Modularity of Galois representations. imaginary quadratic fields

Transcription

1 over imaginary quadratic fields Krzysztof Klosin (joint with T. Berger) City University of New York December 3, 2011

2 Notation F =im. quadr. field, p # Cl F d K, fix p p

3 Notation F =im. quadr. field, p # Cl F d K, fix p p Σ = finite set of finite places of F, p, p Σ, G Σ = Gal(F Σ /F );

4 Notation F =im. quadr. field, p # Cl F d K, fix p p Σ = finite set of finite places of F, p, p Σ, G Σ = Gal(F Σ /F ); O= ring of integers in a finite extension of Q p, ϖ=uniformizer, F = O/ϖ;

5 Notation F =im. quadr. field, p # Cl F d K, fix p p Σ = finite set of finite places of F, p, p Σ, G Σ = Gal(F Σ /F ); O= ring of integers in a finite extension of Q p, ϖ=uniformizer, F = O/ϖ; Ψ= (unramified) Hecke character of -type z z, Ψ p : G Σ O the associated Galois character, χ 0 = Ψ p (mod ϖ)

6 Main Theorem Theorem (Berger-K., 2011) Suppose that dim F Ext 1 F[G Σ ] (χ 0, 1) = 1.

7 Main Theorem Theorem (Berger-K., 2011) Suppose that dim F Ext 1 F[G Σ ] (χ 0, 1) = 1. Let ρ : G Σ GL 2 (Q p ) be continuous and irreducible. Suppose that:

8 Main Theorem Theorem (Berger-K., 2011) Suppose that dim F Ext 1 F[G Σ ] (χ 0, 1) = 1. Let ρ : G Σ GL 2 (Q p ) be continuous and irreducible. Suppose that: det ρ = Ψ p

9 Main Theorem Theorem (Berger-K., 2011) Suppose that dim F Ext 1 F[G Σ ] (χ 0, 1) = 1. Let ρ : G Σ GL 2 (Q p ) be continuous and irreducible. Suppose that: det ρ = Ψ p ρ ss = 1 χ 0

10 Main Theorem Theorem (Berger-K., 2011) Suppose that dim F Ext 1 F[G Σ ] (χ 0, 1) = 1. Let ρ : G Σ GL 2 (Q p ) be continuous and irreducible. Suppose that: det ρ = Ψ p ρ ss = 1 χ 0 ρ is crystalline (or ordinary if p splits) ρ is minimally ramified.

11 Main Theorem Theorem (Berger-K., 2011) Suppose that dim F Ext 1 F[G Σ ] (χ 0, 1) = 1. Let ρ : G Σ GL 2 (Q p ) be continuous and irreducible. Suppose that: det ρ = Ψ p ρ ss = 1 χ 0 ρ is crystalline (or ordinary if p splits) ρ is minimally ramified. Then ρ is modular, i.e., L(ρ γ, s) = L(s, π) for some automorphic representation π of GL 2 (A F ).

12 Remarks to the Main Theorem 1 At the moment this is the only known modularity result for an imaginary quadratic field.

13 Remarks to the Main Theorem 1 At the moment this is the only known modularity result for an imaginary quadratic field. 2 This is similar to a result of Skinner and Wiles which applies to Q or a totally real field, but their method fails for F =imaginary quadratic. An important step in their method is the existence of an ordinary, minimal deformation [ ] ρ = : G 0 Q GL 2 (O) of the residual representation [ ] 1 ρ 0 = = 1 χ 0 χ 0 0 (uses Kummer theory).

14 Remarks to the Main Theorem 1 At the moment this is the only known modularity result for an imaginary quadratic field. 2 This is similar to a result of Skinner and Wiles which applies to Q or a totally real field, but their method fails for F =imaginary quadratic. An important step in their method is the existence of an ordinary, minimal deformation [ ] ρ = : G 0 Q GL 2 (O) of the residual representation [ ] 1 ρ 0 = = 1 χ 0 χ 0 0 (uses Kummer theory). But Theorem (Berger-K.) No such deformation ρ exists for F.

