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

Terms

a(0) =0a(1) =1a(2) =4a(3) =13a(4) =40a(5) =121a(6) =364a(7) =1093a(8) =3280a(9) =9841a(10) =29524a(11) =88573a(12) =265720a(13) =797161a(14) =2391484a(15) =7174453a(16) =21523360a(17) =64570081a(18) =193710244a(19) =581130733a(20) =1743392200a(21) =5230176601a(22) =15690529804a(23) =47071589413a(24) =141214768240a(25) =423644304721