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

Terms

a(0) =1a(1) =5a(2) =41a(3) =365a(4) =3281a(5) =29525a(6) =265721a(7) =2391485a(8) =21523361a(9) =193710245a(10) =1743392201a(11) =15690529805a(12) =141214768241