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

Terms

a(0) =1a(1) =5a(2) =29a(3) =185a(4) =1241a(5) =8525a(6) =59189a(7) =412865a(8) =2885681a(9) =20186645a(10) =141267149a(11) =988751945a(12) =6920909321a(13) =48445302365a(14) =339113927909