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Let be an infinite sum-free set - that is, there are no solutions to
with . How small can be? Is it possible that ?
Source: erdosproblems.com/876
A full solution has been claimed but not yet accepted. Settled in another form, for example when its parts resolve differently or the question is open-ended.
The site's label is OPEN; the derived standing is claimed, through a pending full claim on the site's proof-claims tab that would settle the gap questions if accepted. What the refereed sources give: a sum-free set has density zero, and , and some sum-free set has (Erdős 1962, Theorems I--III and the construction); a sum-free set has for infinitely many , and for every some sum-free set has for all large (Łuczak and Schoen 2000); the reciprocal sum is below an absolute constant that the authors say "seems certain" to be below (Benkoski and Erdős 1974), below by Erdős's 1977 restatement, and below by Sullivan, who conjectured a maximum "only a little greater than " (Erdős 1977, the site's figures; Sullivan's work is not held). None of these decides the gap questions: the site's near-linear gap statement (Graham, as reported by Erdős 1998) and the construction (Deshouillers, Erdős and Melfi 1999) rest on papers not held and are reported from the site. Two claims on the site's proof-claims tab, neither adopted by the site, bear on the gap questions and have claim pages: Price's partial claim of 18 July 2026, credited to GPT 5.6 Sol Pro, that every sum-free sequence has , a negative answer to the second question; and Korsky's full claim of 22 September 2026, credited to GPT Astra, with two theorems: for a nondecreasing slowly varying , a sum-free sequence with gaps of the order exists if and only if the integral is finite (its claim page states how that reading of the summary's is reached); and the smallest for which a sum-free sequence can have gaps and infinitely many bounded gaps is . Both are unreviewed; the pending full claim makes the derived standing claimed, while the site's label stays OPEN. Neither touches the reciprocal-sum question. The refereed sources alone give a bounded negative finding, not a certificate of openness.