a(n) = (2*(-1)^n - 2^(n+1) + 3*n*2^n)/9.

Terms

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