a(n) = 11^n + 3^n - 1.

Terms

a(0) =1a(1) =13a(2) =129a(3) =1357a(4) =14721a(5) =161293a(6) =1772289a(7) =19489357a(8) =214365441a(9) =2357967373a(10) =25937483649a(11) =285311847757