20Mxx Semigroups

20M05  Free semigroups, generators and relations, word problems
20M07  Varieties of semigroups
20M10  General structure theory
20M11  Radical theory
20M12  Ideal theory
20M14  Commutative semigroups
20M15  Mappings of semigroups
20M17  Regular semigroups
20M18  Inverse semigroups
20M19  Orthodox semigroups
20M20  Semigroups of transformations, etc., See also {47D03,
               47H20, 54H15}
20M25  Semigroup rings, multiplicative semigroups of rings, See
               also {16S36, 16Y60}
20M30  Representation of semigroups
20M35  Semigroups in automata theory, linguistics, etc., See Also {
               03D05, 68Qxx, 68S05}
20M50  Connections of semigroups with homological algebra and
               category theory
20M99  None of the above but in this section