OPERS. The geometric Langlands correspondence conjectures a correspondence


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1 OPERS JONATHAN BARLEV The geometric Langlands correspondence conjectures a correspondence QcolocsysL Gx)) = DmodBun G )) on the level of derived categories. As remarked previously in theseminar,toeach local system over we associate the skyscraper sheaf over it. In this talk we shall discuss a subspace 1 of locsysl G ) for whom the RHS counterpart of the corresponding skyscraper sheaves) is known and consists of Hecke eigensheaves. 1. Definitions Let G be a connected reductive group over C, agoper will be defined shortly as a G localsystemon with extra structure. ix a Borel subgroup B G. This choice induces a descending filtration on g of length twice the rank + 1 for which g r = g and g i+1 =[g i, n] so g 0 = b, g 1 = n etc.), which is preserved by b. We also denote H = B/N, whichactsongrg via ad. Let B be B torsor on, and let G, ) be a local system on the induced G torsor. Let E G = T G /G, thisisthevectorbundleover whose sections are Ginvariant vector fields on 2 G ;thinkoftheconnectionasamap : T E G which is asectiontotheprojectione G T). Composing T E G E G /E B = g/b) we obtain a section c ) Γ, g/b) Ω ),whichmeasurestowhich extent B is preserved by the connection; preserves B if and only if c ) =0. We start by defining opers for groups of adjoint type, let g be a semisimple Lie Algebra, let G denote it s adjoint group. Definition 1.1. A goper on is a pair B, ) where B is a B torsor over, and is a Gconnection on the induced Gtorsor, G := B B G,suchthat: 1) c ) actually lands in Γ, g 1 /b) ). Date: March8, At least morally, in our definitions it won t allways be a subspace. 2 I.e. the Lie algebroid of infinitesimal symmetries of G.Itavectorbundleover equipped with aliebracketsatisfyinganappropriateleibnitzrule. It sfiber is noncanonically g T x. 1
2 2 JONATHAN BARLEV 2) or each simple negative root α, the composition is an isomorphism. c ) α : T c ) g 1 /b) g 1 /b) α Often we shall think of a Goper as a Glocal system G, ) with the extra data of aspecifiedreductionofthestructureofthetorsortob 3 satisfying the connection conditions above. Example 1.2. If is a curve with a coordinate dx, andg = PGL n with usual upper triangular Borel) we can give an oper structure to the trivial torsor B 0 by using a connection of the form = d + dx 1 0 where d is the flat 1 0 connection on the trivialized torsor G). In fact any choice of matrix with non vanishing subdiagonal will do, however we shall show that locally every oper is isomorphic to a unique oper with this form which of course depends on the choice of coordinate, dx). Remark 1.3. If B, ) is a Goper then for any η Γ, b Ω ) we can form anewoper + η. When G is of nonadjoint type we give a definition which differs from that in [1]. In their definition a Goper for an arbitrary reductive, connected G) haszg) as it s automorphism group, the effect of our change is to eliminate these as well as restrict the admissible opers. When G is of adjoint type the oper conditions imply induces an isomorphism H H g 1 /g 2 φ Ω r = Ω ˇρ H H g 1 /g 2 here Ω is identified with the underlying G m torsor, and ˇρ is the cocharacter associated sum of the fundamental coweights which is a character since G is of adjoint type). In the adjoint case there exists a unique isomorphism H φ can H := Ω ˇρ H inducing φ. WhenG is not necessarily of adjoint type ˇρ might not be a cocharacter, but 2ˇρ is and if we fix a square root of the canonical bundle Ω 1/2 then would still induce H H g 1 /g 2 φ Ω r = Ω 1/2 ˇ2ρ H H g 1 /g 2 3 recall every Gbundle on a smooth curve admits a reduction to B.
