2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/164423Let $A_2$ be a free associative or polynomial algebra of rank two over a field $K$ of characteristic zero. Based on the degree estimate of Makar-Limanov and J.-T.Yu, we prove: 1) An element $p \in A_2$ is a test element if $p$ does not belong to any proper retract of $A_2$; 2) Every endomorphism preserving the automorphic orbit of a nonconstant element of $A_2$ is an automorphism.11 pagesRings and Algebras16S10, 16W20 (Primary) 13B10, 13F20 (Secondary)text