Array read by antidiagonals: T(r,n) = number of two-stack sortable r-permutations.

Terms

a(0) =1a(1) =1a(2) =2a(3) =1a(4) =3a(5) =6a(6) =1a(7) =4a(8) =15a(9) =22a(10) =1a(11) =5a(12) =28a(13) =95a(14) =91a(15) =1a(16) =6a(17) =45a(18) =252a(19) =690a(20) =408a(21) =1a(22) =7a(23) =66a(24) =525a(25) =2618a(26) =5481a(27) =1938a(28) =1a(29) =8