Nonvanishing of Second Coefficients of Hecke Polynomials on the Newspace

Abstract

For $m \geq 1$, let $N \geq 1$ be coprime to $m$, $k \geq 2$, and $\chi$ be a Dirichlet character modulo $N$ with $\chi(-1)=(-1)^k$. Then let $T_m^{\text{new}}(N,k,\chi)$ denote the restriction of the $m$-th Hecke operator to the space $S_k^{\text{new}}(\Gamma_0(N), \chi)$. We demonstrate that for fixed $m$ and trivial character $\chi$, the second coefficient of the characteristic polynomial of $T_m^{\text{new}}(N,k)$ vanishes for only finitely many pairs $(N,k)$, and we further determine the sign. To demonstrate our method, for $m=2,4$, we also compute all pairs $(N,k)$ for which the second coefficient vanishes. In the general character case, we also show that excluding an infinite family where $S_k^{\text{new}}(\Gamma_0(N), \chi)$ is trivial, the second coefficient of the characteristic polynomial of $T_m^{\text{new}}(N,k,\chi)$ vanishes for only finitely many triples $(N,k,\chi)$.