2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/77451We classify groups generated by powers of 2 Dehn twists which are 1) free or 2) have no ``unexpected'' reducible elements. We give some sufficient conditions in the case of groups generated by powers of more than two twists.16 pagesGeometric Topology20F38text