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

Terms

a(0) =1a(1) =4a(2) =9a(3) =20a(4) =37a(5) =76a(6) =141a(7) =280a(8) =541a(9) =1072a(10) =2097a(11) =4192a(12) =8289a(13) =16548a(14) =32953a(15) =65860a(16) =131397a(17) =262764a(18) =524909a(19) =1049736a(20) =2098381a(21) =4196560a(22) =8390865a(23) =16781696a(24) =33558929a(25) =67117460a(26) =134226585a(27) =268452580a(28) =536888037a(29) =1073775900