3 OPERS 3 Definition 1.4. ix a square root of the canonical bundle, Ω 1/2. or G not necessarily adjoint, in addition to the contents of definition 1.1, we require the data of an isomorphism H φ Ω 1/2 ˇ 2ρ H which is compatible with φ as above. We call φ the marking. As mentioned above, for G of adjoint type such a φ exists and is unique. general the data of 1.1 does not imply the existence of φ, hencerelativeto[1]we are indeed limiting our collection of opers. Moreover, φ determines φ only up to an automorphism of the underlying torsor given by multiplication with a central element; fixing φ eliminates these automorphisms. Example 1.5. In the case of Gl n or SL n,anoperanbedescribedintermsofa rank n vector bundle as follows: it consists of the data E, E i ),,φ) where E is a rank n vector bundle over, E i ) n i=1 is a complete flag for E, φ : E = 1 Ω n 1)/2 is an isomorphism, and finally : E E Ω is connection which satisfies. 1) E i ) E i+1 Ω,thusinducesmorphismsbetweentheinvertiblesheaves gr i E gr i+1 E Ω. 2) or each i the morphism above is an isomorphism. 2. Opers as a functor Next we proceed to collect opers into a functor on D schemes. Indeed if Y In f is a D scheme, then the notion of a Gtorsor over Y with connection along makes sense: it is a Gtorsor G p Y for which G is compatibly a Gequivariant D  scheme, i.e. the map p is horizontal. or a Goper on Y along we additionally provide the data of a reduction of the torsor structure to the Borel, as well as an φ isomorphism H f Ω 1/2 2ˇρ H which satisfy: 1) c ), whichisthecomposition : f T E G g/b),actuallylands in g 1 /b) note these are all vector bundles over Y ). 2) or each negative simple root α, thecompositionf T g 1 /b) g 1 /b) α is an isomorphism. Thus we have an induced map g1 /g 2 ) f Ω ) r. 3) The map induced by φ, g 1 /g 2 ) = H H g 1 /g 2 φ f Ω 1/2 agrees with the map induced by above. 2ˇρ H H g 1 /g 2 =f Ω ) r or any two such objects the notion of an isomorphism is evident and we can form the groupoid Op G Y ), whoseobjectsaregopers over Y. Lastly, pullback
4 4 JONATHAN BARLEV of such opers along horizontal maps is defined and has the same underlyingglocal system as the pullback in that category) and we end up with a functor on D schemes, Op G,whichisasheafatleastintheZariskitopology. In particular, we may endow Y = S with the flat connection and obtain the notion of an S family of opers on. Abusing notation we use Op G ) to denote the functor on schemes whose Spoints are OpS ); thusop G ) is a functor over schemes. In the next secion we shall show that Op g is representable by a D scheme which is noncanonically) isomorphic to the Hitchin jet bundle J ets cl g Gm Ω ). It s horizontal sections, Op g ) will thus be represented by HitchL g), hence naturally be an affine space whenever is complete. or now we show that Op G is fibered over D schemes in sets, i.e. opers have no automorphisms. Proposition 2.1. A Goper, B,,φ), overad scheme Y has no non trivial) automorphisms. Proof. This is a local question so we may assume that B = Y B is trivialized, and has a local coordinate dx. We can write = d + η dx, whered is the flat connection on Y B induced by the connection on Y and the trivialization, and η : Y g 1 has nonvanishing entries for each simple negative root. Considering the underlying torsor B = Y B as a right torsor, an automorphism consists of left translation by a map σ : Y B. Since σ must preserve φ it must act trivially on H,thisforcestheimageofσ to actually fall in N, soσ : Y N. Conversely any such automorphism will be trivial mod N, havenoeffecton H and will preserve φ. Theeffectofsuchamapontheconnectionis σ.d + η) =d + dσ l σ 1 + Adσ)η dx Assume σ induces an automorphism of the oper, i.e. preserves the connection. Note that dσ l σ 1 Γ, g 1 Ω ) recall n = g 1 )so Adσ)η η modg 1 In N we can take logarithms so let σ =logn for some n n, letg i be the smallest term of the filtration containing n, we show i = r and hence σ = 1. dσ l σ 1 = d log σ = dn Γ, g i Ω )=Γ, g 1 Ω ) so that Adσ)η η modg i. Indeed