15 Remarks to the Main Theorem We do not follow [SW]-strategy. Instead we develop a commutative algebra criterion that allows one to reduce the problem of modularity of all deformations of ρ 0 to that of modularity of the reducible deformations of ρ 0.

16 Remarks to the Main Theorem We do not follow [SW]-strategy. Instead we develop a commutative algebra criterion that allows one to reduce the problem of modularity of all deformations of ρ 0 to that of modularity of the reducible deformations of ρ 0. The condition dim F Ext 1 F[G Σ ] (χ 0, 1) = 1 is (probably) essential (work in progress).

17 Remarks to the Main Theorem We do not follow [SW]-strategy. Instead we develop a commutative algebra criterion that allows one to reduce the problem of modularity of all deformations of ρ 0 to that of modularity of the reducible deformations of ρ 0. The condition dim F Ext 1 F[G Σ ] (χ 0, 1) = 1 is (probably) essential (work in progress). The unramifiedness of Ψ-condition can be replaced by demanding that H 2 c (S Kf, Z p ) tors = 0.

18 Method [ ] 1 Let ρ 0 = : G 0 χ Σ GL 2 (F) be a non-semisimple 0 residual representation.

19 Method [ ] 1 Let ρ 0 = : G 0 χ Σ GL 2 (F) be a non-semisimple 0 residual representation. We study the crystalline (or ordinary if p splits) deformations of ρ 0.

20 Method [ ] 1 Let ρ 0 = : G 0 χ Σ GL 2 (F) be a non-semisimple 0 residual representation. We study the crystalline (or ordinary if p splits) deformations of ρ 0. There exists a universal couple (R Σ, ρ Σ : G Σ GL 2 (R Σ ))

21 Method [ ] 1 Let ρ 0 = : G 0 χ Σ GL 2 (F) be a non-semisimple 0 residual representation. We study the crystalline (or ordinary if p splits) deformations of ρ 0. There exists a universal couple (R Σ, ρ Σ : G Σ GL 2 (R Σ )) One gets a surjection φ : R Σ T Σ

22 Method [ ] 1 Let ρ 0 = : G 0 χ Σ GL 2 (F) be a non-semisimple 0 residual representation. We study the crystalline (or ordinary if p splits) deformations of ρ 0. There exists a universal couple (R Σ, ρ Σ : G Σ GL 2 (R Σ )) One gets a surjection φ : R Σ T Σ Goal: Show that φ is an isomorphism.

23 Ideal of reducibility I re := the smallest ideal I of R Σ such that for χ 1, χ 2 characters. tr ρ Σ = χ 1 + χ 2 (mod I )

24 Ideal of reducibility I re := the smallest ideal I of R Σ such that for χ 1, χ 2 characters. tr ρ Σ = χ 1 + χ 2 (mod I ) R Σ /I re controls the reducible deformations Key idea: Reduce the problem to that of modularity of reducible lifts.

25 Commutative algebra criterion Theorem (Berger-K.) Let R, S be commutative rings. Choose r R such that n r n R = 0. Let A be a domain and suppose that S is a finitely generated free module over A. Suppose we have a commutative diagrams of ring maps: R φ S R/rR φ S/φ(r)S.

26 Commutative algebra criterion Theorem (Berger-K.) Let R, S be commutative rings. Choose r R such that n r n R = 0. Let A be a domain and suppose that S is a finitely generated free module over A. Suppose we have a commutative diagrams of ring maps: R φ S R/rR φ S/φ(r)S. If rk A S/φ(r)S = 0, then φ is an isomorphism.

27 Commutative algebra criterion Theorem (Berger-K.) Let R, S be commutative rings. Choose r R such that n r n R = 0. Let A be a domain and suppose that S is a finitely generated free module over A. Suppose we have a commutative diagrams of ring maps: R φ S R/rR φ S/φ(r)S. If rk A S/φ(r)S = 0, then φ is an isomorphism. The rank condition can be replaced by a condition rr r 2 R = φ(r)s φ(r) 2 S and then the theorem gives an alternative to the criterion of Wiles and Lenstra.

