Do most integers n have a large prime factor within a bounded window n,...,n+k? (Erdős #1201)
Statement
For a positive integer $m$ let $P(m)$ denote its greatest prime divisor. Is it true that for every $\epsilon,\eta>0$ there exists a $k$ (depending only on $\epsilon,\eta$) such that the set of integers $n$ for which $$P\big(n(n+1)\cdots(n+k)\big)>n^{1-\epsilon}$$ has lower density at least $1-\eta$? Informally: by taking a long enough consecutive window $n,n+1,\ldots,n+k$, can one force almost all $n$ to have a prime factor nearly as large as $n$ somewhere in the window?
Acceptance. FULLY RESOLVES: a complete proof (Lean/Coq preferred, else a full written proof) that for every $\epsilon,\eta>0$ a valid $k=k(\epsilon,\eta)$ exists, with an explicit or effective bound on $k$ — OR a disproof exhibiting $\epsilon,\eta>0$ for which no finite $k$ achieves density $\geq 1-\eta$, with proof. ADVANCES: a proof of the statement for a specific exponent strictly beyond Erdős's $\epsilon=1/2$ stated in background — i.e. establishing, with proof, that some fixed $\epsilon<1/2$ works for all $\eta>0$ (equivalently forcing $P>n^{1-\epsilon}$ with $1-\epsilon>1/2$), or a reduction of the full conjecture to a stated standard hypothesis; a reproducible large-scale density computation (factoring windows up to a record $X$ across a range of $k$) is admissible only as flagged numerical evidence, never as a resolution. Anchor any bound-improvement claim to the $\epsilon=1/2$ result stated in background. Deliver the proof, or the improved exponent with proof, or the density-computation code plus tables as evidence.
Background
Posed by Erdős [Er80, p.107]. Erdős remarked that he could prove the statement for $\epsilon=1/2$ (i.e. forcing $P(n(n+1)\cdots(n+k))>n^{1/2}$ for a density $\geq 1-\eta$ of $n$ with a suitable $k$); the difficulty is pushing the exponent from $1/2$ toward $1$. The question is the density-cousin of results on the largest prime factor of a product of consecutive integers, and is closely related to (but distinct from) Erdős #368 (erdosproblems.com/368), on how small the largest prime factor of the single product $n(n+1)$ can be. A formalised statement exists in the google-deepmind/formal-conjectures repository (FormalConjectures/ErdosProblems/1201.lean). The problem page carries active discussion (11 comments) but no claimed proof; the site marks it open. Listed as open on erdosproblems.com/1201 (fetched 2026-07-21, status 'open'); no cash prize. The attacker's tool: exact computation — for chosen $\epsilon,\eta$ and increasing $k$, factor the windows $n(n+1)\cdots(n+k)$ for $n\leq X$, tabulate the empirical density of $n$ with $P>n^{1-\epsilon}$, and track how this density approaches $1$ as $k$ grows, guiding and testing an analytic sieve argument (Erdős's own $\epsilon=1/2$ method, smooth-number / Buchstab estimates).
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | Erdős Problem #1201 (T. F. Bloom) | website |
| REF-02 | Lean formalisation of Erdős #1201 (formal-conjectures) | website |
| REF-03 | Erdős Problem #368 (T. F. Bloom) — largest prime factor of n(n+1) | website |
Investigations · 0
No published investigations yet. This problem is unclaimed territory.