(n^4 - 10*n^2 + 15*n - 6)/2.

Terms

a(0) =0a(1) =0a(2) =15a(3) =75a(4) =222a(5) =510a(6) =1005a(7) =1785a(8) =2940a(9) =4572a(10) =6795a(11) =9735a(12) =13530a(13) =18330a(14) =24297a(15) =31605a(16) =40440a(17) =51000a(18) =63495a(19) =78147a(20) =95190a(21) =114870a(22) =137445a(23) =163185a(24) =192372a(25) =225300a(26) =262275a(27) =303615a(28) =349650a(29) =400722