A p-adic Birch and Swinnerton-Dyer conjecture for modular abelian varieties

Transcription

1 A p-adic Birch and Swinnerton-Dyer conjecture for modular abelian varieties Steffen Müller Universität Hamburg joint with Jennifer Balakrishnan and William Stein Rational points on curves: A p-adic and computational perspective Oxford University Tuesday, September 25, 2012

2 Notation f(z) = n=1 a ne 2πinz S 2 (Γ 1 (N)) newform, K f = Q(...,a n,...), A f = J 1 (N)/Ann T (f)j 1 (N) abelian variety /Q associated to f, g = [K f : Q] dimension of A f, G f = {σ : K f C}, f σ (z) = n=1 σ(a n)e 2πinz for σ G f, L(A f,s) = σ G f L(f σ,s) Hasse-Weil L-function of A f. L (A f,1) leading coefficient of series expansion of L(A f,s) in s = 1. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 2 / 23

3 BSD conjecture Conjecture (Birch, Swinnerton-Dyer, Tate) We have r := rk(a f (Q)) = ord s=1 L(A f,s) and L (A f,1) = Reg(A f/q) (A f /Q) v c v A f (Q) tors A f (Q). tors Ω + A f Ω + A f : real period A f (R) η, η Néron differential, Reg(A f /Q): Néron-Tate regulator, c v : Tamagawa number at v, v finite prime, (A f /Q): Shafarevich-Tate group. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 3 / 23

4 Shimura periods Let p > 2 be a prime such that A f has good ordinary reduction at p. We want to find a p-adic analogue of the BSD conjecture. First need to construct a p-adic L-function. Theorem. (Shimura) For all σ G f there exists Ω + f σ C such that we have (i) L(fσ,1) K Ω + f, f σ ( (ii) σ ) L(f,1) Ω + f = L(fσ,1) Ω + f σ, (iii) an analogue of (i) for twists of f by Dirichlet characters. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 4 / 23

5 Modular symbols Fix a Shimura period Ω + f. Fix a prime p of K f such that p p. Let α be the unit root of x 2 a p x+p (K f ) p [x]. For r Q, the plus modular symbol is [r] + f := πi Ω + f ( i r f(z)dz + i r ) f(z)dz K f. Define a measure on Z p : µ + f,α (a+pn Z p ) = 1 α n [ ] a + p n f 1 α n+1 [ a p n 1 ] + f Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 5 / 23

6 Mazur/Swinnerton-Dyer p-adic L-function Write x Z p as ω(x) x where ω(x) p 1 = 1 and x 1+pZ p. Define L p (f,s) := Z p x s 1 dµ + f,α (x) for all s Z p. Fix a topological generator γ of 1+pZ p. Convert L p (f,s) into a p-adic power series L p (f,t) in terms of T = γ s 1 1. Let ǫ p (f) := (1 α 1 ) 2 be the p-adic multiplier. Then we have the following interpolation property (due to Mazur-Tate-Teitelbaum): L p (f,0) = L p (f,1) = ǫ p (f) [0] + f = ǫ p(f) L(f,1) Ω +. f Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 6 / 23

7 Mazur-Tate-Teitelbaum conjecture All of this depends on the choice of Ω + f! If A f = E is an elliptic curve, a canonical choice is given by Ω + f = Ω+ E. Conjecture. (Mazur-Tate-Teitelbaum) If A f = E is an elliptic curve, then we have r := rk(e/q) = ord T=0 (L p (f,t)) and L p(f,0) = Reg γ(e/q) (E/Q) v c v ǫ p (f) E(Q) tors 2. L p(f,0): leading coefficient of L p (f,t), Reg γ (E/Q) = Reg p (E/Q)/log(γ) r, where Reg p (E/Q) is the p-adic regulator (due to Schneider, Néron, Mazur-Tate, Coleman Gross, Nekovář). Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 7 / 23

8 Extending Mazur-Tate-Teitelbaum An extension of the MTT conjecture to arbitrary dimension g > 1 should be equivalent to BSD in rank 0, reduce to MTT if g = 1, be consistent with the main conjecture of Iwasawa theory for abelian varieties. Problem. Need to construct a p-adic L-function for A f! Idea: Define L p (A f,s) := σ G f L p (f σ,s). But to pin down L p (f σ,s), first need to fix a set {Ω + f σ } σ Gf of Shimura periods. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 8 / 23

9 p-adic L-function associated to A f Theorem. If {Ω + f } σ σ Gf are Shimura periods, then there exists c Q such that Ω + A f = c Ω + f. σ σ G f So we can fix Shimura periods {Ω + f } σ σ Gf such that Ω + A f = Ω + f. (1) σ σ G f With this choice, define L p (A f,s) := σ G f L p (f σ,s). Then L p (A f,s) does not depend on the choice of Shimura periods, as long as (1) holds. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 9 / 23

