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

Terms

a(0) =1a(1) =-1a(2) =9a(3) =-20a(4) =62a(5) =-174a(6) =556a(7) =-1660a(8) =4911a(9) =-14693a(10) =44357a(11) =-133053a(12) =398389a(13) =-1195207a(14) =3587853a(15) =-10763270a(16) =32283452a(17) =-96850386a(18) =290570104a(19) =-871710994a(20) =2615074146a(21) =-7845220010a(22) =23535839600a(23) =-70607518824a(24) =211822017739a(25) =-635466060265