A255279(2^n-1).

Terms

a(0) =1a(1) =7a(2) =25a(3) =107a(4) =413a(5) =1615a(6) =6241a(7) =24227a(8) =93973a(9) =364743a(10) =1415289a(11) =5491643a(12) =21307533a(13) =82673535a(14) =320773713a(15) =1244606995a(16) =4829092037a(17) =18736941431a(18) =72699560681a(19) =282075165611