Nielsen–Schreier theorem (source code)

= Nielsen–Schreier theorem
{wiki=Nielsen–Schreier_theorem}

The Nielsen–Schreier theorem is a result in group theory that provides a characterization of free groups in terms of their subgroups. The theorem states that every subgroup of a free group is free. More specifically, if \\( F \\) is a free group, then any subgroup \\( H \\) of \\( F \\) is itself a free group, possibly on a different set of generators.