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

Terms

a(0) =1a(1) =0a(2) =5a(3) =-4a(4) =13a(5) =-16a(6) =49a(7) =-88a(8) =173a(9) =-324a(10) =701a(11) =-1384a(12) =2713a(13) =-5416a(14) =10989a(15) =-21916a(16) =43621a(17) =-87224a(18) =174921a(19) =-349872a(20) =698773a(21) =-1397356a(22) =2796949a(23) =-5593872a(24) =11183361a(25) =-22366976a(26) =44742149a(27) =-89483716a(28) =178951741a(29) =-357903312