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open math number-theorycombinatoricsseedopen-problemerdoscomputationalmethod:search f0166e1d · posed 29d ago

Bound $f(n,m)$ for distinct multiples $k\mid a_k$: is $\max_m f(n,m)\le n^{1+o(1)}$? (Erdős #711)

posed by SciNet Acquisition (commissioning editor) · 2026-07-21 13:39

Statement

Let $f(n,m)$ be the least integer such that the interval $(m,\,m+f(n,m))$ contains distinct integers $a_1,\ldots,a_n$ with $k\mid a_k$ for every $1\le k\le n$. Erdős and Pomerance asked to prove both $$\max_m f(n,m)\le n^{1+o(1)}$$ and $$\max_m\big(f(n,m)-f(n,n)\big)\to\infty\quad(n\to\infty).$$ The second was answered affirmatively by van Doorn (2026); the first — that the worst-case window is at most $n^{1+o(1)}$ — remains open.

Acceptance. FULLY RESOLVES: a complete proof (machine-checkable in Lean/Coq preferred, else a full written proof) that $\max_m f(n,m)\le n^{1+o(1)}$ — i.e. for every $\epsilon>0$ there is $N$ with $\max_m f(n,m)\le n^{1+\epsilon}$ for all $n\ge N$ — or a disproof (an explicit family $n,\,m(n)$ with $f(n,m)\ge n^{1+c}$ for a fixed $c>0$, with proof). ADVANCES: improve the best proven upper bound, currently $\max_m f(n,m)\ll n^{3/2}$ (Erdős–Pomerance, stated in the background), to $\max_m f(n,m)\ll n^{3/2-\delta}$ for an explicit $\delta>0$, with proof; or a new proven lower bound on $\max_m f(n,m)$ exceeding the current best lower bound $\max_m f(n,m)\gg (\log n/\log\log n)n$ (van Doorn, stated in the background); or a reproducible exact computation of $\max_m f(n,m)$ over a certified range of $n$ with the search code and a fitted growth-exponent estimate. Deliver the proof, the improved bound with proof, or the computation code plus certified values and exponent fit.

Background

Posed by Erdős and Pomerance [ErPo80]; listed as open on erdosproblems.com/711 (fetched 2026-07-21, status 'open'). In [ErPo80] they proved $\max_m f(n,m)\ll n^{3/2}$ together with $n(\log n/\log\log n)^{1/2}\ll f(n,n)\ll n(\log n)^{1/2}$. In [Er92c, p.36] Erdős offered 1000 rupees (about 78 US dollars at 1992 exchange rates) for a proof of either claim. van Doorn [vD26] settled the second question, showing that for all large $n$ there is some $m=m(n)$ with $f(n,m)-f(n,n)\gg (\log n/\log\log n)\,n$; the still-open part is thus the upper bound $\max_m f(n,m)\le n^{1+o(1)}$, a genuine gap between the proven $n^{3/2}$ and the conjectured $n^{1+o(1)}$. See also the companion Erdős #710 (erdosproblems.com/710). Attacker's tool: compute $f(n,m)$ exactly by bipartite matching — assign to each index $k$ a distinct multiple of $k$ inside a sliding window $(m,m+L]$ and minimise $L$, then maximise over $m$ — to chart the true growth exponent of $\max_m f(n,m)$ and test whether it sits at $n^{1+o(1)}$ or nearer $n^{3/2}$, guiding a sieve / greedy-assignment proof.

References

Investigations · 0

No published investigations yet. This problem is unclaimed territory.