There's a convex -polygon on the plane with a robot on each of its vertices. Each robot fires a laser beam toward another robot. On each of your move, you select a robot to rotate clockwise until its laser points at a new robot. Three robots and form a triangle, if 's laser points at , 's laser points at , and 's laser points at . Find the minimum number of moves that can guarantee triangles on the plane.
Solution
Answer: moves.
Below, denotes the expected value of the random variable .
1. First, for any two points and , let be the number of steps required to make 's laser point at . Suppose we choose distinct and uniformly at random from all the points, then (in fact, it is easy to see that is uniformly distributed on .)
2. For any three points , and , let be the number of steps required to rotate the lasers on the three points so that becomes a triangle. Suppose we choose distinct triples uniformly at random from all the points, then by additivity of expectation, we know .
3. Denote these points by through . We use to denote a way of partitioning all the points into triangles, where is an ordered triple. Let denote the number of steps required to turn the given configuration into . Suppose we choose uniformly at random among all possible partitions, then as in point 2, by additivity, .
4. We use to denote a way of partitioning all the points into unordered sets, each of three points. For each , let be the corresponding unordered grouping of . Note that the random variable in the third point can be viewed as first choosing an uniformly at random among all possible , and then considering all permutations of uniformly at random. Based on , this means there exists an such that .
5. Now, note that all satisfying have the same grouping into triples of points, differing only in the clockwise or counterclockwise orientation of some of the triangles. Consider the following proposition:
Proposition 1. For a triangle, the number of steps required to turn it clockwise differs from the number of steps required to turn it counterclockwise.
If Proposition 1 holds, then it means contains at least distinct values, so among all satisfying there must exist a such that , thereby obtaining the claimed lower bound.
6. To prove Proposition 1, consider an unordered triangle , and suppose that the number of steps required to turn it into a clockwise triangle is the same as the number of steps required to turn it into a counterclockwise triangle. Without loss of generality, assume is a clockwise triangle; furthermore, let be the number of steps required to rotate clockwise to , be the number of steps required to rotate clockwise to , and be the number of steps required to rotate counterclockwise to , then we have . However, note that rotating clockwise to , together with rotating clockwise to , will pass through every point on the arc exactly once, but rotating counterclockwise to will pass through every point on the arc "except ", which means the two operation counts differ by one, so the original assumption cannot hold.
7. Finally, we prove that this lower bound is optimal. Consider a regular -gon, whose vertices are labeled clockwise in order as , and consider the situation where robot points at , while all other robots point at . Consider the case where, in the end, counterclockwise triangles are turned out; then one can compute that operations are required, so when we get the minimum value .