a(n) = 9*5^n-1.

Terms

a(0) =8a(1) =44a(2) =224a(3) =1124a(4) =5624a(5) =28124a(6) =140624a(7) =703124a(8) =3515624a(9) =17578124a(10) =87890624a(11) =439453124a(12) =2197265624a(13) =10986328124a(14) =54931640624a(15) =274658203124