28 Applying the commutative algebra criterion Corollary Set S = T Σ, R = R Σ.

29 Applying the commutative algebra criterion Corollary Set S = T Σ, R = R Σ. Suppose I re = rr and #T Σ /φ(i re )T Σ <.

30 Applying the commutative algebra criterion Corollary Set S = T Σ, R = R Σ. Suppose I re = rr and #T Σ /φ(i re )T Σ <. If the map φ : R Σ T Σ induces an isomorphism then φ is an isomorphism. R Σ /I re = TΣ /φ(i re ),

31 Applying the commutative algebra criterion Corollary Set S = T Σ, R = R Σ. Suppose I re = rr and #T Σ /φ(i re )T Σ <. If the map φ : R Σ T Σ induces an isomorphism then φ is an isomorphism. R Σ /I re = TΣ /φ(i re ), Upshot: To show R Σ = T Σ it suffices to prove:

32 Applying the commutative algebra criterion Corollary Set S = T Σ, R = R Σ. Suppose I re = rr and #T Σ /φ(i re )T Σ <. If the map φ : R Σ T Σ induces an isomorphism then φ is an isomorphism. R Σ /I re = TΣ /φ(i re ), Upshot: To show R Σ = T Σ it suffices to prove: I re is principal,

33 Applying the commutative algebra criterion Corollary Set S = T Σ, R = R Σ. Suppose I re = rr and #T Σ /φ(i re )T Σ <. If the map φ : R Σ T Σ induces an isomorphism then φ is an isomorphism. R Σ /I re = TΣ /φ(i re ), Upshot: To show R Σ = T Σ it suffices to prove: I re is principal, R Σ /I re = TΣ /φ(i re ), i.e., that every reducible deformation of ρ 0 is modular.

34 Applying the commutative algebra criterion Corollary Set S = T Σ, R = R Σ. Suppose I re = rr and #T Σ /φ(i re )T Σ <. If the map φ : R Σ T Σ induces an isomorphism then φ is an isomorphism. R Σ /I re = TΣ /φ(i re ), Upshot: To show R Σ = T Σ it suffices to prove: I re is principal, R Σ /I re = TΣ /φ(i re ), i.e., that every reducible deformation of ρ 0 is modular. Remark: The criterion removes the condition p B 2,ω k 2 from a modularity result for residually reducible Galois representations over Q due to Calegari.

35 Principality of the ideal of reducibility Theorem (Bellaïche-Chenevier, Calegari) If dim F Ext 1 F[G Σ ] (χ 0, 1) = dim F Ext 1 F[G Σ ] (1, χ 0) = 1, then I re is principal.

36 Principality of the ideal of reducibility Theorem (Bellaïche-Chenevier, Calegari) If dim F Ext 1 F[G Σ ] (χ 0, 1) = dim F Ext 1 F[G Σ ] (1, χ 0) = 1, then I re is principal. Theorem (Berger-K.) Let A be a Noetherian local ring with 2 A. Set S = A[G Σ ]. Let ρ : S M 2 (A) be an A-algebra map with ρ = ρ 0 mod m A.

37 Principality of the ideal of reducibility Theorem (Bellaïche-Chenevier, Calegari) If dim F Ext 1 F[G Σ ] (χ 0, 1) = dim F Ext 1 F[G Σ ] (1, χ 0) = 1, then I re is principal. Theorem (Berger-K.) Let A be a Noetherian local ring with 2 A. Set S = A[G Σ ]. Let ρ : S M 2 (A) be an A-algebra map with ρ = ρ 0 mod m A. If A is reduced, infinite, but #A/I re,a <, then I re,a is principal.

38 Modularity of reducible deformations Goal is to show that φ : R Σ /I re T Σ /φ(i re ) is an isomorphism.

39 Modularity of reducible deformations Goal is to show that φ : R Σ /I re T Σ /φ(i re ) is an isomorphism. Two steps: Show #R Σ /I re #O/L value (Iwasawa Main Conjecture - Rubin).

