a(n) = Sum_{k=1..n} (-1)^(n-k)*binomial(n,k)*sigma(k).

Terms

a(0) =1a(1) =1a(2) =-2a(3) =5a(4) =-14a(5) =40a(6) =-111a(7) =293a(8) =-731a(9) =1726a(10) =-3882a(11) =8408a(12) =-17796a(13) =37423a(14) =-79337a(15) =170917a(16) =-373812a(17) =823585a(18) =-1809844a(19) =3934750a(20) =-8424747a(21) =17749392a(22) =-36874749a(23) =75862016a(24) =-155359339a(25) =318331170a(26) =-655146929a(27) =1356952623a(28) =-2828151136a(29) =5920984735