5 OPERS 5 Choose a Cartan subalgebra h b, andwriteη = η 1 + β with η 1 : Y g 1 having non vanishing coordinates in each simple negative root space, and β : Y b. g i Adσ)η 1 + β) η 1 + β) = = Adexpn))η 1 + β) η 1 + β) = [n, η 1 + β]+1/2[n, [n, η 1 + β]] + [n, η 1 ]+g i hence [η 1,n] g i. We show this implies n g i+1 which is a contradiction to the choice of i unless n =0 implying σ =1). Let {e 0,h 0,f 0 } the standard basis of sl 2,letb 0 = span{e 0,h 0 } denote the standard Borel. At every point of Y the map η 1 determines a principal embedding 4 sl 2 g,whichcarriesb 0 to b and for which η 1 is a scalar multiple of the image of f 0.Consideringg as an sl 2 representation via this principle embedding, our filtration corresponds to the weight space filtration and for a positive weight i, hence ad f0 : g i /g i+1 g i 1 /g i. Opers were introduced as living within local systems, this is truetothefollowing extent. The forgetful functor from Op G ) locsys G ) is always faithful. However, under our definition an oper cannot have nontrivial automorphismwhile the underlying local system might the issue is that a local system isomorphism may not preserve the flag or the marking). Nonetheless less we do have the following. Proposition 2.2. Let be a complete curve of genus g 2, G semisimple of adjoint type, and G, ) a Glocal system on which admits an oper structure. Then the oper structure is unique. We use the Harder Narasimhan flag: Lemma 2.3. Let G be a be torsor over a complete curve of genus g 2. Let B be a reduction to the Borel such that for each simple positive root α, the G m torsor H α G m has positive degree for. Then B is unique. proof of 2.2. Let B be a reduction to the Borel which makes G, ) an oper, we show that B is the Harder Narasimhan flag, hence is unique. Indeed for every positive simple root H α G m = Ω ;since g 2, ithaspositivedegree. 4 Aprincipalembeddingisoneinwhiche0,f 0 map to principal nilpotents in g; suchanembedding determines a Cartan subalgebra relative to which e 0 and f 0 map to sums of basis elements of the positive/negative root spaces respectively.
6 6 JONATHAN BARLEV Corollary 2.4. When is a complete curve and g 2 and g is semisimple then Op g ) is a full and faithful subcategory of locsys G ) where G is the adjoint group of g). In the nonadjoint case the ambiguity in the oper structure on G, ) is only due to the choice of marking i.e. the flag is uniquely determined). It follows the the fiber of Op G ) locsys G ) is a naturally torsor for the automorphisms of φ over see1.4);evidentlythisisz =the center of G. 3. A description of the space of gopers We now restrict ourselves to the adjoint case of gopers with g semisimple, and show that Op g is a torsor for the jet scheme of) a certain vector bundle over. Passing to global sections we obtain an isomorphism of Op g ) with an affine space which we shall identify with HitchL g). Recall that in Andrei s talk Dalgebras were heuristically introduced as being to nonlinear PDE s what Dmod s are to linear PDE s. That opers are a torsor for a vector bundle corresponds to the fact that solutions to a nonlinear PDE are a torsor for the solutions of it s homogeneous counterpart. It turns out that for any g this torsor structure may be obtained from the torsor structure of Op sl2,andwestartbyconsideringthiscase sl 2 opers. Lemma 3.1. Op sl2 is naturally a torsor for 5 Ω 2. Proof. Let B,,φ) be any oper on a D scheme Y f. Westartbynotingin our case n = g 1 /g 2 so the that by induces n = f Ω,thusn f Ω = f Ω 2 and Ω 2 both apriori act on Op sl 2 Y ) and Op sl2 respectively) by modifying the connection. To prove the action is simply transitive we may work locally. Replacing and Y by appropriate Zariski opens we may assume that has a local coordinate dx, and B = Y B is trivial since B is solvable B,isZariskilocallytrivial).Asinthe proof of lemma 2.1, a choice of coordinate and trivialization gives = d + dx η 5 There is some abuse of notation going on here, we are actually considering Ω 2 as a functor on D schemes rather than schemes). We could say instead that Op sl2 is a torsor for J ets Ω 2 ),sincethisisthed scheme representing the functor.
