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MathNet 2023-part-05 — 191 problems (EN–VI) Question 91

Math Olympiad Vault · MathNet 2023-part-05 — 191 problems (EN–VI)

Question 91

Define $M = \{x \in \mathbb{R} \mid \lfloor nx \rfloor + \lfloor (n+1)x \rfloor = \lfloor (2n+1)x \rfloor, \forall n \in \mathbb{N}\}$.

a) Prove that, if $x \in M \cap [\frac{1}{2}, 1)$, then $\lfloor nx \rfloor = \lfloor \frac{n}{2} \rfloor, \forall n \in \mathbb{N}$.

b) Find the cardinal of the set $M \cap [0, 2023]$.