20L05 Groupoids (i.e. small categories in which all morphisms are isomorphisms) {For sets with a single binary operation, see <a href="#code:20N02">20N02</a>; for topological groupoids, see <a href="#code:22A22">22A22</a>, <a href="#code:58H05">58H05</a>}

20L99 None of the above, but in this section

20M05 Free semigroups, generators and relations, word problems [See also <a href="#code:03D40">03D40</a>, <a href="#code:08A50">08A50</a>, <a href="#code:20F10">20F10</a>]

20M07 Varieties and pseudovarieties of semigroups

20M20 Semigroups of transformations, etc. [See also <a href="#code:47D03">47D03</a>, <a href="#code:47H20">47H20</a>, <a href="#code:54H15">54H15</a>]

20M25 Semigroup rings, multiplicative semigroups of rings [See also <a href="#code:16S36">16S36</a>, <a href="#code:16Y60">16Y60</a>]

20M30 Representation of semigroups; actions of semigroups on sets

20M35 Semigroups in automata theory, linguistics, etc. [See also <a href="#code:03D05">03D05</a>, <a href="#code:68Q70">68Q70</a>, <a href="#code:68T50">68T50</a>]

20M50 Connections of semigroups with homological algebra and category theory

20M99 None of the above, but in this section

20N02 Sets with a single binary operation (groupoids)

20N05 Loops, quasigroups [See also <a href="#code:05Bxx">05Bxx</a>]

20N10 Ternary systems (heaps, semiheaps, heapoids, etc.)

20N25 Fuzzy groups [See also <a href="#code:03E72">03E72</a>]

20N99 None of the above, but in this section

20Pxx Probabilistic methods in group theory [See also <a href="#code:60Bxx">60Bxx</a>]

20P05 Probabilistic methods in group theory  [See also <a href="#code:60Bxx">60Bxx</a>]

20P99 None of the above, but in this section

22-XX Topological groups, Lie groups {For transformation groups, see <a href="#code:54H15">54H15</a>, <a href="#code:57Sxx">57Sxx</a>, <a href="#code:58-XX">58-XX</a>. For abstract harmonic analysis, see <a href="#code:43-XX">43-XX</a>}

22-00 General reference works (handbooks, dictionaries, bibliographies, etc.)

22-01 Instructional exposition (textbooks, tutorial papers, etc.)