a(n) = 6*8^n-1.

Terms

a(0) =5a(1) =47a(2) =383a(3) =3071a(4) =24575a(5) =196607a(6) =1572863a(7) =12582911a(8) =100663295a(9) =805306367a(10) =6442450943a(11) =51539607551a(12) =412316860415