a(n) = 6^n/3 - 1.

Terms

a(0) =1a(1) =11a(2) =71a(3) =431a(4) =2591a(5) =15551a(6) =93311a(7) =559871a(8) =3359231a(9) =20155391a(10) =120932351a(11) =725594111a(12) =4353564671a(13) =26121388031a(14) =156728328191a(15) =940369969151