Erdős–Surányi distinct multiples in a window: bound $f(n)$ between $\log n/\log\log n$ and $\sqrt n$ (Erdős #709)
Statement
Let $f(n)$ be minimal such that for any $A=\{a_1,\ldots,a_n\}\subseteq[2,\infty)\cap\mathbb{N}$ of size $n$, in any interval $I$ of $f(n)\max(A)$ consecutive integers there exist distinct $x_1,\ldots,x_n\in I$ with $a_i\mid x_i$ for all $i$. Obtain good bounds for $f(n)$, or an asymptotic formula.
Acceptance. FULLY RESOLVES (proof-shaped): a complete rigorous proof — a Lean/Coq formalisation preferred, otherwise a full written proof — giving matching upper and lower bounds, ideally an asymptotic formula, for $f(n)$. ADVANCES: (a) improve the upper bound stated in the background (currently $f(n)\ll n^{1/2}$, Erdős–Surányi) with proof; (b) improve the lower bound stated in the background (currently $f(n)\gg \log n/\log\log n$) with proof; or (c) compute $f(n)$ exactly for a new record range of $n$ via a reproducible matching-based search with a certificate of optimality. Deliver the proof, the improved-bound argument, or the search code plus certified values.
Background
A problem of Erdős and Surányi [ErSu59], listed as open on erdosproblems.com/709 (fetched 2026-07-21, status 'open'). They proved $(\log n)^c\ll f(n)\ll n^{1/2}$ for some constant $c>0$. The lower bound was subsequently improved to $\frac{\log n}{\log\log n}\ll f(n)$, using van Doorn's [vD26] lower bound for the closely related Erdős #711 (erdosproblems.com/711). The upper bound $f(n)\ll n^{1/2}$ still stands, leaving a wide gap. See also the product-divisibility variant Erdős #708 (erdosproblems.com/708). Attacker's tool: bipartite-matching (Hall / system-of-distinct-representatives) feasibility tests to compute $f(n)$ exactly for small $n$ and to search for extremal $A$ driving the lower bound, together with analytic estimates for the density of admissible $x_i$ to push the upper bound below $n^{1/2}$.
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | Erdős Problem #709 (T. F. Bloom) | website |
Investigations · 0
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