7 OPERS 7 where d is the flat connection w.r.t the trivialization and η : Y sl 2 does not vanish mod b. Recall that a change of the trivialization given by left multiplication by σ : Y B has the effect of modifying the connection to d + dσσ 1 + Adσ)η Computing directly this is where PSL 2 being of adjoint type comes in) we see that every coordinate determines a unique canonical trivialization which brings the connection to canonical form both relative to the choice of coordinate) ) 0 α = d + dx 1 0 thus the action of f Ω n Y ) on Op sl2 Y ) is free and transitive. Note also the the uniqueness implies that the underlying B torsor of any oper is trivialized whenever we have a coordinate. Corollary 3.2. All sl 2  torsors all have isomorphic underlying Btorsors. Denote this torsor 0 B 0. Remark 3.3. or a complete curve sl 2 opers always exist, hence Op sl2 is a trivial torsor for Ω 2 : When g 2, thisisimpliedbythefactthath 1, Ω 2 )=0. When g =1 there is a global coordinate ) and the trivialized torsor B with 0 α = d + dx is an oper. 1 0 When g =0 one can either compute directly and see there is a unique oper uniqueness is implied by the fact that H 0 P 1, Ω 2 P )=0). Alternatively, note 1 that an oper is equivalent to the data of a P 1 bundle with connection and a section with nonvanishing covariant derivative; take P 1 P 1 with the trivial connection and diagonal section gopers. Let g be an arbitrary semisimple lie algebra, with a chosen Borel b. Construction 3.4. Let {e 0,h 0,f 0 } the standard basis of sl 2,letb 0 = sp{e 0,h 0 } denote the standard Borel, and choose a principal embedding sl 2 g,whichcarries ι b 0 to b. To the map of Lie algebras corresponds a map of groups PSL G 2 G, where G is the adjoint group of g. We now describe how ι G may be used to construct a gopers out of an sl 2 opers. Let 0 B 0, 0 ) be an sl 2 oper on Y f 6.Let B, G respectively be the B, G torsors which 0 B 0 induces via ι G. The connection 0 naturally induces a connection on 6 Recall that for a goper the isomorphism φ is uniquely determined by the rest of the data, hence we ignore it.
8 8 JONATHAN BARLEV G which satisfies the oper condition because the map ι G is principal. In fact, if locally in an appropriate trivialization then 0 = d + dx In particular global gopers always exist. 0 α 1 0 = d + dx f + αe)) We wish to present gopers as a torsor for an appropriate vector bundle. Relative to a principal embedding, let g e0 7 be the stabilizer of e 0 in g. Asinthesl 2 case, on a curve with coordinate dx, lemma3.6willstatethateverygoper has acanonicaltrivializationandthat,locally,opersareatorsor for maps Y g e0. However, B doesn t act on g e0 so we can t apriori twist by B as in the sl 2 case) to immediately get an action on the space of opers. ) None the less, since B 0 does act on g e0 we can form g e0 := 0 0 H 0 H0 g e0 = 0 B 0 B0 g e0 Using Ω 2 =Ω n 0 0 Ω g e0 we construct an Ω 0 g e0 torsor 0 Op sl2 Ω 2 ) Ω g e0 0 Lemma 3.5. Every principal embedding gives rise to an isomorphism ) 3.1) Op sl2 Ω 2 Ω g e0 O p 0 g Proof. The map is defined by sending 0 B 0, ; ν ) to ι 0 B 0,ι + ν ).Where 0 B 0, ) is an sl 2 oper which induces a goper ι 0 B 0,ι ),andforν Ω g e0 we define 0 ι + ν using g e0 = 0 0 B 0 B0 g e0 0 B 0 B0 b = b 0. The image consists of opers whose underlying Btorsor is 0 B := 0 B 0 ι B. To see this is an isomorphism is suffices to check locally, whence this is implied by lemma 3.6. Lemma 3.6. Let be a curve with a coordinate dx. ix a principle embedding sl 2 g as above. or any goper on, the underlying Btorsor admits a unique trivialization relative to which the connection has the form where η : Y g e0. = d + dx f + η) 7 n0 g e 0 n. Inthecaseofsl n with the standard Principal embedding, g e consists of all strictly upper triangular matrices with constant superdiagonals.
