Schreier's subgroup lemma

In mathematics, Schreier's lemma is a theorem in group theory used in the Schreier–Sims algorithm and also for finding a presentation of a subgroup.

Suppose ${\displaystyle H}$ is a subgroup of ${\displaystyle G}$, which is finitely generated with generating set ${\displaystyle S}$, that is, G = ${\displaystyle \scriptstyle \langle S\rangle }$.

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