Let $k = \mathbb{ F}_{ q}(T)$ be the rational function field over a finite field $\mathbb{ F}_{ q}$, where $q$ is a power of $2$. In this paper we solve the problem of averaging the quadratic $L$-functions $L(s, \chi_{ u})$ over fundamental discriminants. Any separable quadratic extension $K$ of $k$ is of the form $K = k(x_{ u})$, where $x_{ u}$ is a zero of $X^2+X+u=0$ for some $u\in k$. We characterize the family $\mathcal I$ (resp. $\mathcal F$, $\mathcal F'$) of rational functions $u\in k$ such that any separable quadratic extension $K$ of $k$ in which the infinite prime $\infty = (1/T)$ of $k$ ramifies (resp. splits, is inert) can be written as $K = k(x_{ u})$ with a unique $u\in\mathcal I$ (resp. $u\in\mathcal F$, $u\in\mathcal F'$). For almost all $s\in\mathbb C$ with ${ \rm Re}(s)\ge \frac{ 1}2$, we obtain the asymptotic formulas for the summation of $L(s,\chi_{ u})$ over all $k(x_{ u})$ with $u\in \mathcal I$, all $k(x_{ u})$ with $u\in \mathcal F$ or all $k(x_{ u})$ with $u\in \mathcal F'$ of given genus. As applications, we obtain the asymptotic mean value formulas of $L$-functions at $s=\frac{ 1}2$ and $s=1$ and the asymptotic mean value formulas of the class number $h_{ u}$ or the class number times regulator $h_{ u} R_{ u}$.
Categories: math.NT