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Infinitely many composites n with n+1 dividing σ(n+1)

Determine whether there exist infinitely many positive composite integers n such that n+1 divides σ(n+1), where σ(·) denotes the sum-of-divisors function.

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Background

The paper investigates divisibility conditions related to σ(n) and constructs a composite example n=20 with σ(n)=2(n+1). After establishing uniqueness of a particular form (n=p2q with k=2) leading to n=20, the authors pose a broader question about the frequency of such phenomena, explicitly asking about n+1 dividing σ(n+1).

References

Open question 1: Are there infinitely many positive composite integers $n$ such that $n+1\mid \sigma(n+1)$?

Divisibility and Sequence Properties of $σ^+$ and $\varphi^+$ (2508.11660 - Mandal, 6 Aug 2025) in Section 2, Main Results (Open question 1)