[1] Z. HEDRLÍN A. PULTR, V. TRNKOVÁ:
Concerning a categorial approach to topological and algebraic theories. Proceedings of the Second Prague Topological Symposium 1966, Praha 1967, 176-181.
MR 0230784
[2] Z. HEDRLÍN A. PULTR:
On full embeddings of categories of algebras. Illinois J. of Math. 10 (1966), 392-406.
MR 0191858
[3] P. VOPĚNKA A. PULTR Z. HEDRLÍN:
A rigid relation exists on any set. Comment. Math. Univ. Carolinae, vol. 6 (1965), 149-155.
MR 0183647