Improved first moments of primitive quartic and sextic Dirichlet $L$-functions

Abstract

We study the smoothed first moments of primitive quartic and sextic Dirichlet $L$-functions for which every nontrivial power of the character remains primitive. Assuming the generalized Riemann hypothesis for the Dirichlet and finite order Hecke $L$-functions occurring in the proof, we obtain asymptotic formulas with errors $O(Q^{\frac{49}{58}+\eps})$ and $O(Q^{\frac{23}{26}+\eps})$, respectively. We follow the double Dirichlet series approach of Gao and Zhao. The main ingredient is a joint estimate for the two sums arising from the removal of rational prime divisors, with the Möbius coefficients retained. This gives a quotient of Dirichlet $L$-functions. Under GRH, we bound both the Dirichlet $L$-functions and their reciprocals. Combining the joint estimate with estimates for the generating series of Gauss sums gives larger regions of meromorphic continuation. These regions do not separate the possible secondary terms.