a(n) = 9*4^n - 1.

Terms

a(0) =8a(1) =35a(2) =143a(3) =575a(4) =2303a(5) =9215a(6) =36863a(7) =147455a(8) =589823a(9) =2359295a(10) =9437183a(11) =37748735a(12) =150994943a(13) =603979775a(14) =2415919103a(15) =9663676415a(16) =38654705663a(17) =154618822655a(18) =618475290623