p y ÞÑ B p AS pprxyq r xq P A. If so then it is automatically central. We can check pź pź źpr AS pprxyq r xq prxb ri q r x p prxb i q r x p prxb i qx p. Remark 5.8. Can turn this calculation into a proof for large p by torus localization. But, it is true for all p, by direct geometric argument. Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA address: gusl@mit.edu
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