Is there a composite $n+k$ with least prime factor $p(n+k)>k^2$ for all large $n$? (Erdős #681)
Statement
Let $p(m)$ denote the least prime factor of $m$. Is it true that for all large $n$ there exists $k$ such that $n+k$ is composite and $$p(n+k)>k^2?$$ That is, near every large $n$ one can find an index $k$ for which the shifted integer $n+k$ is composite yet has least prime factor exceeding $k^2$ (the compositeness requirement rules out the trivial case where $n+k$ is itself prime and $p(n+k)=n+k$).
Acceptance. FULLY RESOLVES: a complete proof — a Lean/Coq formalisation matching the DeepMind formal-conjectures statement for #681 is preferred, otherwise a full written proof — that for all large $n$ there is a $k$ with $n+k$ composite and $p(n+k)>k^2$, or a proof that this fails. ADVANCES: prove the statement under a precisely stated prime-distribution hypothesis; or prove the generalisation with $k^2$ replaced by $k^d$ for a specific $d>1$ (conditional or unconditional, clearly flagged); or computationally verify that every $n$ up to a record $N$ admits such a composite $n+k$, supplying the search program and the attained $N$ strictly beyond any bound recorded here, and/or extend OEIS A389680. Deliver the proof or the verification code with its attained $N$.
Background
Posed by Erdős [Er79d]; listed as open on erdosproblems.com/681 (fetched 2026-07-21, status 'open'; page last edited 01 February 2026), with a machine-formalised statement in the Google DeepMind formal-conjectures project. The problem is related to questions of Erdős, Eggleton, and Selfridge, and is expected to remain true with $k^2$ replaced by $k^d$ for any fixed $d$. It is the composite-restricted companion of erdosproblems.com/680 (which uses $k^2+1$ without requiring $n+k$ composite and adds a heuristic upper-bound question) and is also grouped with erdosproblems.com/682; the related integers are tabulated in OEIS A389680. Attacker's tool: since a composite $n+k$ has $p(n+k)\le\sqrt{n+k}$, the condition $p(n+k)>k^2$ forces $k^2<\sqrt{n+k}$, i.e. $k\lesssim n^{1/4}$, so for each $n$ the search over $k$ is finite — verify the statement holds for every $n$ up to a record bound $N$ and extend OEIS A389680, while the unconditional proof needs analytic input on least prime factors in short intervals.
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | Erdős Problem #681 (T. F. Bloom) | website |
| REF-02 | Lean formalisation of Erdős #681 (DeepMind formal-conjectures) | website |
| REF-03 | OEIS A389680 — related to Erdős #681 | website |
Investigations · 0
No published investigations yet. This problem is unclaimed territory.