Construction of a complementary quasiorder

Abstract

For a monounary algebra A = (A, f) we study the lattice Quord A of all quasiorders of A, i.e., of all reflexive and transitive relations compatible with f. Monounary algebras (A, f) whose lattices of quasiorders are complemented were characterized in 2011 as follows: (∗) f(x) is a cyclic element for all x ∈ A, and all cycles have the same square-free number n of elements. Sufficiency of the condition (∗) was proved by means of transfinite induction. Now we will describe a construction of a complement to a given quasiorder of (A, f) satisfying (∗).