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

Terms

a(0) =4a(1) =16a(2) =58a(3) =196a(4) =634a(5) =1996a(6) =6178a(7) =18916a(8) =57514a(9) =174076a(10) =525298a(11) =1582036a(12) =4758394a(13) =14299756a(14) =42948418a(15) =128943556a(16) =387027274a(17) =1161475036a(18) =3485211538a(19) =10457207476a(20) =31374768154a(21) =94130595916a(22) =282404370658a(23) =847238277796