a(n) = 7*9^n - 1.

Terms

a(0) =6a(1) =62a(2) =566a(3) =5102a(4) =45926a(5) =413342a(6) =3720086a(7) =33480782a(8) =301327046a(9) =2711943422a(10) =24407490806a(11) =219667417262