2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/65052In this thesis, we apply the stack sorting operator to $r$-permutations and construct the functional equation for the generating function of two-stack-sortable $k$-tuple $r$-permutations counted by descents by using a factorization similar to Zeilberger's. We solve the functional equation and give explicit formulas for the number of two-stack-sortable $r$-permutations.PH.D Thesis, 57 pagesCombinatoricstext