Directly decomposable tolerances on direct products of lattices and semilattices Ivan Chajda, Bohdan Zelinka (1983) Czechoslovak Mathematical Journal
Does 𝐒𝐏 K ⊇ 𝐏𝐒 K imply axiom of choice? Hajnal Andréka, István Németi (1980) Commentationes Mathematicae Universitatis Carolinae