Let a path be any line AiAj and a wall be any line AkBk. We say that a path and a wall intersect if the path goes through an inner point of the wall. A path is good if there is no wall to intersect it, and a wall is irrelevant if it does not intersect any path.
Claim 1. For any i∈{1,2,…,n} there exists at least one good path from point Ai.
Proof. We will prove the claim for point A1, and it holds analogously for all points Ai. Define the distance between a point A and a wall w as
T∈wmin∣AT∣.
Since all walls are mutually disjoint, we know that none of the walls A2B2,…,AnBn contain point A1. Let AkBk be the wall closest to A1. We claim that the path A1Ak is good. If we assume the contrary, that means that there exists a wall which intersects A1Ak, but then this wall is closer to A1 than wall AkBk, which is a contradiction. □
Claim 2. There exists at least one irrelevant wall.
Proof. Without loss of generality, we can assume that points A1,A2,…,Ak are the vertices of the convex hull of {A1,A2,…,An}, and that they are labelled clockwise in that order as the vertices of polygon P=A1A2…Ak. If the wall w1=A1B1 is irrelevant, we are done. Therefore, assume that w1 intersects some path. That means that this wall goes through the inner points of polygon P, and since the walls are mutually disjoint, we conclude that w1 must also intersect a path which is the side of P, because it cannot pass through any of the vertices of P except A1. Let C be the intersection of wall w1 and a side of P. Consider the arc \overarcA1C (clockwise from point A1 to point C), and note that it contains points A2,…,Al. We can repeat this inference for wall A2B2. Since all walls are mutually disjoint, its corresponding arc is strictly smaller than the arc of w1. Therefore, if all of the walls A2B2,…,Al−1Bl−1 are relevant, then wall AlBl must surely be irrelevant. □
Finally, we prove the problem statement by mathematical induction on n. The claim obviously holds for n=1. Assume the claim holds for some positive integer n. For n+1 it follows from Claim 2 that there exists a point Ai such that the wall AiBi is irrelevant. By the inductive assumption, we can conclude that the claim holds for all points except possibly point Ai. However, by Claim 1, point Ai is connected by a good path with at least one of the remaining points, which proves the claim for n+1.