20Exx Structure and classification of infinite or finite groups

20Fxx Special aspects of infinite or finite groups

20Gxx Linear algebraic groups and related topics (For arithmetic theory, see 11E57, 11H56; for geometric theory, see 14Lxx, 22Exx; for other methods in representation theory, see 15A30, 22E45, 22E46, 22E47, 22E50, 22E55)

20Hxx Other groups of matrices [See also 15A30]

20Jxx Connections with homological algebra and category theory

20Kxx Abelian groups

20Lxx Groupoids (i.e. small categories in which all morphisms are isomorphisms) (For sets with a single binary operation, see 20N02; for topological groupoids, see 22A22, 58H05)

20Mxx Semigroups

20Nxx Other generalizations of groups

20Pxx Probabilistic methods in group theory [See also 60Bxx]