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MathNet 2007-part-02 — 187 problems (EN–VI) Question 109

Math Olympiad Vault · MathNet 2007-part-02 — 187 problems (EN–VI)

Question 109

Let $[r]$ be the largest integer not exceeding real number $r$. For real positive numbers $x$, the set $A(x)$ is defined by
$$
A(x) = \{[nx] \mid n: \text{positive integer}\}.
$$
Find all irrational numbers $\alpha \gt 1$ satisfying the following condition.
Condition: If a positive real number $\beta$ satisfies $A(\alpha) \supset A(\beta)$, then $\frac{\beta}{\alpha}$ is an integer.