An explicit type number formula for quaternion orders of level $(N_1,N_2)$

Abstract

We give an explicit type number formula for the quaternion orders of level $(N_1,N_2)$ specified in this paper, where $N_1=p_1^{2u_1+1}\cdots p_w^{2u_w+1}$, the primes $p_i$ are distinct, $u_i\ge 0$, $w$ is odd, and $\gcd (N_1,N_2)=1$. The formula includes Eichler orders and the orders with odd prime-power ramified level considered by Boyd, and allows nonmaximal local orders at several ramified primes, including $2$. We express the answer in terms of a generalized modified Hurwitz class number and explicit local correction factors. The proof combines the correspondence between quaternion orders and ternary quadratic forms with the Siegel--Weil formula and local representation densities. We tabulate class and type numbers for $N_1{N}_2\le 100$. As an application, a mass bound and a finite exact calculation determine the $27$ pairs in this family with type number one.

arXiv commentarXiv admin note: text overlap with arXiv:2402.17443