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

Terms

a(0) =1a(1) =5a(2) =49a(3) =245a(4) =2401a(5) =12005a(6) =117649a(7) =588245a(8) =5764801a(9) =28824005a(10) =282475249a(11) =1412376245a(12) =13841287201a(13) =69206436005a(14) =678223072849