a(n) = (7*8^n + (-8)^n)/8.

Terms

a(0) =1a(1) =6a(2) =64a(3) =384a(4) =4096a(5) =24576a(6) =262144a(7) =1572864a(8) =16777216a(9) =100663296a(10) =1073741824a(11) =6442450944a(12) =68719476736a(13) =412316860416