Relating elliptic curve point-counting and solutions of quadratic forms with congruence conditions

Abstract: In this paper, we analyze the theta series associated to the quadratic form $Q(\vec{x}) = x_1^2+x_2^2+x_3^2+x_4^2$ with congruence conditions on $x_i$ modulo $2,3,4$ and $6$. By employing special operators on modular, non-holomorphic Eisenstein series of weight 2, we construct a basis for Eisenstein space for levels $2^k, k\leq 7$, $3^{\ell}, \ell\leq 3$ and $p$, for odd prime $p$. By analyzing cusp form part of theta series corresponding to $Q(\vec{x})$, we prove a linear relation between the number of integer solutions to the equation $Q(\vec{x}) = p$ under the congruence condition $x_i \equiv 1 \pmod{3}$ and the number of $\mathbb{F}_p$-rational points on the elliptic curve $y^2=x^3+1$ for primes $p \equiv 1 \pmod{6}$.