a(n) = Sum_{k=1..n} 2^(floor(n/k) - 1).

Terms

a(0) =1a(1) =3a(2) =6a(3) =12a(4) =21a(5) =41a(6) =74a(7) =144a(8) =275a(9) =541a(10) =1054a(11) =2102a(12) =4151a(13) =8281a(14) =16484a(15) =32938a(16) =65707a(17) =131391a(18) =262464a(19) =524878a(20) =1049201a(21) =2098291a(22) =4195444a(23) =8390860a(24) =16779477a(25) =33558743a(26) =67113306a(27) =134226304a(28) =268444033a(29) =536887965