On inverse semigroups the closure of whose set of idempotents is a Clifford semigroup. B. Billhardt — 1992 Semigroup forum
On a wreath product embedding and idempotent pure congruences on inverse semigroups. B. Billhardt — 1992 Semigroup forum
On a semigroup theoretic generalization of the Kaluznin Krasner Theorem and normal extensions of inverse semigroups. B. Billhardt — 1992 Semigroup forum