Triangle read by rows: T(n,k) = binomial(3*n-k,n-k).

Terms

a(0) =1a(1) =3a(2) =1a(3) =15a(4) =5a(5) =1a(6) =84a(7) =28a(8) =7a(9) =1a(10) =495a(11) =165a(12) =45a(13) =9a(14) =1a(15) =3003a(16) =1001a(17) =286a(18) =66a(19) =11a(20) =1a(21) =18564a(22) =6188a(23) =1820a(24) =455a(25) =91a(26) =13a(27) =1a(28) =116280a(29) =38760