Monday, 8 November 2010

nt.number theory - Are there pairs of consecutive integers with the same sum of factors?

Background/Motivation



I was planning to explain Ruth-Aaron pairs to my son, but it took me a few moments to remember the definition. Along the way, I thought of the mis-definition, a pair of consecutive numbers with the same sum of divisors. Well, that's actually two definitions, depending on whether you are looking only at proper divisors. Suppose all divisors. I quickly found (14,15) which both have a divisor sum (sigma function) of 24. Some more work provided (206,207) and then a search on OEIS gave sequence A002961.



What about proper divisors only? (2,3) comes quickly, but then nothing for a while. Noting that the parity of this value (sigma(n)n) is the same as that of n unless n is a square or twice a square, any solution pair must include one number of that form. With that much information in hand, I posted this problem at the reference desk on Wikipedia. User PrimeHunter determined that there were no solutions up to 1012, but there were no general responses.



Aside from the parity issue, I haven't found other individual constraints that would filter the candidates--the number of adjacent values identical modulo p for other small primes is at least as great as would be expected by chance, and there are a fair number of pairs that are arithmetically close.




Other than (2,3), are there pairs of consecutive integers such that sigma(n)n=sigma(n+1)(n+1)?


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