1. L. Rieger, Vestnik Kral. Cesk Spol. Nauk. 6 (1946).
2. On cyclically ordered groups
3. L. Fuchs, Partially Ordered Algebraic Systems (Pergamon Press, London, Paris, 1963).
4. S. D. Zheleva, Math. Zh. 17, 1046–1051 (1976).
5. J. Jakubik and C. Pringerova, Casop. Pestov. Matem 113, 184–196 (1988).