  In 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.

Combinatorics