a(1) = 1; a(n) = -Sum_{d|n, d < n} d * a(d).

Terms

a(0) =1a(1) =-1a(2) =-1a(3) =1a(4) =-1a(5) =4a(6) =-1a(7) =-3a(8) =2a(9) =6a(10) =-1a(11) =-24a(12) =-1a(13) =8a(14) =7a(15) =21a(16) =-1a(17) =-38a(18) =-1a(19) =-58a(20) =9a(21) =12a(22) =-1a(23) =288a(24) =4a(25) =14a(26) =-16a(27) =-108a(28) =-1a(29) =-180