a(n) = (3^n - 7)/2.

Terms

a(0) =1a(1) =10a(2) =37a(3) =118a(4) =361a(5) =1090a(6) =3277a(7) =9838a(8) =29521a(9) =88570a(10) =265717a(11) =797158a(12) =2391481a(13) =7174450a(14) =21523357a(15) =64570078a(16) =193710241a(17) =581130730a(18) =1743392197a(19) =5230176598a(20) =15690529801a(21) =47071589410a(22) =141214768237a(23) =423644304718