a(n) = 9*binomial(n,4) = 3*n*(n-1)*(n-2)*(n-3)/8.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =9a(5) =45a(6) =135a(7) =315a(8) =630a(9) =1134a(10) =1890a(11) =2970a(12) =4455a(13) =6435a(14) =9009a(15) =12285a(16) =16380a(17) =21420a(18) =27540a(19) =34884a(20) =43605a(21) =53865a(22) =65835a(23) =79695a(24) =95634a(25) =113850a(26) =134550a(27) =157950a(28) =184275a(29) =213759