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

Terms

a(0) =0a(1) =1a(2) =8a(3) =57a(4) =400a(5) =2801a(6) =19608a(7) =137257a(8) =960800a(9) =6725601a(10) =47079208a(11) =329554457a(12) =2306881200a(13) =16148168401a(14) =113037178808a(15) =791260251657