Triangle read by rows: T(n,k) is the number of 2-stack sortable n-permutations with k runs.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =4a(5) =1a(6) =1a(7) =10a(8) =10a(9) =1a(10) =1a(11) =20a(12) =49a(13) =20a(14) =1a(15) =1a(16) =35a(17) =168a(18) =168a(19) =35a(20) =1a(21) =1a(22) =56a(23) =462a(24) =900a(25) =462a(26) =56a(27) =1a(28) =1a(29) =84