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arXiv · 0906.3380

Common values of the arithmetic functions phi and sigma

Kevin Ford·Florian Luca·Carl Pomerance
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Abstract

We show that the equation phi(a)=σ(b) has infinitely many solutions, where phi is Euler's totient function and sigma is the sum-of-divisors function. This proves a 50-year old conjecture of Erdos. Moreover, we show that there are infinitely many integers n such that phi(a)=n and sigma(b)=n each have more than n^c solutions, for some c>0. The proofs rely on the recent work of the first two authors and Konyagin on the distribution of primes p for which a given prime divides some iterate of phi at p, and on a result of Heath-Brown connecting the possible existence of Siegel zeros with the distribution of twin primes.

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Kevin Ford, Florian Luca, Carl Pomerance. 2010-10-26. Common values of the arithmetic functions phi and sigma. https://doi.org/10.1112/blms%2Fbdq014

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