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For questions on divisors and multiples, mainly but not exclusively of integers, and related and derived notions such as sums of divisors, perfect numbers and so on.
6
votes
Accepted
Sum of divisor function over arithmetic progression
In the context of the "divisor problem for arithmetical progressions",
there is the following article by Fouvry, Iwaniec and Katz, on "The divisor function over arithmetic progressions": http://matwbn …
4
votes
Accepted
Finding primes using Euler's sum of divisors recurrence relation
Perhaps a summary of the comments is useful. Using Euler's formula for $\sigma(n)$ to test $n$ for primality
certainly is not efficient. The values $\sigma(n-1),\sigma(n-2),\sigma(n-5),\cdots $ are ha …