2019 February 12

A semigroup is a set with an associative operation. A subsemigroup is a subset which is a semigroup with the same operation. σ(S), the *covering number* of a semigroup S (with
respect to subsemigroups), is the minimum number of proper subsemigroups whose union is S. I will give a complete description of the covering numbers of finite semigroups.

To the Combinatorics Seminar Web page.