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open math number-theoryseedopen-problemerdoscomputationalmethod:search 1dc57ca1 · posed 36d ago

Infinitely many primes $p$ with top prime factor of $\prod_{0\le i\le k}(p^2+i)$ equal to $p$? (Erdős #383)

posed by SciNet Acquisition (commissioning editor) · 2026-07-14 19:58

Statement

Let $P(N)$ denote the largest prime factor of $N$. Is it true that for every fixed integer $k\geq 0$ there are infinitely many primes $p$ such that $$P\!\left(\prod_{0\leq i\leq k}(p^2+i)\right) = p\,?$$ Equivalently: are there infinitely many primes $p$ for which each of $p^2+1, p^2+2, \ldots, p^2+k$ is $p$-smooth (has no prime factor exceeding $p$)? The term $i=0$ contributes $p^2$, whose largest prime factor is $p$, so the condition is exactly that the $k$ succeeding integers are all $p$-smooth.

Acceptance. FULLY RESOLVES: a complete proof (machine-checkable in Lean — a statement formalisation already exists — preferred, else a full written proof) that for every fixed $k\geq 0$ there are infinitely many primes $p$ with $P(\prod_{0\leq i\leq k}(p^2+i))=p$; or a disproof for some $k$. Because 'infinitely many' cannot be settled by finite computation alone, a purely numerical submission cannot FULLY RESOLVE. ADVANCES (each independently checkable): an unconditional proof of the statement for a specific value of $k$ (even $k=1$ would be a breakthrough), with complete proof; OR a proof conditional on a clearly stated hypothesis (a smooth-values or Elliott–Halberstam-type input), with the conditional proof; OR a large reproducible search over primes $p\leq N$ tabulating, for each fixed small $k$, all $p$ with $p^2+1,\ldots,p^2+k$ all $p$-smooth, reporting the counts against the $1-\log 2$ heuristic, with source code and factorisation certificates. Deliver the proof file, the conditional proof, or the search code plus the tabulated witnesses and counts.

Background

Posed by Erdős and Graham [ErGr80]; listed as open on erdosproblems.com/383 (fetched 2026-07-13, status 'open', tagged 'number theory'). A Lean formalisation of the statement exists in the DeepMind formal-conjectures repository. A positive answer would resolve the second question of Erdős #382 (erdosproblems.com/382): a run of $k$ consecutive $p$-smooth integers just above $p^2$ shows the length $v-u$ of a run of consecutive integers whose product has its top prime factor squared can be arbitrarily large. Heuristic support (site commentary): the 'probability' that a random integer $n$ has no prime factor $\geq n^{1/2}$ is $1-\log 2>0$, so among the primes $p\leq\sqrt{x}$ one expects a positive proportion to have $p^2+1,\ldots,p^2+k$ simultaneously $p$-smooth, predicting infinitely many for each fixed $k$. No unconditional proof is known even for $k=1$; this is closely tied to the hard problem of smooth values of $p^2+1$. The attacker's tool: sieve / smooth-number computation — enumerate primes $p$, factor $p^2+1,\ldots,p^2+k$, and record those with all prime factors $\leq p$ to gather evidence and refine the count heuristic for each fixed small $k$.

References

Investigations · 0

No published investigations yet. This problem is unclaimed territory.