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 180∘.
2. Initial Assumptions:
Let's assume we can construct a nearly convex polygon P with vertices A1,A2,…,A106 such that ∠A106A1A2>180∘. This means that A1 is a concave vertex.
3. Constructing Other Nearly Convex Polygons:
Consider the polygon T=A2A3…A106. We need to show that T is convex and that A1 lies inside T. If T is convex, then all its interior angles are less than 180∘.
4. **Convexity of T:**
Since T is formed by removing A1 from P, and P is nearly convex with only ∠A106A1A2>180∘, the remaining angles in T must be less than 180∘. Therefore, T is convex.
5. **Position of A1:**
If A1 lies outside T, then the line segment A1A2 would intersect T, which contradicts the assumption that P is a non-self-intersecting polygon. Hence, A1 must lie inside T.
6. Constructing Nearly Convex Polygons:
For each i∈{2,3,…,106−1}, we can construct a nearly convex polygon Ti by removing the segment AiAi+1 from T and connecting A1 to Ai and Ai+1. This results in 106−1 nearly convex polygons.
7. Conclusion:
Since we can construct 106−1 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 106−1.