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

Terms

a(0) =2a(1) =7a(2) =22a(3) =69a(4) =216a(5) =671a(6) =2066a(7) =6313a(8) =19180a(9) =58035a(10) =175110a(11) =527357a(12) =1586144a(13) =4766599a(14) =14316154a(15) =42981201a(16) =129009108a(17) =387158363a(18) =1161737198a(19) =3485735845a(20) =10458256072a(21) =31376865327a(22) =94134790242a(23) =282412759289a(24) =847255055036