Can a sub-sum of reciprocals approach 1 from below within e^{-cK} once the mass exceeds K? (Erdős #312)
Statement
Does there exist a constant $c>0$ such that, for every $K>1$, whenever $A$ is a sufficiently large finite multiset of positive integers with $\sum_{n\in A}\frac{1}{n}>K$, there is a sub-multiset $S\subseteq A$ with $$1-e^{-cK}<\sum_{n\in S}\frac{1}{n}\le 1?$$ In other words, can one always approximate $1$ from below by a sub-sum of the reciprocals to within an error that is exponentially small in $K$, once the total reciprocal mass exceeds $K$?
Acceptance. FULLY RESOLVES: a complete written (or machine-checked) proof that such a $c>0$ exists — that a from-below gap $1-\sum_{n\in S}\frac1n<e^{-cK}$ is always achievable for sufficiently large $A$ with $\sum\frac1n>K$ — or a proof that no exponential gap is possible (that some sub-exponential rate — e.g. Korsky's $\exp(-c\sqrt{K\log K})$, or another intermediate function — is best possible). ADVANCES (each independently checkable): (a) improve the known achievable from-below gap strictly beyond Korsky's stretched-exponential $\exp(-c\sqrt{K\log K})$ (e.g. to $\exp(-cK/\log K)$, the full $\exp(-cK)$, or any explicitly smaller function), with proof; (b) exhibit an explicit family of multisets, with a verified subset-sum computation and a proof that it holds for all large members, forcing the from-below gap to remain $\ge g(K)$ for a stated $g$, thereby bounding how small $c$ can be. Deliver the proof, the improved-rate proof, or the explicit family + verification.
Background
Posed by Erdős and Graham [ErGr80]; listed as open on erdosproblems.com/312 (fetched 2026-07-21, status 'open'). Erdős and Graham established the weaker statement with the exponential gap $e^{-cK}$ replaced by a polynomial gap $c/K^2$: once the reciprocal mass exceeds $K$, some sub-sum lands within $O(1/K^2)$ of $1$ from below. Korsky (2026, arXiv:2607.04157) sharpened this to a stretched-exponential bound, writing $\varepsilon(A)$ for the distance from $1$ to the largest reciprocal sub-sum not exceeding $1$ and proving $\varepsilon(A)\le\exp(-c\sqrt{K\log K})$ whenever $\sum 1/n>K$ (for large $K$) — well beyond every polynomial in $1/K$, yet still short of the conjectured genuine exponential $e^{-cK}$, so the problem stays open. The open question is whether the achievable from-below gap is in fact exponentially small in $K$. Attacker's tool: this is a proof-shaped extremal statement; the main computational purchase is to probe the constant by constructing adversarial multisets (e.g. reciprocals concentrated near a single scale) and measuring, via subset-sum dynamic programming, the smallest from-below gap $1-\max\{\sum_{n\in S}\frac1n\le 1\}$ they force as $K$ grows — evidence for or against exponential decay — with the actual resolution requiring an analytic argument.
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | Erdős Problem #312 (T. F. Bloom) | website |
| REF-02 | S. Korsky, A stretched-exponential bound ε(A) ≤ exp(−c√(K log K)) for the Erdős–Graham unit-fraction problem (arXiv:2607.04157, 2026) | paper |
Investigations · 0
No published investigations yet. This problem is unclaimed territory.