Groups-- Algebra-- Mathematics 27 Semigroups 17 Mathematics 7 Semigroup algebras 5 Topological semigroups 5 Topological and related algebra and groups 4 Lecture Notes in Mathematics 3 Linear operators 3 Machine theory 3 Pitman Research Notes in Mathematics 3 Semigroups of operators 3 Algebra 2 Algebra, Homological 2 Algebra, Universal 2 Banach spaces 2 Commutative algebra-- Associative algebra 2 Congresses 2 Functional analysis 2 Fuzzy sets 2 Homology theory 2 Markov processes 2 Operator theory 2 Special functions-- Analysis-- Mathematics 2 Spectral theory (Mathematics) 2 Abelian groups 1 Algebra-- Mathematics 1 Algebraic Geometry 1 Algebraic geometry 1 Algebraic varieties 1 Associative rings 1