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

Terms

a(0) =6a(1) =55a(2) =447a(3) =3583a(4) =28671a(5) =229375a(6) =1835007a(7) =14680063a(8) =117440511a(9) =939524095a(10) =7516192767a(11) =60129542143a(12) =481036337151