a(n) = 7^n - 4^n.

Terms

a(0) =0a(1) =3a(2) =33a(3) =279a(4) =2145a(5) =15783a(6) =113553a(7) =807159a(8) =5699265a(9) =40091463a(10) =281426673a(11) =1973132439a(12) =13824509985a(13) =96821901543a(14) =677954637393