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

Terms

a(0) =4a(1) =9a(2) =25a(3) =75a(4) =229a(5) =699a(6) =2125a(7) =6435a(8) =19429a(9) =58539a(10) =176125a(11) =529395a(12) =1590229a(13) =4774779a(14) =14332525a(15) =43013955a(16) =129074629a(17) =387289419a(18) =1161999325a(19) =3486260115a(20) =10459304629a(21) =31378962459a(22) =94138984525a(23) =282421147875a(24) =847271832229