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

Terms

a(0) =0a(1) =1a(2) =5a(3) =19a(4) =65a(5) =211a(6) =665a(7) =2059a(8) =6305a(9) =19171a(10) =58025a(11) =175099a(12) =527345a(13) =1586131a(14) =4766585a(15) =14316139a(16) =42981185a(17) =129009091a(18) =387158345a(19) =1161737179a(20) =3485735825a(21) =10458256051a(22) =31376865305a(23) =94134790219a(24) =282412759265a(25) =847255055011