a(n) = (7^(n+1) - 4) / 3.

Terms

a(0) =1a(1) =15a(2) =113a(3) =799a(4) =5601a(5) =39215a(6) =274513a(7) =1921599a(8) =13451201a(9) =94158415a(10) =659108913a(11) =4613762399a(12) =32296336801a(13) =226074357615