Mennicke symbol

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 W_q 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 W_q with values in a group C is a function (a, b) → [b
a] from W_q to C such that

• [0
  1] = 1, [bc
  a] = [b
  a][c
  a]
• [b
  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 C_q such that any Mennicke symbol with values in C can be obtained by composing the universal Mennicke symbol with a unique homomorphism from C_q to C.

References

• Bass, Hyman; Milnor, John Willard; Serre, Jean-Pierre (1967), "Solution of the congruence subgroup problem for SL_n (n ≥ 3) and Sp_2n (n ≥ 2)", Publications Mathématiques de l'IHÉS (33): 59–137, ISSN 1618-1913, MR0244257, http://www.numdam.org/item?id=PMIHES_1967__33__59_0  Erratum
• Mennicke, Jens L. (1965), "Finite factor groups of the unimodular group", Annals of Mathematics. Second Series 81: 31–37, ISSN 0003-486X, JSTOR 1970380, MR0171856