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There are two well-known L-functions attached to Siegel modular forms of degree two. These are the Spin and the standard L-function. They have been studied by Andrianov, Böcherer, Shimura, and others. Other L-functions associated to degree two modular forms include the adjoint L-function (degree 10), and two others (degree 14 and degree 16).

The Spin L-function is of particular interest in the cases of paramodular forms of level t and weight k. Examples of paramodular forms can be made using the Gritsenko lift, a generalization of the Saito-Kurokawa lifting. The Gritsenko lift is actually a map defined on Jacobi forms, but it may also be considered a lift of classical newforms \mathcal{S}_{2k-2}^{new-}(\Gamma_0(t)), where the space \mathcal{S}_{2k-2}^{new-}(\Gamma_0(t)) consists of newforms such that the sign in the functional equation of their L-function is -1. Skoruppa and Zagier defined an isomorphism from the space J_{k,t}^{cusp, new} of cuspidal Jacobi newforms to \mathcal{S}_{2k-2}^{new-}(\Gamma_0(t)). So one takes a classical newform in the “minus space,” maps it to the space of Jacobi forms using the inverse of the Skoruppa-Zagier map, then lifts it with the Gritsenko lift to the space of paramodular forms S_k(K(t)). We will also call the composition of these two maps the Gritsenko lift.

Euler factors of the Spin L-function for paramodular forms of square-free level t=N were computed by Schmidt using representation theoretic methods. At the time of his paper, the local Langlands correspondence for GSp(4) was unverified, but Gan and Takeda have since proven its existence and so Schmidt’s results hold. He had shown that for f\in \mathcal{S}_{2k-2}^{new-}(\Gamma_0(N)), the lifting produces a cusp form F\in S_k(K(N)) of degree 2 whose completed Spin L-function is given by

L(s,F, {\rm spin})=\dfrac{1}{4\pi}\left(s-\dfrac{1}{2}\right)Z\left(s+\dfrac{1}{2}\right)Z\left(s-\dfrac{1}{2}\right)L(s,f)

where Z is the completed Riemann zeta function. Moreover, this lifting preserves the Atkin-Lehner eigenvalues, i.e., \eta_pF=\varepsilon_pF for every prime p.

Schmidt also determined the local components of the automorphic representation \pi_f associated to a classical newform f\in\mathcal{S}_{2k-2}^{new-}(\Gamma_0(N)). Moreover, he explained that the Gritsenko lift and similar lifts are predicted by Langlands functoriality. Schmidt also computed the Euler factors of the Spin L-function for the space of paramodular forms of square-free level that are lifts using representation theoretic methods since the lift is functorial. Schmidt and Roberts later determined the possible Euler factors of any paramodular representation, but precise information for the non-square-free level case remained unknown.

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Let C be a smooth projective variety of genus g\leq3 over \mathbb{Q}. Arithmetic information about the curve is encoded in its L-function L(C,s). The conjectures of Birch and Swinnerton-Dyer about elliptic curves over \mathbb{Q} were generalized to arbitrary abelian varieties over number fields by John Tate.

In the case of hyperelliptic curves (genus 2 curves) over \mathbb{Q}, the first conjecture is that the order of vanishing of the L-function of the Jacobian at s=1 (the analytic rank) is equal to the Mordell-Weil rank of the Jacobian. The second is

\lim_{s\rightarrow1}(s-1)^{-r}L(J,s)=\Omega\cdot{\rm Reg}\cdot\prod_{p}c_p\cdot\# Ш (J,\mathbb{Q})\cdot(\# J(\mathbb{Q})_{\rm tors})^{-2}

where L(J,s) is the L-series of J and r is its analytic rank. \Omega denotes the integral over J(\mathbb{R}) of a certain differential 2-form, Reg is the regulator of J(\mathbb{Q}), c_p = \# J(\mathbb{Q}_p)/J^0(\mathbb{Q}) is the Tamagawa number, Ш(J,\mathbb{Q}) is the Tate-Shafarevich group of J over \mathbb{Q}, and J(\mathbb{Q})_{tors} is the torsion subgroup of J(\mathbb{Q}). Here, J^0(\mathbb{Q}) is the subgroup of the Jacobian isomorphic to \mathcal{J}^0(\mathbb{Z}_p), where \mathcal{J}^0 is the open subgroup scheme of the closed fiber of the Néron model of J over \mathbb{Z}_p

The L-series of the curve C is given as both an Euler product and a Dirichlet series.

L(C,s)=\prod_p L_p(p^{-s})^{-1}=\sum_{n=1}^\infty a_n n^{-s}.

If C has good reduction at the prime p, the factor in the Euler product at this prime is determined by a polynomial L_p(T) of degree 4. It appears in the local zeta function of the curve over the finite field \mathbb{F}_p of order p.

