f(f(n+1))-f(f(n)), where f(0)=0, and for m>0, f(m) = sigma(m) = A000203(m).

Terms

a(0) =1a(1) =3a(2) =3a(3) =1a(4) =4a(5) =16a(6) =-13a(7) =9a(8) =-10a(9) =25a(10) =-11a(11) =28a(12) =-32a(13) =36a(14) =0a(15) =-28a(16) =7a(17) =17a(18) =-14a(19) =54a(20) =-33a(21) =28a(22) =-31a(23) =108a(24) =-136a(25) =64a(26) =-6a(27) =30a(28) =-48a(29) =123