Least prime factor spikes: is $p(n+k)>k^2+1$ solvable for every large $n$? (Erdős #680)
Statement
Let $p(m)$ denote the least prime factor of $m$. Is it true that for all sufficiently large $n$ there exists some $k$ such that $$p(n+k)>k^2+1?$$ Can one further prove that this fails if $k^2+1$ is replaced by $e^{(1+\epsilon)\sqrt{k}}+C_\epsilon$ for every $\epsilon>0$, where $C_\epsilon>0$ is a suitable constant? The first part asks that near every large $n$ one can find an index $k$ whose shifted integer $n+k$ has an unusually large least prime factor; the second asks how far this phenomenon can be pushed before it must break down.
Acceptance. FULLY RESOLVES: a complete unconditional proof — a Lean/Coq formalisation matching the DeepMind formal-conjectures statement for #680 is preferred, otherwise a full written proof — that for all large $n$ some $k$ satisfies $p(n+k)>k^2+1$; and/or a proof of the second part, showing the $e^{(1+\epsilon)\sqrt{k}}+C_\epsilon$ version fails for every $\epsilon>0$. ADVANCES: prove the first statement conditionally under a precisely stated prime-distribution hypothesis (an explicit form of prime $k$-tuples / short-interval conjectures, improving on the informal 'plausible assumptions'); 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 $k$, supplying the search program and the attained $N$ strictly beyond any bound recorded here. Deliver the proof or the verification code with its attained $N$.
Background
Posed by Erdős [Er79d]; listed as open on erdosproblems.com/680 (fetched 2026-07-21, status 'open'), with a machine-formalised statement in the Google DeepMind formal-conjectures project. The first statement follows from plausible assumptions on the distribution of primes (as does the variant with $k^2$ replaced by $k^d$ for any fixed $d$); the challenge is an unconditional proof. Erdős observed that Cramér's conjecture $\limsup_{k\to\infty}(p_{k+1}-p_k)/(\log k)^2=1$ implies that for every $\epsilon>0$ and all large $n$ there is a $k$ with $p(n+k)>e^{(1-\epsilon)\sqrt{k}}$. There is now evidence that Cramér's conjecture is inaccurate: Granville's refined heuristic [Gr95] suggests the $\limsup$ is instead $2e^{-\gamma}\approx 1.119\cdots$, so the $1+\epsilon$ in the second question should perhaps read $2e^{-\gamma}+\epsilon$. This problem is grouped with erdosproblems.com/681 (the closely related variant using $k^2$ with $n+k$ required composite) and erdosproblems.com/682. Attacker's tool: for each fixed $n$ the required $k$ is bounded (since $k^2+1<p(n+k)\le n+k$ forces $k\lesssim\sqrt{n}$), so the first statement is a finite check per $n$ — verify it holds for every $n$ up to a record bound $N$ — while the conditional and refined parts draw on prime-gap heuristics (Cramér, Granville).
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | Erdős Problem #680 (T. F. Bloom) | website |
| REF-02 | Lean formalisation of Erdős #680 (DeepMind formal-conjectures) | website |
Investigations · 0
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