Z(C/\mathbb{F}_p;T)=exp\left(\sum_{k=1}^\infty N_kT^k/k\right)=\dfrac{L_p(T)}{(1-T)(1-pT)},

where N_k is the number of \mathbb{F}_{q^k}-points on C.

By a theorem of Weil, the polynomial L_p(T)=\sum_{n=0}^4 a_nT^n, can be determined by counting points on C only over \mathbb{F}_p and \mathbb{F}_{p^2} since the coefficients must satisfy a_0=1, a_3=pa_1, and a_4=p^2. We consider five special hyperelliptic curves (those associated to paramodular forms taken from Brumer and Kramer’s paper) and compute their discriminant and local L-factor at p=7.

Curve: y^2=x^6 + 4x^5 + 4x^4 + 2x^3 + 1
Conductor: 249
Discriminant: 261095424 = 2^{20} \cdot 3 \cdot 83
Local L-factor at p=7: L_7(7^{-s})^{-1}=\dfrac{1}{7^{2-4 s}+8\cdot 7^{1-3 s}+44\cdot     7^{-2 s}+8\cdot 7^{-s}+1}

Curve: y^2=x^6 + 2x^5 + 3x^4 + 4x^3 - x^2 - 2x + 1
Conductor: 277
Discriminant: 290455552 = 2^{20} \cdot 277
Local L-factor at p=7: L_7(7^{-s})^{-1}=\dfrac{1}{7^{2-4 s}+6\cdot 7^{1-3 s}+54\cdot     7^{-2 s}+6\cdot 7^{-s}+1}

Curve: y^2=x^6 - 2x^3 - 4x^2 + 1
Conductor: 295
Discriminant: 309329920 = 2^{20} \cdot 5 \cdot 59
Local L-factor at p=7: L_7(7^{-s})^{-1}=\dfrac{1}{7^{2-4 s}+6\cdot 7^{1-3 s}+48\cdot     7^{-2 s}+6\cdot 7^{-s}+1}

Curve: y^2=x^6 - 2x^5 + 3x^4 - x^2 - 2x + 1
Conductor: 349
Discriminant: 365953024 = 2^{20} \cdot 349
Local L-factor at p=7: L_7(7^{-s})^{-1}=\dfrac{1}{7^{2-4 s}+9\cdot 7^{1-3 s}+69\cdot     7^{-2 s}+9\cdot 7^{-s}+1}

Curve: y^2=x^6 + 2x^5 + 5x^4 + 2x^3 + 2x^2 + 1
Conductor: 353
Discriminant: 370147328 = 2^{20} \cdot 353
Local L-factor at p=7: L_7(7^{-s})^{-1}=\dfrac{1}{7^{2-4 s}+7^{2-3     s}+7^{1-s}+37\cdot 7^{-2 s}+1}

If one were to attempt to find an automorphic representation \pi=\otimes\pi_p that could be associated to these varieties, one could use this information about the local L-factors to rule out possible local components. For example, Schmidt determined the local L-factors of the representations \Pi({\rm St}\otimes 1), \Pi(\xi{\rm St}\otimes 1), \Pi({\rm St}\otimes{\rm St}):

\Pi({\rm St}\otimes 1) : L_p(s,\Pi_p)^{-1} = (1-p^{-s-1/2})^2(1-p^{-s+1/2}),
\Pi(\xi{\rm St}\otimes 1) : L_p(s,\Pi_p)^{-1} = (1-p^{-s-1/2})(1-p^{-s+1/2})(1+p^{-s-1/2}),
\Pi({\rm St}\otimes{\rm St}) : L_p(s,\Pi_p)^{-1} = (1-p^{-s-1/2})^2.

Comparing these with the L-factors of the curves we found at p=7, we can say that the local component at p=7 of an associated automorphic representation is not one of these.

A. Brumer and K. Kramer, Paramodular abelian varieties of odd conductor, arXiv:1004.4699v2 (2010).

E. Flynn, F. Leprévost, et. al., Empirical evidence for the Birch and Swinnerton-Dyer conjectures for modular Jacobians of genus 2 curves, Math. Comp. 70 (2001), no. 236, 1675-1697

K. Kedlaya and A. Sutherland, Computing L-series of hyperelliptic curves, Algorithmic number theory, 312–326, Lecture Notes in Comput. Sci., 5011, Springer, Berlin, 2008.

R. Schmidt, On classical Saito-Kurokawa liftings, J. Reine Angew. Math. 604 (2007), 211-236.

J. Tate, On the conjectures of Birch and Swinnerton-Dyer and a geometric analog, Séminaire Bourbaki, 306 1965/1966.

A. Weil, Number of solutions of equations in finite fields, Bull. Amer. Math. Soc. 55, (1949), 497-508.

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