a(n) = 102*2^n - 78.

Terms

a(0) =24a(1) =126a(2) =330a(3) =738a(4) =1554a(5) =3186a(6) =6450a(7) =12978a(8) =26034a(9) =52146a(10) =104370a(11) =208818a(12) =417714a(13) =835506a(14) =1671090a(15) =3342258a(16) =6684594a(17) =13369266a(18) =26738610a(19) =53477298a(20) =106954674a(21) =213909426a(22) =427818930a(23) =855637938a(24) =1711275954a(25) =3422551986a(26) =6845104050a(27) =13690208178a(28) =27380416434a(29) =54760832946