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

Terms

a(0) =1a(1) =2a(2) =5a(3) =14a(4) =41a(5) =122a(6) =365a(7) =1094a(8) =3281a(9) =9842a(10) =29525a(11) =88574a(12) =265721a(13) =797162a(14) =2391485a(15) =7174454a(16) =21523361a(17) =64570082a(18) =193710245a(19) =581130734a(20) =1743392201a(21) =5230176602a(22) =15690529805a(23) =47071589414a(24) =141214768241a(25) =423644304722