Über das Radikal in potenzverbundenen Halbgruppen
Über die Funktionalgleichung f(f(x) * f(y)) = f(x*y).
Über die Funktionalgleichung f(f(x)*f(y)) = f(x*y). (Short Communication).
Über die kleinste separative Kongruenz auf einer Halbgruppe
Union and symmetry preserving endomorphisms of the semigroup of all binary relations on a set
Union and symmetry preserving homomorphisms of semigroups of closed relations.
Union preserving homomorphisms of semigroups of closed relations.