a(n) = (7^(n+1) - (-6)^(n+1))/13.

Terms

a(0) =1a(1) =1a(2) =43a(3) =85a(4) =1891a(5) =5461a(6) =84883a(7) =314245a(8) =3879331a(9) =17077621a(10) =180009523a(11) =897269605a(12) =8457669571a(13) =46142992981a(14) =401365114963