a(0) = 1 and a(n) = (3^n - (-1)^n)/2 for n >= 1.

Terms

a(0) =1a(1) =2a(2) =4a(3) =14a(4) =40a(5) =122a(6) =364a(7) =1094a(8) =3280a(9) =9842a(10) =29524a(11) =88574a(12) =265720a(13) =797162a(14) =2391484a(15) =7174454a(16) =21523360a(17) =64570082a(18) =193710244a(19) =581130734a(20) =1743392200a(21) =5230176602a(22) =15690529804a(23) =47071589414a(24) =141214768240a(25) =423644304722