132-avoiding Two-stack Sortable Permutations, Fibonacci Numbers, and Pell Numbers

Eric S. Egge, Toufik Mansour

In 1990 West conjectured that there are $2(3n)!/((n+1)!(2n+1)!)$ two-stack sortable permutations on $n$ letters. This conjecture was proved analytically by Zeilberger in 1992. Late…