The Epstein zeta function for a matrix of a positive definite real quadratic form and a complex variable with (where denotes the real part) is defined by
(1)
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where the sum is over all column vectors with integer coordinates and the prime means the summation excludes the origin (Terras 1973). Epstein (1903) derived the analytic continuation, functional equation, and so-called Kronecker limit formula for this function.
Epstein (1903) defined this function in the course of an effort to find the most general possible function satisfying a functional equation similar to that satisfied by the Riemann zeta function (Glasser and Zucker 1980, p. 68).
A slightly different notation is used in theoretical chemistry, where the Epstein zeta function arises in connection with lattice sums. Let be a positive definite quadratic form
(2)
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where with , ... is a symmetric matrix. Then the Epstein zeta function can be defined as
(3)
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where and are arbitrary vectors, the sum runs over a -dimensional lattice, and is omitted if is a lattice vector (Glasser and Zucker 1980, p. 69).