40 Modularity of reducible deformations Goal is to show that φ : R Σ /I re T Σ /φ(i re ) is an isomorphism. Two steps: Show #R Σ /I re #O/L value (Iwasawa Main Conjecture - Rubin). Show #T Σ /φ(i re ) #O/L value (congruences - Berger).

41 Higher-dimensional context Let F be a number field, G Σ = Gal(F Σ /F );

42 Higher-dimensional context Let F be a number field, G Σ = Gal(F Σ /F ); τ j : G Σ GL nj (F) be absolutely irreducible;

43 Higher-dimensional context Let F be a number field, G Σ = Gal(F Σ /F ); τ j : G Σ GL nj (F) be absolutely irreducible; [ ] τ1 ρ 0 = : G 0 τ Σ GL n1 +n 2 (F) be non-semisimple. 2

44 Higher-dimensional context Let F be a number field, G Σ = Gal(F Σ /F ); τ j : G Σ GL nj (F) be absolutely irreducible; [ ] τ1 ρ 0 = : G 0 τ Σ GL n1 +n 2 (F) be non-semisimple. 2 Study crystalline deformations of ρ 0, get (R Σ, ρ Σ ).

45 Higher-dimensional context Let F be a number field, G Σ = Gal(F Σ /F ); τ j : G Σ GL nj (F) be absolutely irreducible; [ ] τ1 ρ 0 = : G 0 τ Σ GL n1 +n 2 (F) be non-semisimple. 2 Study crystalline deformations of ρ 0, get (R Σ, ρ Σ ). Then I re is principal for essentially self-dual deformations (Berger-K., 2011);

46 Higher-dimensional context Let F be a number field, G Σ = Gal(F Σ /F ); τ j : G Σ GL nj (F) be absolutely irreducible; [ ] τ1 ρ 0 = : G 0 τ Σ GL n1 +n 2 (F) be non-semisimple. 2 Study crystalline deformations of ρ 0, get (R Σ, ρ Σ ). Then I re is principal for essentially self-dual deformations (Berger-K., 2011); Commutative algebra criterion still works;

47 Higher-dimensional context One needs to prove R Σ /I re = T Σ /φ(i re ).

48 Higher-dimensional context One needs to prove R Σ /I re = T Σ /φ(i re ). This uses the Bloch-Kato conjecture for the module Hom( τ 2, τ 1 ), where τ j are the unique lifts of τ j to O,

49 Higher-dimensional context One needs to prove R Σ /I re = T Σ /φ(i re ). This uses the Bloch-Kato conjecture for the module Hom( τ 2, τ 1 ), where τ j are the unique lifts of τ j to O, Congruences among automorphic forms on higher-rank groups (results of Agarwal, Böcherer, Dummigan, Schulze-Pillot and K. on congruences to the Yoshida lifts on Sp 4 allow us to prove that certain 4-dimensional Galois represenentations arise from Siegel modular forms.)

50 Another modularity result N=square-free, k=even, p > k 4, F = Q. Assume that every prime l N satisfies l 1 mod p. Let f S 2 (N), g S k (N), Σ = {l N, p}. Assume that ρ f and ρ g are absolutely irreducible. Theorem (Berger-K.) Suppose: dim F H 1 Σ (Q, Hom(ρ g, ρ f (k/2 1))) = 1; R ρf (k/2 1) = R ρg = O; the B-K conjecture holds for H 1 Σ (Q, Hom(ρ g, ρ f (k/2 1))). Let ρ : G Q,Σ GL 4 (Q p ) be continuous, irreducible and such that: ρ ss = ρ f (k/2 1) ρ g ; ρ is crystalline at p and essentially self-dual. Then ρ is modular. More precisely, there exists a Siegel modular form of weight k/2 + 1, level Γ 0 (N) and trivial character such that ρ = ρ F.

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