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Abstract

We compute the first moment of cubic Hecke $L$-functions over $\mathbb{Q}(\sqrt{-3})$ evaluated at any $s$ inside the critical strip. The first moment for $s<\frac{1}{2}$ is particularly interesting, and we show there is a phase transition at $s=\frac{1}{3}$. This extends the analogue result of David-Meisner for the first moment over function fields. As in their work, the computation of the moment at $s=\frac{1}{3}$ relies on a cancellation between two terms which are a priori not related: a main term of the principal sum which comes from cubes, and the contribution from infinitely many residues of Dirichlet series of cubic Gauss sums to the dual sum. The cancellation also improves the error term and exhibits a secondary term for all $s$. In particular, at $s=\frac{1}{2}$, we prove the existence of a secondary term of size $Q^{5/6}$, where the size of the family is $Q$. We conjecture that a similar behaviour would hold for higher order Hecke $L$-functions attached to $\ell^{th}$ order residue symbols, refining a function field conjecture of David and Meisner. The proof follows the steps of writing $L(s,\chi)$ as two finite sums with the approximate functional equation. Two main ingredients are then exploited: the bound on the second moment of $L(1/2+it,\chi)$ that follows from Heath-Brown's cubic large sieve, and the deep work of Kubota and Patterson which connects the Dirichlet series of cubic Gauss sums to metaplectic forms, and gives a formula for its residues in terms of the Fourier coefficients of metaplectic theta functions. This is only known for cubic Gauss sums, and not for general Gauss sums of order $\ell\geq 4$.