a(n) = (4*7^n - 1)/3.

Terms

a(0) =1a(1) =9a(2) =65a(3) =457a(4) =3201a(5) =22409a(6) =156865a(7) =1098057a(8) =7686401a(9) =53804809a(10) =376633665a(11) =2636435657a(12) =18455049601a(13) =129185347209a(14) =904297430465