> Is A031363 the set of all numbers n such that sigma_2(n) is not
> divisible by 5? If not, where is the first difference?
11^4.
The first few differences are multiples of p^4,
where p is in {11, 19, 29, 31, 41}.
Those might be the primes == 1 or 4 mod 5 (almost A038872).
--
Don Reble djr at nk.ca