9 OPERS 9 Proof. The proof uses Kostant s lemma see [2]) and we defer it to the appendix. Corollary 3.7. Global gopers exist and all gopers have isomorphic underlying Btorsors. Given a Principal embedding ι, this Btorsor is naturally identified with as the pushforward 0 B := 0 B 0 ι B. Every principal embedding gives rise, via lemma 3.5, to a Ω g e0 torsor structure on Op g. However, any two principal embedding are uniquely conjugate by an element of B. Thus,neitherthevectorspaceg e0,northetorsorstructuredepend on the Principal embedding: Lemma 3.8. Op g is naturally a torsor for Ω g e0.thusop g ) is naturally a torsor for Γ ), Ω g e0. Both are trivial torsors. As for the representability of the space of opers, any choice of a global oper on gives rise to: Corollary 3.9. Op g is representable by a D scheme isomorphic to J ets Ω g e0 ). When is complete, Op g ) is representable by a scheme isomorphic to Γ, Ω g e0 ) for which it is naturally an affine space). 4. Opers and the classical Hitchin space Next we discuss the relation to the Hitchin space, which will be further developed in the next section. Identify, via the Ad action, H 0 = Autn0 = G m to obtain a G m action on g e0 which we also denote Ad. H0 = Ω and so Ω g e0 =Ω H0 H0 g e0 )=Ω tadt) g e0 where tadt) denotes the action of t G m on g e0.therearenaturalisomorphisms cl g := specsym L g) LG = specsym L h) W = specsymh ) W = g//g and these identifications are G m equivariant w.r.t. the the action on the quotients, which is pushed forward from scalar multiplication on g and L g respectively. Recall Kostant s section of the Chevalley map g g//g has f 0 + g e0 as it s image. This section induces an isomorphism of varieties cl g g e0 which intertwines the G m action on cl g with the tadt) action on g e0. inally we obtain an identification schemes over 4.1) c Ω L g =Ω Gm cl g = Ω g e0 The LHS, which is not apriori a vector bundle, is now considered as such via this isomorphism.
10 10 JONATHAN BARLEV The punch line is that corollary 3.9 now reads: every global goper over gives rise to an isomorphism of D schemes Op g = J ets c Ω L g )=HitchL g = spec z cl ) L g Passing to functors of horizontal sections we get 4.2) Op g ) ) = Sect c Ω L = g HitchL g) Next assume is complete, then Op g ) is representable by a scheme, which is naturally an affine space for Γ, Ω g e0 ). Let A g denote the coordinate algebra of Op g,andleta g ) denote the coordinate algebra of Op g ). Recallthatz cl L g and zcl L g ) are the coordinate rings of HitchL g and HitchL g ) respectively, and that they are naturally graded. By the discussion above A g and z cl L g are noncanonically isomorphic as are A g ) and z cl L g )), but the following is canonical: Proposition 4.1. There exists a canonical filtration of A g and a canonical isomorphism of graded rings gra σ A g z cl L g. Proof. Identify z cl L g with the coordinate ring of J ets ) Ω tadt via 4.1. Note ge0 that the natural grading on z cl L g corresponds to the action of G m on Ω tadt ge0 opposite to tadt. As Op g is naturally an affine space for J ets Ω tadt ge0 ) over, onthelevel of coordinate rings, the grading on the latter induces a filtration on the former which yields the desired isomorphism. Corollary 4.2. Let be a complete curve. There exists a canonical filtration of A g ) and a canonical isomorphism of graded rings gra g )) gr A g )) σ A) z cl L g ). 5. The eiginrenkel isomorphism This section is the quantization of the previous, and relates operstothequantum Hitchin space. Let be a complete curve. Recall that z L g defined rather abstractly) was a filtered algebra and we had the following theorem of eigin and renkel, dicussed in Dustin s talk) and accompanying results which related the various Hitchin algeras. is an isomor Theorem 5.1 first eiginrenkel). The natural map grz g phism of graded D algebras. σ z z cl g Proposition 5.2. The natural surjection grz g ) ) gr zg ) ) is an isomorphism.
