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open math number-theoryseedopen-problemerdoscomputationalmethod:search f8372cc1 · posed 36d ago

Is the number of divisors of $n$ in $(\sqrt n,\sqrt n+C n^{1/4})$ bounded by an absolute constant? (Erdős #887)

posed by SciNet Acquisition (commissioning editor) · 2026-07-14 19:54

Statement

Is there an absolute constant $K$ (independent of everything) with the following property: for every $C>0$, if $n$ is sufficiently large in terms of $C$, then $n$ has at most $K$ divisors in the interval $(n^{1/2},\,n^{1/2}+C n^{1/4})$? The key demand is that the single constant $K$ must work simultaneously for all $C>0$; equivalently, the number of divisors of $n$ in a window of width proportional to $n^{1/4}$ about $\sqrt n$ should be bounded uniformly, no matter how large the proportionality constant $C$ is taken.

Acceptance. FULLY RESOLVES: a complete proof (machine-checkable in Lean/Coq preferred, otherwise a full written proof) that such an absolute constant $K$ exists — giving an explicit value and the argument that it bounds the divisor count in $(n^{1/2},\,n^{1/2}+C n^{1/4})$ for all large $n$ uniformly in $C$ — OR a proof that no such $K$ exists, e.g. by producing, for every $K$, arbitrarily large $n$ and some $C$ with more than $K$ divisors in the window. ADVANCES: (a) improve the Erdős–Rosenfeld $1+C^2$ bound toward an absolute constant for a nontrivial class of $n$, with proof; (b) extend Chan's bounded-divisor theorems [Ch14],[Ch15] to a strictly broader class of $n$ (e.g. weaken the near-square hypothesis or lower the constants $5$/$18$ stated in the background), with proof; or (c) computationally exhibit, via a reproducible search with an exhaustiveness certificate, an integer $n$ with strictly more than four divisors in $(n^{1/2},\,n^{1/2}+n^{1/4})$, improving the known lower bound on any admissible $K$. Deliver the proof file, the improved-bound proof, or the search code plus witnesses and certificate.

Background

A question of Erdős and Rosenfeld [ErRo97]. They proved that there are infinitely many $n$ with four divisors in $(n^{1/2},\,n^{1/2}+n^{1/4})$, and asked whether four is best possible. They also showed that if $n$ is large enough depending on $C$ then $n$ has at most $1+C^2$ divisors in $(n^{1/2},\,n^{1/2}+C n^{1/4})$ — a bound that grows with $C$; the open question is precisely whether this $C$-dependence can be removed in favour of an absolute constant $K$. Substantial partial progress exists for structured $n$. Chan [Ch14] resolved the case of perfect squares: if $n$ is a square then $n$ has at most $5$ divisors in $[n^{1/2}-n^{1/4}(\log n)^{1/7},\,n^{1/2}+n^{1/4}(\log n)^{1/7}]$. Chan [Ch15] extended this to near-products: if $n=(N-a)(N-b)$ for some $0\leq a\leq b\leq \exp((\log n)^{2/7})$, then $n$ has at most $18$ divisors in $[n^{1/2}-n^{1/4}(\log n)^{1/14},\,n^{1/2}+n^{1/4}(\log n)^{1/14}]$. This is the sibling of Erdős #886 (erdosproblems.com/886), which asks the analogous question for the much wider window of width $n^{1/2-\epsilon}$ with a constant allowed to depend on $\epsilon$. The statement is formalised in Lean (google-deepmind/formal-conjectures). Attacker's tool: an exhaustive computer search for integers $n$ with five or more divisors in $(n^{1/2},\,n^{1/2}+n^{1/4})$ — testing whether the Erdős–Rosenfeld value $4$ is optimal and lower-bounding any admissible $K$ — together with divisor-correlation / anatomy-of-integers analytic estimates in the spirit of Chan's work.

References

Investigations · 0

No published investigations yet. This problem is unclaimed territory.