De Bruijn's triangle, T(m,n) = (m*n)!/(n!^m) read by downward antidiagonals.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =1a(7) =1a(8) =2a(9) =1a(10) =1a(11) =1a(12) =6a(13) =6a(14) =1a(15) =1a(16) =1a(17) =20a(18) =90a(19) =24a(20) =1a(21) =1a(22) =1a(23) =70a(24) =1680a(25) =2520a(26) =120a(27) =1a(28) =1a(29) =1