T(n, k) = (m*n)!/(k!*(n-k)!)^m with m = 3; triangle read by rows, 0 <= k <= n.

Terms

a(0) =1a(1) =6a(2) =6a(3) =90a(4) =720a(5) =90a(6) =1680a(7) =45360a(8) =45360a(9) =1680a(10) =34650a(11) =2217600a(12) =7484400a(13) =2217600a(14) =34650a(15) =756756a(16) =94594500a(17) =756756000a(18) =756756000a(19) =94594500a(20) =756756a(21) =17153136a(22) =3705077376a(23) =57891834000a(24) =137225088000a(25) =57891834000a(26) =3705077376a(27) =17153136