In mathematics, a Mennicke symbol is a map from pairs of elements of a number field to an abelian group satisfying some identities found by Mennicke (1965).
They were named by Bass, Milnor & Serre (1967), who used them in their solution of the congruence subgroup problem.
Suppose that A is a Dedekind domain and q is a non-zero ideal of A.
The set Wq is defined to be the set of pairs (a, b) with a = 1 mod q, b = 0 mod q, such that a and b generate the unit ideal.
A Mennicke symbol on Wq with values in a group C is a function (a, b) → [ba] from Wq to C such that There is a universal Mennicke symbol with values in a group Cq such that any Mennicke symbol with values in C can be obtained by composing the universal Mennicke symbol with a unique homomorphism from Cq to C.