Maths Olympiad Prep

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Combinatorics Difficulty 6.6 National olympiad Find the answer

A non-self-intersecting polygon is nearly convex if precisely one of its interior angles is greater than 180180^\circ.

One million distinct points lie in the plane in such a way that no three of them are collinear. We would like to construct a nearly convex one-million-gon whose vertices are precisely the one million given points. Is it possible that there exist precisely ten such polygons?

Solution

1. Understanding the Problem:
We are given one million distinct points in the plane such that no three of them are collinear. We need to determine if it is possible to construct exactly ten nearly convex polygons using these points as vertices. A nearly convex polygon is defined as a non-self-intersecting polygon with exactly one interior angle greater than 180180^\circ.

2. Initial Assumptions:
Let's assume we can construct a nearly convex polygon PP with vertices A1,A2,,A106A_1, A_2, \ldots, A_{10^6} such that A106A1A2>180\angle A_{10^6}A_1A_2 > 180^\circ. This means that A1A_1 is a concave vertex.

3. Constructing Other Nearly Convex Polygons:
Consider the polygon T=A2A3A106T = A_2A_3 \ldots A_{10^6}. We need to show that TT is convex and that A1A_1 lies inside TT. If TT is convex, then all its interior angles are less than 180180^\circ.

4. **Convexity of TT:**
Since TT is formed by removing A1A_1 from PP, and PP is nearly convex with only A106A1A2>180\angle A_{10^6}A_1A_2 > 180^\circ, the remaining angles in TT must be less than 180180^\circ. Therefore, TT is convex.

5. **Position of A1A_1:**
If A1A_1 lies outside TT, then the line segment A1A2A_1A_2 would intersect TT, which contradicts the assumption that PP is a non-self-intersecting polygon. Hence, A1A_1 must lie inside TT.

6. Constructing Nearly Convex Polygons:
For each i{2,3,,1061}i \in \{2, 3, \ldots, 10^6-1\}, we can construct a nearly convex polygon TiT_i by removing the segment AiAi+1A_iA_{i+1} from TT and connecting A1A_1 to AiA_i and Ai+1A_{i+1}. This results in 106110^6 - 1 nearly convex polygons.

7. Conclusion:
Since we can construct 106110^6 - 1 nearly convex polygons, it is not possible to have exactly ten such polygons. The number of nearly convex polygons must be either zero or at least 106110^6 - 1.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.