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USAMO 2026 — Days 1 and 2 Question 1

USAMO · USAMO 2026 — Days 1 and 2

Question 1

Fix an integer $n \ge 2$.
For which real numbers $x$ is

$$\lfloor nx\rfloor-\sum_{k=1}^n\frac{\lfloor kx\rfloor}{k}$$

maximal, and what is the maximal value that this expression can take?

Proposed by Titu Andreescu and Gabriel Dospinescu.

Manual essay; 1 editorial point, not official contest scoring. Automatic answer fields are intentionally empty.