a(n) = Sum_{k=0..n} k^p*q^k, where p=1, q=-2.

Terms

a(0) =0a(1) =-2a(2) =6a(3) =-18a(4) =46a(5) =-114a(6) =270a(7) =-626a(8) =1422a(9) =-3186a(10) =7054a(11) =-15474a(12) =33678a(13) =-72818a(14) =156558a(15) =-334962a(16) =713614a(17) =-1514610a(18) =3203982a(19) =-6757490a(20) =14214030a(21) =-29826162a(22) =62448526a(23) =-130489458a(24) =272163726a(25) =-566697074a(26) =1178133390a(27) =-2445745266a(28) =5070447502a(29) =-10498808946