In mathematics, a Mennicke symbol is a map from pairs of elements of a number field to an abelian group satifying some identities found by Menncike (1965). They were named by Bass, Milnor & Serre (1967), who used them in their solution of the congruence subgroup problem.
Definition
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) → [b
a] from Wq to C such that
1] = 1, [bc
a] = [b
a][c
a]
a] = [b + ta
a] if t is in q, [b
a] = [b
a + tb] if t is in A.
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.