a(n) = 2*7^n-3^n.

Terms

a(0) =1a(1) =11a(2) =89a(3) =659a(4) =4721a(5) =33371a(6) =234569a(7) =1644899a(8) =11523041a(9) =80687531a(10) =564891449a(11) =3954476339a(12) =27682042961a(13) =193776426491