a(n) = (3^n + 1)^2/4.

Terms

a(0) =1a(1) =4a(2) =25a(3) =196a(4) =1681a(5) =14884a(6) =133225a(7) =1196836a(8) =10764961a(9) =96864964a(10) =871725625a(11) =7845353476a(12) =70607649841a(13) =635467254244