[1] P. GORALČÍK: Problem. Coll. Math. Soc. J. Bolyai 17, Contribution to universal algebra, North Holland, 1977, p. 604,
[2] B. JÓNSS0N:
Topics in universal algebra. Springer-Verlag, Berlin - Heidelberg - New York 1972.
MR 0345895
[3] P. P. PÁLFY:
On certain congruence lattices of finite unary algebras. Comment. Math. Univ. Carolinae 19 (1978), 88-95.
MR 0472656