On $L$-functions of quadratic $\mathbb{Q}$-curves

TL;DR

This paper develops methods to compute and analyze the special values of L-functions of quadratic Q-curves, verifying parts of the Birch and Swinnerton-Dyer conjecture under certain conjectural assumptions.

Contribution

It introduces an effective approach to compute the L-value at 1 for quadratic Q-curves and verifies the weak BSD conjecture for specific cases.

Findings

Proved that L(E,1) is zero for certain curves when a computed integer condition is met.

Developed an algorithm to find the associated newform for L-function factorization.

Verified the weak BSD conjecture for some rank 2 Q-curves.

Abstract

Let $K$ be a quadratic number field of discriminant $\Delta_K$, let $E$ be a $\mathbb Q$-curve without CM completely defined over $K$ and let $\omega_E$ be an invariant differential on $E$. Let $L(E,s)$ be the $L$-function of $E$. In this setting, it is known that $L(E,s)$ possesses an analytic continuation to $\mathbb C$. The period of $E$ can be written (up to a power of $2$) as the product of the Tamagawa numbers of $E$ with $\Omega_E/\sqrt{|\Delta_K|}$, where $\Omega_E$ is a quantity, independent of $\omega_E$, which encodes the real periods of $E$ when $K$ is real and the covolume of the period lattice of $E$ when $K$ is imaginary. In this paper we compute, under the generalized Manin conjecture, an effective nonzero integer $Q=Q(E,\omega_E)$ such that if $L(E,1)\neq 0$ then $L(E,1)\cdot Q\cdot\sqrt{|\Delta_K|}/\Omega_E$ is an integer. Computing $L(E,1)$ up to sufficiently high precision, our result allows us to prove that $L(E,1)=0$ whenever this is the case and to compute the $L$-ratio $L(E,1)\cdot\sqrt{|\Delta_K|}/\Omega_E$ when $L(E,1)\neq 0$. An important ingredient is an algorithm to compute a newform $f$ of weight $2$ level $\Gamma_1(N)$ such that $L(E,s)=L(f,s)\cdot L({}^{\sigma\!} f,s)$, for ${}^{\sigma\!} f$ the unique Galois conjugate of $f$. As an application of these results, we verify the validity of the weak BSD conjecture for some $\mathbb Q$-curves of rank $2$ and we will compute the $L$-ratio of a curve of rank $0$.