Triangle read by rows: T(n, k) = binomial(3*n+1-k, n-k) for n, k >= 0.

Terms

a(0) =1a(1) =4a(2) =1a(3) =21a(4) =6a(5) =1a(6) =120a(7) =36a(8) =8a(9) =1a(10) =715a(11) =220a(12) =55a(13) =10a(14) =1a(15) =4368a(16) =1365a(17) =364a(18) =78a(19) =12a(20) =1a(21) =27132a(22) =8568a(23) =2380a(24) =560a(25) =105a(26) =14a(27) =1a(28) =170544a(29) =54264