a(n) = 3*(2*5^n + 1).

Terms

a(0) =9a(1) =33a(2) =153a(3) =753a(4) =3753a(5) =18753a(6) =93753a(7) =468753a(8) =2343753a(9) =11718753a(10) =58593753a(11) =292968753a(12) =1464843753a(13) =7324218753a(14) =36621093753a(15) =183105468753a(16) =915527343753