[1] G. Grätzer J. Płonka, A. Sekanina: On the number of polynomials of a universal algebra I. Colloquium Mathematicum XXII (1970), 9-11. MR 0294216

[2] E. Marczewski: Remaгk on Symmetrical and Quasi-symmetгical Operations. Bulletin de ľАcademie Polonaise des Sciences, Serie des sc. math., astr. et phys., XII (1964), p. 735-737. MR 0175831

[3] J. Płonka: On the number of independent elements in finite abstract algebras having a binary operation. Coloquium Mathematicum, XIV (1966), p. 189-201. MR 0191861

[4] J. Płonka: On the number of independent elements in finite abstract algebras with two binary symmetrical operations. Colloquium Mathematicum, XIX (1968), p. 9-21. MR 0223290

[5] J. Płonka: Diagonal algebгas. Fundamenta Mathematicae, 58 (1966), p. 309-322.

[6] J. Płonka,: On algebras with n distinct essentially $n$-aгy operations. Аlgebra Universalis, I (1971), p. 73-79.

[7] K. Urbanik: On algebraic opeгations in idempotent algebras. Colloquium Mathematicum, XIII (1965), p. 129-157. MR 0179119