a(n) = 8*Sum_{k=0..n} 7^k.

Terms

a(0) =8a(1) =64a(2) =456a(3) =3200a(4) =22408a(5) =156864a(6) =1098056a(7) =7686400a(8) =53804808a(9) =376633664a(10) =2636435656a(11) =18455049600a(12) =129185347208a(13) =904297430464