a(n) = 4*9^n-1.

Terms

a(0) =3a(1) =35a(2) =323a(3) =2915a(4) =26243a(5) =236195a(6) =2125763a(7) =19131875a(8) =172186883a(9) =1549681955a(10) =13947137603a(11) =125524238435