Vector-Valued Period Polynomials and Zeta Values of Quadratic Fields

TL;DR

This paper derives explicit formulas for vector-valued period polynomials associated with quadratic forms, connecting them to zeta values of quadratic fields and providing new evaluations and divisor-sum formulas for these zeta values.

Contribution

It provides a closed-form expression for vector-valued period polynomials linked to quadratic forms, revealing their algebraic and zeta components, and evaluates differences of zeta values in terms of Bernoulli numbers.

Findings

Explicit formula for vector-valued period polynomial separating algebraic and zeta parts.

Evaluation of zeta value differences at odd weights using Bernoulli numbers.

Finite divisor-sum formula for Dedekind zeta values under certain conditions.

Abstract

Let $k\ge 2$ and $N\ge 1$ be integers. Let $D$ be a positive integer that is congruent to a square modulo $4N$, and fix $\rho$ with $\rho^2\equiv D\pmod{4N}$. In this paper, we consider two weight $2k$ cusp forms $f^{\pm}_{k,N,D,\rho}$ on $\Gamma_0(N)$ defined by sums over binary quadratic forms, and investigate the vector-valued period polynomial arising from these forms. Our first main result gives a closed formula for this vector-valued period polynomial. The identity component of this formula is particularly explicit: it separates as the sum of a finite \textit{algebraic part} coming from some binary forms and a \textit{zeta part} involving the values at $s=k$ of certain zeta functions. Using this formula together with a symmetry of vector-valued period polynomials, we explicitly evaluate, for odd $k$, the difference between the zeta values corresponding to the two choices of square root of $D$ modulo $4N$, in terms of Bernoulli numbers and a finite quadratic-form sum. Finally, under a vanishing condition on Fricke-invariant cusp forms at lower levels, we obtain a finite divisor-sum formula for the Dedekind zeta values $\zeta_{\mathbb{Q}(\sqrt{D})}(k)$ at even integers $k$.