Michael Friedberg (1971)

Mathematische Zeitschrift

J. Shafer (1974)

Semigroup forum

K.D. jr. Magill, P.R. Misra (1993)

Semigroup forum

D.B. McAlister (1971)

Semigroup forum

K.E. Osondu (1980)

Semigroup forum

Volker Diekert, Axel Möbus (1989)

RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications

D.A. Bredikhin (1996)

Semigroup forum

P. Goralčík, A. Goralčíková, V. Koubek (1986)

RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications

Bruno Bosbach (1989)

Czechoslovak Mathematical Journal

K. Denecke, J. Koppitz (1994)

Semigroup forum

Walter Taylor (1981)

Aequationes mathematicae

George M. Bergman (1981)

Aequationes mathematicae

George M. Bergman (1981)

Aequationes mathematicae

Maksa, Gyula, Páles, Zsolt (2001)

Acta Mathematica Academiae Paedagogicae Nyí regyháziensis. New Series [electronic only]