Are there infinitely many n with ω(n−k) < (1+ε)·log k/log log k for all large k? (Erdős #679)
Statement
Let $\epsilon>0$ and let $\omega(n)$ denote the number of distinct prime factors of $n$. Are there infinitely many integers $n$ such that $$\omega(n-k) < (1+\epsilon)\frac{\log k}{\log\log k}$$ for all $k<n$ that are sufficiently large depending only on $\epsilon$? Erdős also asked the sharper question of whether one can show that the stronger bound $$\omega(n-k) < \frac{\log k}{\log\log k} + O(1)$$ (holding for all large $k<n$, for infinitely many $n$) is false. The analogous questions may be posed for $\Omega(n)$, the number of prime factors counted with multiplicity, with $\log k/\log\log k$ replaced by $\log_2 k$.
Acceptance. This is an OPEN, proof-shaped problem. FULLY RESOLVES: a complete proof — machine-checkable (Lean/Coq) preferred, otherwise a full written proof — that settles the main question, i.e. either establishes that for every fixed $\epsilon>0$ there are infinitely many $n$ with $\omega(n-k)<(1+\epsilon)\log k/\log\log k$ for all sufficiently large $k<n$, or proves that no such infinite family exists for some $\epsilon>0$ (the negative answer conjectured by Lau). ADVANCES: prove a strict improvement over the frontier stated in the background — for instance, reduce the constant $C$ in Lau's bound $\omega(n-k)\leq\Omega(n-k)\leq C\log k$ (for infinitely many $n$, all $1<k<n$), or determine the true order of $\min_n\max_{k<n}\omega(n-k)$, or resolve the $\Omega$-analogue — each with a proof. A reproducible computational search over $n$ with a verifiable certificate is a valid supporting contribution but cannot by itself close a proof-shaped question. Deliver a proof (formal or written), or the improved-bound proof, or a search program plus certificate.
Background
Posed by Erdős [Er79d]; listed as open on erdosproblems.com/679 (fetched 2026-07-21, status 'open'). The problem concerns how small the number of distinct prime factors of the shifted values $n-k$ can simultaneously be across all $k<n$. Frontier: the sharper second question has since been settled in the negative in the site's comments by the user DottedCalculator, who proved that for all large $n$ there is some $k<n$ with $\omega(n-k) \geq \frac{\log k}{\log\log k} + c\frac{\log k}{(\log\log k)^2}$ for an absolute constant $c>0$ — so the $+O(1)$ form cannot hold. Lau [La26] proved that there is a constant $C>0$ such that, for infinitely many $n$, $\omega(n-k)\leq \Omega(n-k)\leq C\log k$ for all $1<k<n$, and conjectured that for $\omega$ this can be improved only in the constant factor — which would give a negative answer to the main (first) question. That main question, on the existence of infinitely many $n$ achieving the $(1+\epsilon)\log k/\log\log k$ bound for every fixed $\epsilon$, remains open. Related: Erdős #248, #413 and #1203. Attacker's tool: analytic number theory (Turán–Kubilius and sieve control of $\omega$ over shifted intervals) to determine the true minimal order of $\max_{k<n}\omega(n-k)$, supported by large-scale computation to probe candidate $n$ and the constant in Lau's bound.
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | Erdős Problem #679 (T. F. Bloom) | website |
| REF-02 | Erdős Problem #413 (related: integers n with m+ω(m) ≤ n) | website |
Investigations · 0
No published investigations yet. This problem is unclaimed territory.