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Difficulty: 9/102025 USAMO 2025 (Q3)

Alice and Bob play game with cities and roads. Bob places cities with distance ≥ 1 and no three collinear. Roads are built based on similarity of triangles to set S chosen by Alice. Determine who has winning strategy.

Options:

  • A.

    Bob has a winning strategy.

  • B.

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

  • C.

    No such configuration exists under the given conditions.

  • Alice has a winning strategy.

Guide / Hint

Hint 1: Consider what happens if Alice chooses to be a highly restrictive set of triangles (e.g., very thin or specific non-constructible angles).

Hint 2: Analyze the road placement condition. How does the distance constraint prevent cities from being placed too close?

Hint 3: Show that Alice can restrict the road graph to be a planar forest, preventing Bob from creating cycles or achieving connectivity.

Solution

Step 1 (Game Analysis): Bob places cities (points) in the plane such that no three are collinear and all pairwise distances are . Alice selects a set of triangle shapes. Roads (edges) are built between cities if the triangle they form with a third city is similar to one in . We want to determine who has a winning strategy.

Step 2 (Alice's Strategy - Planar Graph Constraint): Alice wants to select a set of triangles that severely restricts the roads Bob can build. Specifically, Alice can choose to contain only triangles with extremely sharp angles or specific transcendental angle ratios. By doing so, the condition for road placement ensures that the graph of roads cannot contain any cycle, or must be planar and bipartite.

Step 3 (Forcing Planarity): Since the road graph is planar and has bounded degree due to the distance constraint , Bob cannot complete any complex interlocking network or achieve his goal. Alice's selection of a highly restrictive similarity set guarantees that she can block Bob's moves indefinitely, resulting in a win for Alice.

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