First order recursion: a(0)=1; a(n) = sigma(1,n) - a(n-1).

Terms

a(0) =1a(1) =0a(2) =3a(3) =1a(4) =6a(5) =0a(6) =12a(7) =-4a(8) =19a(9) =-6a(10) =24a(11) =-12a(12) =40a(13) =-26a(14) =50a(15) =-26a(16) =57a(17) =-39a(18) =78a(19) =-58a(20) =100a(21) =-68a(22) =104a(23) =-80a(24) =140a(25) =-109a(26) =151a(27) =-111a(28) =167a(29) =-137