10 Interpolation Convert L p (A f,s) into a p-adic power series L p (A f,t) in terms of T = γ s 1 1. Let ǫ p (A f ) := σ ǫ p(f σ ) be the p-adic multiplier. Then we have the following interpolation property L p (A f,0) = ǫ p (A f ) L(A f,1) Ω + A f. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 10 / 23

11 More notation L p(a f,0): leading coefficient of L p (A f,t), Reg γ (A f /Q) = Reg p (A f /Q)/log(γ) r, where r = rk(a f (Q)) and Reg p (A f /Q) is the p-adic regulator. If A f is not principally polarized, then Reg p (A f /Q) is only defined up to ±1. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 11 / 23

12 The conjecture We make the following p-adic BSD conjecture (with the obvious sign ambiguity if A f is not principally polarized): Conjecture. The Mordell-Weil rank r of A f /Q equals ord T=0 (L p (A f,t)) and L p(a f,0) = Reg γ(a f /Q) (A f /Q) v c v ǫ p (A f ) A f (Q) tors A f (Q). tors This conjecture is equivalent to BSD in rank 0, reduces to MTT if g = 1, is consistent with the main conjecture of Iwasawa theory for abelian varieties, via work of Perrin-Riou and Schneider. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 12 / 23

13 Computing the p-adic L-function To test our conjecture in examples, we need an algorithm to compute L p (A f,t). The modular symbols [r] + f σ can be computed efficiently in a purely algebraic way up to a rational factor (Cremona, Stein), To compute L p (A f,t) to n digits of accuracy, can use (i) approximation using Riemann sums (similar to Stein-Wuthrich) exponential in n or (ii) overconvergent modular symbols (due to Pollack-Stevens) polynomial in n. Both methods are now implemented in Sage. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 13 / 23

14 Normalization To find the correct normalization of the modular symbols, can use the interpolation property. Find a Dirichlet character ψ associated to a quadratic number field Q( D) such that D > 0 and L(B,1) 0, where B is A f twisted by ψ, gcd(pn,d) = 1. We have Ω + B η ψ = D g/2 Ω + A f for some η ψ Q. Can express [r] + B := σ [r]+ f σ ψ in terms of modular symbols [r] + f σ. The correct normalization factor is L(B, 1) Ω + B [0]+ B = η ψ L(B,1) D g/2 Ω + A f [0] +. B Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 14 / 23

15 Coleman-Gross height pairing Suppose A f = Jac(C), where C/Q is a hyperelliptic curve of genus g. The Coleman-Gross height pairing is a symmetric bilinear pairing h : Div 0 (C) Div 0 (C) Q p, which can be written as a sum of local height pairings h = v h v over all finite places v of Q and satisfies h(d,div(g)) = 0 for g k(c). Techniques to compute h v depend on v: v p: intersection theory (M., Holmes) v = p: logarithms, normalized differentials, Coleman integration (Balakrishnan-Besser) Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 15 / 23

16 Local heights away from p D,E Div 0 (C) with disjoint support, suppose v p, X/Spec(Z v ): regular model of C, (. ) v : intersection pairing on X, D, E Div(X): extensions of D,E to X such that (D.F) v = (E.F) v = 0 for all vertical divisors F Div(X). Then we have h v (D,E) = (D.E) v log p (v). Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 16 / 23

17 Computing local heights away from p Regular models can be computed using Magma; divisors on C and extensions to X can be represented using Mumford representation; intersection multiplicities of divisors on X can be computed algorithmically using linear algebra and Gröbner bases (M.) or resultants (Holmes). All of this is implemented in Magma. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 17 / 23

18 Local heights at p Let ω D be a normalized differential associated to D. The local height pairing at p is given by h p (D,E) = ω D. Suppose C/Q p is given by an odd degree model y 2 = g(x). Let P, Q C(Q p ). If P Q (mod p), then it is easy to compute Q P ω D. The work of Balakrishnan-Besser gives a method to extend this to the rigid analytic space C an C p, using analytic continuation along Frobenius. Need to compute matrix of Frobenius, e.g. using Monsky-Washnitzer cohomology. This has been implemented by Balakrishnan in Sage. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 18 / 23 E

