Nonreversible trees having a removable edge


Nenad Morača




A relational structure is said to be reversible iff every bijective homomorphism (condensation) of that structure is an automorphism. In the case of a binary structure X = 〈X, ρ〉, that is equivalent to the following statement: whenever we remove finite or infinite number of edges from X, thus obtaining the structure X′, we have that X′ X. In this paper, we prove that if a nonreversible tree X = 〈X, ρ〉 has a removable edge (i.e. if there is 〈x, y〉 ∈ ρ such that 〈X, ρ〉 〈X, ρ {〈x, y〉}〉, then it has infinitely many removable edges. We also show that the same is not true for arbitrary binary structure by constructing nonreversible digraphs having exactly n removable edges, for n ∈N