The algebraic uniqueness of the addition of natural numbers.
The ideal (a) is not generated
We prove that the ideal (a) defined by the density topology is not generated. This answers a question of Z. Grande and E. Strońska.
The maximal regular ideal of the semigroup of binary relations
The structure of an idempotent relation.
Theorems on sets not belonging to algebras.
Three problems of S. M. Ulam with solutions and generalizations
Transitive closures of binary relations. I.
Transitive closures of binary relations II.
Transitive closures of binary relations. III.
Transitive closures of binary relations. IV.
Transitive ternary relations and quasiorderings
Two constructions of compatible relations