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

Terms

a(0) =1a(1) =3a(2) =9a(3) =21a(4) =57a(5) =121a(6) =305a(7) =657a(8) =1505a(9) =3233a(10) =7489a(11) =15169a(12) =34433a(13) =72833a(14) =156929a(15) =325889a(16) =717313a(17) =1462785a(18) =3187713a(19) =6538241a(20) =13846529a(21) =28739585a(22) =61673473a(23) =124063745a(24) =263856129a(25) =544481281a(26) =1139032065a(27) =2326282241a(28) =4920999937a(29) =9924804609