a(n) = 8*7^n-1.

Terms

a(0) =7a(1) =55a(2) =391a(3) =2743a(4) =19207a(5) =134455a(6) =941191a(7) =6588343a(8) =46118407a(9) =322828855a(10) =2259801991a(11) =15818613943a(12) =110730297607a(13) =775112083255