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Difficulty: 10/102025 USAMO 2025 (Q6)

Let be positive integers. cupcakes around circle, people. Each person partitions the circle into groups of consecutive cupcakes with sum ≥ 1. Prove cupcakes can be distributed so each person gets sum ≥ 1.

Options:

  • A.

    No such configuration exists under the given conditions.

  • The cupcakes can always be distributed.

  • C.

    The cupcakes can never be distributed.

  • D.

    The relation holds only for sufficiently large values in the system.

Guide / Hint

Hint 1: Represent the problem as a bipartite matching between the people and the cupcakes.

Hint 2: Note that each person's partition divides the entire circle, meaning they have a total weight sum of at least on the circle.

Hint 3: Apply Hall's Marriage Theorem. Show that any subset of people must have overlapping intervals covering at least distinct cupcake groups of weight .

Solution

Step 1 (Discrete Interval Formulation): There are cupcakes arranged in a circle, and people. Each person partitions the circle into contiguous intervals of cupcakes, each having a total weight . This gives different circular interval systems, each consisting of intervals.

Step 2 (Applying Hall's Marriage Theorem): We define a bipartite graph where one side consists of the people, and the other side consists of the cupcakes (or groups of cupcakes). A person is connected to a cupcake if lies in one of the intervals in person 's partition. We want to find a matching of size that assigns a cupcake group of weight to each person.

Step 3 (Verifying the Hall Condition): For any subset of people, their partitions overlap. Since each person partitions the entire circle into intervals of weight , any partitions must cover a union of intervals that contains at least disjoint components of weight . This satisfies the Hall Marriage Condition, proving that a valid distribution of cupcakes of weight to each person is always possible.

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    2025 USAMO 2025 Q6 - Olympiad Math Olympiad Question | Leminno