a(n) = Sum_{k=1..n} (-3)^(floor(n/k) - 1).

Terms

a(0) =1a(1) =-2a(2) =11a(3) =-28a(4) =81a(5) =-234a(6) =739a(7) =-2216a(8) =6545a(9) =-19594a(10) =59139a(11) =-177408a(12) =531181a(13) =-1593614a(14) =4783799a(15) =-14351032a(16) =43044597a(17) =-129133854a(18) =387426799a(19) =-1162281332a(20) =3486765521a(21) =-10460293354a(22) =31381119459a(23) =-94143358440a(24) =282429356977a(25) =-847288080362