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

Terms

a(0) =1a(1) =-1a(2) =6a(3) =-8a(4) =17a(5) =-27a(6) =70a(7) =-136a(8) =255a(9) =-491a(10) =1046a(11) =-2082a(12) =4063a(13) =-8131a(14) =16476a(15) =-32882a(16) =65423a(17) =-130845a(18) =262372a(19) =-524818a(20) =1048149a(21) =-2096045a(22) =4195412a(23) =-8390820a(24) =16775029a(25) =-33550477a(26) =67113210a(27) =-134225588a(28) =268427597a(29) =-536854983