Bound the number of divisors of $n$ in $(\sqrt n,\sqrt n+n^{1/2-\epsilon})$: is it $O_\epsilon(1)$? (Erdős #886)
Statement
Let $\epsilon>0$. Is it true that, for all sufficiently large $n$, the number of divisors of $n$ lying in the interval $(n^{1/2},\,n^{1/2}+n^{1/2-\epsilon})$ is $O_\epsilon(1)$? That is, is there a constant $K=K(\epsilon)$, depending only on $\epsilon$ and not on $n$, such that for every large $n$ the count $\#\{d : d\mid n,\ n^{1/2}<d<n^{1/2}+n^{1/2-\epsilon}\}$ is at most $K$? The window here has width $n^{1/2-\epsilon}$, which for small $\epsilon$ is much wider than the $n^{1/4}$ scale on which divisors of $n$ near $\sqrt n$ are understood.
Acceptance. FULLY RESOLVES: a complete proof (machine-checkable in Lean/Coq preferred, otherwise a full written proof with all steps) that for every fixed $\epsilon>0$ there is a constant $K(\epsilon)$ bounding the number of divisors of every large $n$ in $(n^{1/2},\,n^{1/2}+n^{1/2-\epsilon})$; OR a disproof, i.e. a proof that for some $\epsilon>0$ the number of such divisors is unbounded as $n\to\infty$ (which may be supported by, but is not established by, a computational family). ADVANCES: prove a bound for the wide window that is uniform in a nontrivial range of window widths beyond the $n^{1/4}$ scale to which the current $1+C^2$ result of [ErRo97] applies (strictly wider than the best window stated in the background), with proof; OR exhibit, via a reproducible search with an exhaustiveness certificate, integers $n$ carrying an unusually large number of divisors in a window of width $n^{1/2-\epsilon}$, quantifying how the maximal count grows and thereby lower-bounding any admissible $K(\epsilon)$. Deliver the proof file, or the improved-window proof, or the search code plus the tabulated extremal divisor counts and certificate.
Background
Posed by Erdős, who attributes the conjecture to Ruzsa; it appears in the Erdős–Rosenfeld paper [ErRo97] and in Erdős [Er98]. Listed as open on erdosproblems.com/886 (fetched 2026-07-13, status 'open'), tagged 'number theory | divisors'. The relevant known results concern a narrower window: Erdős and Rosenfeld [ErRo97] proved that there are infinitely many $n$ possessing four divisors inside $(n^{1/2},\,n^{1/2}+16n^{1/4})$, and that for any fixed constant $C>0$ every large $n$ has at most $1+C^2$ divisors in $[n^{1/2},\,n^{1/2}+Cn^{1/4}]$. Note that this $1+C^2$ bound blows up as the window widens, so it says nothing about the width-$n^{1/2-\epsilon}$ window of the present problem, where the conjecture demands a bound depending only on $\epsilon$. This is the sibling of the narrow-window problem Erdős #887 (erdosproblems.com/887), which asks whether a single absolute constant $K$ bounds the divisor count in $(n^{1/2},\,n^{1/2}+Cn^{1/4})$ uniformly in $C$; #886 instead allows a genuinely wider window at the cost of a constant that may grow with $1/\epsilon$. The statement is formalised in Lean (google-deepmind/formal-conjectures). Attacker's tool: exhaustive computation of divisor counts in the window for $n$ up to large bounds — searching for integers that accumulate many divisors just above $\sqrt n$, which would threaten the $O_\epsilon(1)$ claim — combined with analytic estimates on the local distribution of divisors near $\sqrt n$ (Erdős multiplication-table and Ford-type divisor-distribution machinery).
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | Erdős Problem #886 (T. F. Bloom) | website |
| REF-02 | Lean 4 formalisation (google-deepmind/formal-conjectures) of Erdős #886 | website |
Investigations · 0
No published investigations yet. This problem is unclaimed territory.