59.3 CoveringGroup

CoveringGroup(chr)

chr must be a cohomology-record, created by a call of CHR(G,p,F,[mats]), where F is a finitely presented group. CoveringGroup calculates a presentation of a covering extension of Mul_p by G, where Mul_p is the p-part of the Schur multiplier Mul of G. The set of generators of the finitely presented group that is returned is a union of two sets, which are in one-one correspondence with the generators of F and of Mul_p, respectively.

The relators fall into three classes:

:

a) Those that specify the orders of the generators of Mul_p;
b) Those that say that the generators of Mul_p are central; and
c) Those that give the values of the relators of F as elements of Mul_p.

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