theories.group_theory
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Group theory (especially finite group theory).

* [subgroup](subgroup.lean) : general subgroup theories, quotient group using quot
* [finsubg](finsubg.lean) : finite subgroups (finset and fintype), Lagrange theorem, finite cosets and lcoset_type, normalizer for finite groups, coset product and quotient group based on lcoset_type, semidirect product
* [hom](hom.lean) : homomorphism and isomorphism, kernel, first isomorphism theorem
* [perm](perm.lean) : permutation group
* [cyclic](cyclic.lean) : cyclic subgroup, finite generator, order of generator, sequence and rotation
* [action](action.lean)	: fixed point, action, stabilizer, orbit stabilizer theorem, orbit partition, Cayley theorem, action on lcoset, cardinality of permutation group
* [pgroup](pgroup.lean)	: subgroup with order of prime power, Cauchy theorem, first Sylow theorem