a(n) = -(n - 4)*(n - 5)*(n - 12)/6.

Terms

a(0) =2a(1) =5a(2) =8a(3) =10a(4) =10a(5) =7a(6) =0a(7) =-12a(8) =-30a(9) =-55a(10) =-88a(11) =-130a(12) =-182a(13) =-245a(14) =-320a(15) =-408a(16) =-510a(17) =-627a(18) =-760a(19) =-910a(20) =-1078a(21) =-1265a(22) =-1472a(23) =-1700a(24) =-1950a(25) =-2223a(26) =-2520a(27) =-2842a(28) =-3190a(29) =-3565