Triangle, see Mathematica code.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =1a(6) =1a(7) =6a(8) =6a(9) =1a(10) =1a(11) =30a(12) =90a(13) =30a(14) =1a(15) =1a(16) =120a(17) =1800a(18) =1800a(19) =120a(20) =1a(21) =1a(22) =520a(23) =31200a(24) =156000a(25) =31200a(26) =520a(27) =1a(28) =1a(29) =2184