a(n) = -6 + 2^(n+1)*(3 - 2*n + n^2).

Terms

a(0) =0a(1) =2a(2) =18a(3) =90a(4) =346a(5) =1146a(6) =3450a(7) =9722a(8) =26106a(9) =67578a(10) =169978a(11) =417786a(12) =1007610a(13) =2392058a(14) =5603322a(15) =12976122a(16) =29753338a(17) =67633146a(18) =152567802a(19) =341835770a(20) =761266170a(21) =1686110202a(22) =3716153338a(23) =8153726970a(24) =17817403386a(25) =38788923386