[1] K. Gł­azek J. Michalski: Weak homomorphisms of general algebras. to appear in Commentationes Mathematicae.

[2] J. Niederle L. Polák: The category of non-indexed algebras and weak homomorphisms. to appear in Colloquium Mathematicum.

[3] S. Fajtlowicz: Birkhoff theorem in the category of non-indexed algebras. Bulletin de l' Academie Polonaise des Sciences, Serie des sci. math., astr. et phys. 17 (1969), 273-275. MR 0248064