Erdős Problem #266 — Shifted Reciprocal Series

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Is the following universal assertion false? For every sequence a:N→Na:\mathbb{N}\to\mathbb{N} such that a(n)≥1a(n)\geq1 for every nn and

∑n∈N1a(n)\sum_{n\in\mathbb{N}}\frac1{a(n)}

converges, there exists a natural number t≥1t\geq1 such that

∑n∈N1a(n)+t\sum_{n\in\mathbb{N}}\frac1{a(n)+t}

is irrational. Equivalently, does there exist a sequence a:N→Na:\mathbb{N}\to\mathbb{N} with a(n)≥1a(n)\geq1 for every nn and convergent ∑n∈N1/a(n)\sum_{n\in\mathbb{N}}1/a(n) such that, for every natural number t≥1t\geq1, the series ∑n∈N1/(a(n)+t)\sum_{n\in\mathbb{N}}1/(a(n)+t) is rational?

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