Abstract
Consider a compact zero dimensional (profinite) monoid. While the group of units must be open, a regular D-class need not be open in the ideal it generates. This is the case if and only if the semigroup contains infinitely many copies of a certain semilattice composed of an increasing sequence of idempotents converging to an upper bound.Using compactifications of free products, two generator compact monoids with these properties are constructed.

This publication has 2 references indexed in Scilit: