a(n) = 2*6^n + 1.

Terms

a(0) =3a(1) =13a(2) =73a(3) =433a(4) =2593a(5) =15553a(6) =93313a(7) =559873a(8) =3359233a(9) =20155393a(10) =120932353a(11) =725594113a(12) =4353564673a(13) =26121388033a(14) =156728328193a(15) =940369969153