a(n) = Sum_{k=0..n} k^p*q^k, where p=3, q=-2.

Terms

a(0) =0a(1) =-2a(2) =30a(3) =-186a(4) =838a(5) =-3162a(6) =10662a(7) =-33242a(8) =97830a(9) =-275418a(10) =748582a(11) =-1977306a(12) =5100582a(13) =-12897242a(14) =32060454a(15) =-78531546a(16) =189903910a(17) =-454052826a(18) =1074770982a(19) =-2521320410a(20) =5867287590a(21) =-13554437082