19 Computing the p-adic regulator Suppose P 1,...,P r A f (Q) are generators of A f (Q) mod torsion. Suppose P i = [D i ], D i Div(C) 0 pairwise relatively prime and with pointwise Q p -rational support. Then Reg p (A f /Q) = det((m ij ) i,j ), where m ij = h(d i,d j ). Problem. Given a subgroup H of A f (Q) mod torsion of finite index, need to saturate it. Currently only possible for g = 2 (g = 3 work in progress due to Stoll), so in general only get Reg p (A f /Q) up to a Q-rational square. For g = 2, can use generators of H and compute the index using Néron-Tate regulators to get Reg p (A f /Q) exactly. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 19 / 23

20 Empirical evidence for g = r = 2 From Empirical evidence for the Birch and Swinnerton-Dyer conjectures for modular Jacobians of genus 2 curves (Flynn et al. 01), we considered 16 genus 2 curves of respective level N. For each curve, the associated abelian variety has Mordell-Weil rank 2. N Equation 67 y 2 + (x 3 + x + 1)y = x 5 x 73 y 2 + (x 3 + x 2 + 1)y = x 5 2x 3 + x 85 y 2 + (x 3 + x 2 + x)y = x 4 + x 3 + 3x 2 2x y 2 + (x 3 + x 2 + 1)y = 2x 5 + x 4 + x y 2 + (x 3 + x 2 + 1)y = x 5 + x y 2 + (x 3 + x 2 + 1)y = x 4 x 2 x y 2 + (x 3 + x + 1)y = 2x 3 + x 2 + x 125 y 2 + (x 3 + x + 1)y = x 5 + 2x 4 + 2x 3 + x 2 x y 2 + (x 3 + x 2 + 1)y = x 5 + x 4 2x 3 + 2x 2 2x 147 y 2 + (x 3 + x 2 + x)y = x 5 + 2x 4 + x 3 + x y 2 + (x 3 + x + 1)y = x 3 + 4x 2 + 4x y 2 + (x 3 + x 2 + x)y = x 5 + 2x 4 + 3x 3 + x 2 3x 167 y 2 + (x 3 + x + 1)y = x 5 x 3 x y 2 + (x 3 + x 2 + 1)y = x 5 + x 4 + x y 2 = x 5 x 4 + x 3 + x 2 2x y 2 + (x 3 + x + 1)y = x 3 + x 2 + x Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 20 / 23

21 N = 188 To numerically verify p-adic BSD, need to compute p-adic regulators and p-adic special values. For example, for N = 188, we have: p-adic regulator Reg p (A f /Q) p-adic L-value p-adic multiplier ǫ p (A f ) O(7 8 ) O(7 4 ) O(7 8 ) O(11 7 ) O(11 8 ) O(11 8 ) O(13 8 ) O(13 8 ) O(13 8 ) O(17 8 ) O(17 8 ) O(17 8 ) O(19 8 ) O(19 8 ) O(19 8 ) O(23 8 ) O(23 8 ) O(23 8 ) O(37 8 ) O(37 8 ) O(37 8 ) O(41 8 ) O(41 8 ) O(41 8 ) O(43 8 ) O(43 8 ) O(43 8 ) O(53 8 ) O(53 6 ) O(53 8 ) O(59 8 ) O(59 6 ) O(59 8 ) O(61 8 ) O(61 7 ) O(61 8 ) O(67 8 ) O(67 6 ) O(67 8 ) O(71 8 ) O(71 6 ) O(71 8 ) O(73 8 ) O(73 5 ) O(73 8 ) O(79 8 ) O(79 5 ) O(79 8 ) O(83 8 ) O(83 5 ) O(83 8 ) O(89 8 ) O(89 5 ) O(89 8 ) O(97 8 ) O(97 4 ) O(97 8 ) Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 21 / 23

22 N = 188 normalization The additional BSD invariants for N = 188 are (A f )[2] = 1, A f (Q) tors 2 = 1, c 2 = 9, c 47 = 1. We find that for the quadratic character ψ associated to Q( 233), the twist B of A f by ψ has rank 0 over Q. Algebraic computation yields [0] + B = 144, η ψ = 1, computed by comparing bases for the integral 1-forms on the curve and its twist by ψ. η ψ L(B,1) 233 Ω + A f = 36. So the normalization factor for the modular symbol is 1/4. Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 22 / 23

23 Summary of evidence Theorem. Assume that for the Jacobians of all 16 curves listed above the Shafarevich-Tate group over Q is 2-torsion. Then our conjecture is satisfied up to least 4 digits of precision at all good ordinary p < 100 satisfying the assumptions of our algorithms. The assertion (A f /Q) = (A f /Q)[2] follows from classical BSD (Flynn et al.). We also have a similar result for the Jacobian of a twist of X 0 (31) of rank 4 over Q. Since the twist is odd, we had to use the minus modular symbol associated to J 0 (31). Steffen Müller (Universität Hamburg) p-adic BSD for modular abelian varieties 23 / 23