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Let be an infinite set. Is
irrational?
Source: erdosproblems.com/257
No claim settles this problem.
Open, the site's label (OPEN; page last edited 2026-04-15). No
claim settles the question for every infinite support, so the frontmatter
standing, open/none, follows. Accepted partial claim pages record the
settled classes of supports:
Erdős's 1948 theorem
for and its sets of multiples,
Erdős's 1968 theorem
for pairwise coprime supports with convergent reciprocal sum,
Duverney and Tachiya's theorem
for the sets of products of powers below of a pairwise coprime,
polynomially bounded sequence, such as the squarefree integers, and
Tao and Teräväinen's theorem
for the primes. One pending partial claim,
Cook's 2026 note,
asserts irrationality for every support with convergent reciprocal sum.