a(n) = 3*binomial(n+1, 5).

Terms

a(0) =0a(1) =0a(2) =0a(3) =3a(4) =18a(5) =63a(6) =168a(7) =378a(8) =756a(9) =1386a(10) =2376a(11) =3861a(12) =6006a(13) =9009a(14) =13104a(15) =18564a(16) =25704a(17) =34884a(18) =46512a(19) =61047a(20) =79002a(21) =100947a(22) =127512a(23) =159390a(24) =197340a(25) =242190a(26) =294840a(27) =356265a(28) =427518a(29) =509733