On square values of the product of the Euler totient and sum of divisors functions
Kevin Broughan, Kevin Ford, Florian Luca (2013)
Colloquium Mathematicae
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If n is a positive integer such that ϕ(n)σ(n) = m² for some positive integer m, then m ≤ n. We put m = n-a and we study the positive integers a arising in this way.