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

Terms

a(0) =3a(1) =3a(2) =9a(3) =55a(4) =513a(5) =6251a(6) =93313a(7) =1647087a(8) =33554433a(9) =774840979a(10) =20000000001a(11) =570623341223