Number triangle (3n)!/(3k)!.

Terms

a(0) =1a(1) =6a(2) =1a(3) =720a(4) =120a(5) =1a(6) =362880a(7) =60480a(8) =504a(9) =1a(10) =479001600a(11) =79833600a(12) =665280a(13) =1320a(14) =1a(16) =217945728000a(17) =1816214400a(18) =3603600a(19) =2730a(20) =1a(24) =17643225600a(25) =13366080a(26) =4896a(27) =1