a(n) = 8*6^n - 1.

Terms

a(0) =7a(1) =47a(2) =287a(3) =1727a(4) =10367a(5) =62207a(6) =373247a(7) =2239487a(8) =13436927a(9) =80621567a(10) =483729407a(11) =2902376447a(12) =17414258687a(13) =104485552127a(14) =626913312767