  The two-dimensional inhomogeneous zeta-function series (with homogeneous part
of the most general Epstein type): \[ \sum_{m,n \in \mbox{\bf Z}}
(am^2+bmn+cn^2+q)^{-s}, \] is analytically continued in the variable $s$ by
using zeta-function techniques. A simple formula is obtained, which extends the
Chowla-Selberg formula to inhomogeneous Epstein zeta-functions. The new
expression is then applied to solve the problem of computing the determinant of
the basic differential operator that appears in an attempt at quantizing
gravity by using the Wheeler-De Witt equation in 2+1 dimensional spacetime with
the torus topology.
