Counting cosets of unimodular groups over Dedekind domains

Abstract

In this paper, a formula for the calculation of the number of right cosets contained in a double coset with respect to a unimodular group over a Dedekind domain is developed, and applications of this formula in the theory of congruence subgroups -- an index formula -- and the theory of abstract Hecke algebras -- a reduction theorem and an algorithm for the explicit calculation of products -- are given.