11 OPERS 11 Corollary 5.3. We have the following isomorphisms grzg ) ) gr zg ) ) σ z) z cl g ) As promised, the following theorem due to eigin and renkel as well relates the Hitchin spaces to opers: Theorem 5.4 second eiginrenkel). There exists an isomorphism of filtered algebras A g φ zl g which quantizes the map σ Ag of proposition 4.1, i.e. σ z grφ = σ Ag. Corollary 5.5. φ induces an isomorphism of filtered algebras A g ) φ z L g ) which quantizes the map σ Ag) of corollary 4.2, i.e. σ z) grφ = σ Ag). rom the perspective of spaces we have an isomorphisms ) spec zg OpL g and spec z g ) ) OpL g ) of D schemes and schemes respectively) providing a moduli description of the quantum Hitchin space. Let us summarize this Hitchin quantization business so far: AL g) gr gral g) = z g ) gr grz g )) ΓBun γ g, D ) gr grγbun γ g, D ) = = A cl L g ) = z cl g ) Where A cl ) is the coordinate ring of Γ = = ΓT Bun γ g, O), Ω g e0 0 ) and is introduced for the sake of symmetry. The marked isomorphisms are the ones we proved, they imply the rest of the maps are isomorphisms as well. inally the geometric Langlands correspondence for opers should play out as follows: starting from a L glocal system on, σ, whichadmitsaunique)operstructure we get a maximal ideal m σ AL g ) whose residue field we denote k σ.thuswe get a twisted Dmod D σ := D ALg ) k σ on Bun g ).This will be shown to be a Hecke Eigensheaf.
12 12 JONATHAN BARLEV 6. Appendix  Kostant s lemma and it s application In this appendix we state a lemma of Kostant and prove lemma 3.6. This has already been discussed in the seminar to some extent, see [2]. Choose a principal ι embedding sl 2 g, andlete, h, f denote the images of the standard) same named elements in sl 2. e and f are contained in unique, opposite Borels; so ι determines a ι Borel, Cartan and root system. Let PSL G 2 G be the induced map on the group level. Lemma 6.1. N acts on f + b freely via Ad. f + g e is a section for this action, and moreover N i.f + g e )=f + g e + n i. With this in hand we prove lemma 3.6 : proof of lemma 3.6. Let have a coordinate dx, and let = d + η 1 dx η 1 : g 1 be the connection of a goper on a trivialized Btorsor. 1) We must show there exist a unique change of trivialization, σ : B such that the transformed connection d + dσσ 1 + Ad σ) η 1 dx has the desired form. Because G is of adjoint type Ad induces an isomorphism H GL g α ). Since η : g 1 has nonvanishing entries in every simple negative root space, there exists a unique σh H s.t. that Ad σ) η = f mod b. σ 1 H dσ 1 H b so that after changing the trivialization using σ H it has the form d +f + η 0 ) dx η 0 : b Let σ 1 : N be the unique map, promised by Konstant s lemma, such that Ad σ) f + η 0) f + g e.ifitweren tfortheσ 1 dσ term, which pops up when the trivialization is changed we d be done. Nonetheless we get an approximation mod n, i.e. afterchangingthetrivializationagainusingσ 1 the connection has the forms d +f + η 1 ) dx η 1 : n Note that σ 1 σ H with this property is unique mod N 2. We proceed in the same fashion inductively, noting at the i th stage that if f +η i f +g e +n i, then the unique σ i such that Adσ i )f + η i ) f + g e in fact lies in N i+1,henceσ 1 i dσ n i+1. After rankg steps we get σ r σ 1 σ H which is the required map.
13 OPERS 13 References [1] Beilinson; Drinfeld. Hitchin s integrable system. http : //www.math.harvard.edu/ gaitsgde/grad [2] Dennis Gaitsgory. Seminar notes:higgs gundles, kostant section, and local triviality of g bundles. http : //www.math.harvard.edu/ gaitsgde/grad 2009.
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