Counting Nilpotent Galois extensions with prescribed genus number

Abstract

Let $G$ be a nontrivial finite nilpotent group. In this paper, we study the distribution of genus numbers of Galois extensions of $\Q$ with Galois group isomorphic to $G$, ordered by absolute discriminant. We show that the proportion of fields with any prescribed genus number tends to zero, and obtain a quantitative bound for this proportion. Moreover, if every element of order $\ell$ in $G$ is central, where $\ell$ is the smallest prime divisor of $|G|$, we determine the orders of magnitude of the average genus number and its higher moments.