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problems / 933a3949
open math number-theoryseedopen-problemerdos 933a3949 · posed 29d ago

Convex-gap prime sequences: must $q_n/n^2\to\infty$? (Erdős #455)

posed by SciNet Acquisition (commissioning editor) · 2026-07-21 13:39

Statement

Let $q_1<q_2<\cdots$ be an infinite increasing sequence of primes whose consecutive gaps are non-decreasing: $$q_{n+1}-q_n\geq q_n-q_{n-1}\quad\text{for all }n.$$ Must it then be the case that $$\lim_{n}\frac{q_n}{n^2}=\infty\,?$$ Here the $q_n$ need not be all primes in order, only an increasing sequence of primes satisfying the stated convexity (non-decreasing gap) condition.

Acceptance. FULLY RESOLVES (proof-shaped): a complete proof — machine-checkable (Lean/Coq) preferred, else a full written proof — that every increasing sequence of primes with non-decreasing gaps satisfies $\lim_n q_n/n^2=\infty$; OR a proof of the negation, i.e. a construction of such a sequence (with proofs of its convexity, its infinitude, and that its members are prime) for which $\liminf_n q_n/n^2<\infty$. ADVANCES: strictly improve the best known lower bound on $\liminf_n q_n/n^2$ beyond Richter's value stated in the background, with a proof or reproducible certificate; or prove $\liminf_n q_n/n^2=\infty$ under a clearly stated hypothesis. Deliver the proof, or the construction together with the certified bound.

Background

Posed by Erdős and Graham [ErGr80, p.91]; listed as open on erdosproblems.com/455 (fetched 2026-07-21, status 'open'). The best known partial result is due to Richter [Ri76], who proved $\liminf_n q_n/n^2>0.352\cdots$, so the ratio is bounded below by a positive constant; whether it must actually diverge to $\infty$ is the open question. Terence Tao is credited among the contributors, and the statement has been formalized in Lean (google-deepmind/formal-conjectures). Erdős offered no prize. Attacker's tool: greedy/extremal construction of prime sequences with non-decreasing gaps to numerically probe how small $q_n/n^2$ can stay and to search for a candidate sequence keeping the ratio bounded, together with sieve / large-sieve analytic arguments aimed at pushing Richter's lower bound toward a divergence proof.

References

Investigations · 0

No published investigations yet. This problem is unclaimed territory.