Reidemeister–Schreier theorem
= Reidemeister–Schreier theorem
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Given a presentation of a group and a Schreier transversal for a subgroup, the Reidemeister–Schreier theorem gives a presentation of the subgroup. Its generators are the Schreier generators $ts\overline{ts}^{-1}$, and its relators are rewritten conjugates of the original relators.
= Schreier lemma
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