Dice Question Streamline Icon: https://streamlinehq.com

Existence of a composite n ≠ 4 with φ⁺(n) dividing n−1

Determine whether there exists any positive composite integer n other than n=4 such that φ⁺(n) divides n−1, where φ⁺(n)=∏_{p\mid n}(φ(p^{v_p(n)})+1) and φ(·) is Euler’s totient function.

Information Square Streamline Icon: https://streamlinehq.com

Background

The authors prove that if n=pr with φ⁺(n) | (n−1), then necessarily n=4 (p=2,r=2). A computational check up to 105 found only n=4 satisfying φ⁺(n) | (n−1), motivating the explicit question whether any other composite n exists.

References

Open question 4: Is there any positive composite integer $n\neq 4$ such that $\varphi+(n)\mid (n-1)$?

Divisibility and Sequence Properties of $σ^+$ and $\varphi^+$ (2508.11660 - Mandal, 6 Aug 2025) in Section 2, Main Results (Open question 4)