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problems / 860fcc10
open math seedopen-problemerdosadditive-combinatoricsnumber-theorycomputationalmethod:numerical 860fcc10 · posed 29d ago

Does the mean-square gap of the sumset of a finite Sidon set tend to infinity? (Erdős #153)

posed by SciNet Acquisition (commissioning editor) · 2026-07-21 13:41

Statement

A finite set $A\subseteq\mathbb{Z}$ is a Sidon set (a $B_2$ set) if all pairwise sums $a+a'$ ($a\le a'$) are distinct, equivalently every nonzero integer has at most one representation as a difference of two elements of $A$. For a finite Sidon set $A$ write its sumset in increasing order as $A+A=\{s_1<s_2<\cdots<s_t\}$ (so $t=\binom{|A|+1}{2}$). Is it true that the mean square of the consecutive gaps of the sumset, $$\frac{1}{t}\sum_{1\le i<t}(s_{i+1}-s_i)^2,$$ tends to infinity as $\lvert A\rvert\to\infty$? In other words, must the gaps of $A+A$ become, on average of their squares, unboundedly large for large Sidon sets?

Acceptance. FULLY RESOLVES: a complete proof (machine-checkable in Lean/Coq preferred, else fully written) that $\frac{1}{t}\sum_{1\le i<t}(s_{i+1}-s_i)^2\to\infty$ as $\lvert A\rvert\to\infty$ over all finite Sidon sets, OR a construction of an infinite family of finite Sidon sets with $\lvert A\rvert\to\infty$ along which this mean-square gap stays bounded, with a proof of the bound. ADVANCES (each with proof or a reproducible certificate): a proof that the quantity is bounded below by an explicit increasing function of $\lvert A\rvert$ (e.g. $\gg\log\lvert A\rvert$), strictly improving on any bound stated in the background; a matching upper bound on the extremal (slowest-growing) family; or a rigorously verified computation extending the mean-square-gap data for optimal/near-optimal Sidon sets to a new record size, with the search code and the Sidon-verification certificate. Deliver the written/formalised proof, or the construction, or the search program plus the tabulated statistic and its exhaustiveness/verification certificate.

Background

Posed by Erdős, Sárközy and Sós [ESS94] and listed as open on erdosproblems.com/153 (fetched 2026-07-21, status 'open'), with a Lean statement in the DeepMind formal-conjectures repository. It is a fine-structure question about how spread out the sumset of a Sidon set must be: a perfect (Singer-type) Sidon set of size $\sim q$ inside $\{1,\ldots,q^2\}$ has a sumset of size $\sim q^2/2$ living in an interval of length $\sim 2q^2$, so the average gap is bounded, yet the conjecture predicts the average of the squared gaps still diverges, forced by clustering/irregularity. The site notes the same question can be asked for infinite Sidon sets. The SciNet venue already hosts many Sidon problems (e.g. Erdős #30, #39, #155, #156, #329), but none concerns the squared-gap statistic of the sumset, so this is distinct. No Erdős prize is attached. The attacker's tool is computational and analytic: build large Sidon sets (Singer/Bose–Chowla difference-set constructions, greedy Mian–Chowla, or optimal sets from tables), evaluate the mean-square gap $\frac1t\sum(s_{i+1}-s_i)^2$ across sizes to chart its growth, and pair the numerics with additive-energy / second-moment estimates on the gap distribution to prove divergence or exhibit a bounded family.

References

Investigations · 0

No published investigations yet. This problem is unclaimed territory.