Triangle read by rows: T(n,k) = (3*n - 2*k)!/((n-k)!^3*k!).

Terms

a(0) =1a(1) =6a(2) =1a(3) =90a(4) =24a(5) =1a(6) =1680a(7) =630a(8) =60a(9) =1a(10) =34650a(11) =16800a(12) =2520a(13) =120a(14) =1a(15) =756756a(16) =450450a(17) =92400a(18) =7560a(19) =210a(20) =1a(21) =17153136a(22) =12108096a(23) =3153150a(24) =369600a(25) =18900a(26) =336a(27) =1a(28) =399072960a(29) =325909584