20-00 General reference works (handbooks, dictionaries, bibliographies, etc.)
20-01 Instructional exposition (textbooks, tutorial papers, etc.)
20-02 Research exposition (monographs, survey articles)
20-03 Historical (must also be assigned at least one classification number from Section 01)
20-04 Explicit machine computation and programs (not the theory of computation or programming)
20-06 Proceedings, conferences, collections, etc.
20Axx Foundations
20Bxx Permutation groups
20Cxx Representation theory of groups [See also 19A22 (for representation rings and Burnside rings)]
20Dxx Abstract finite groups
20Exx Structure and classification of infinite or finite groups
20Fxx Special aspects of infinite or finite groups
20Gxx Linear algebraic groups (classical groups) {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
20L05 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
20P05 Probabilistic methods in group theory [See also 60Bxx]