This post marks the official opening of the mini-polymath3 project to solve a problem from the 2011 IMO. I have decided to use Q2, in part to see how the polymath format would cope with a more geometrically themed problem.

Problem 2.Let be a finite set of at least two points in the plane. Assume that no three points of are collinear. Awindmillis a process that starts with a line going through a single point . The line rotates clockwise about the pivot until the first time that the line meets some other point belonging to . This point takes over as the new pivot, and the line now rotates clockwise about , until it next meets a point of . This process continues indefinitely.Show that we can choose a point in and a line going through such that the resulting windmill uses each point of as a pivot infinitely many times.

**All are welcome.**Everyone (regardless of mathematical level) is welcome to participate. Even very simple or “obvious” comments, or comments that help clarify a previous observation, can be valuable.**No spoilers!**It is inevitable that solutions to this problem will become available on the internet very shortly. If you are intending to participate in this project, I ask that you refrain from looking up these solutions, and that those of you have already seen a solution to the problem refrain from giving out spoilers, until at least one solution has already been obtained organically from the project.**Not a race.**This is**not**intended to be a race between individuals; the purpose of the polymath experiment is to solve problems*collaboratively*rather than individually, by proceeding via a multitude of small observations and steps shared between all participants. If you find yourself tempted to work out the entire problem by yourself in isolation, I would request that you refrain from revealing any solutions you obtain in this manner until*after*the main project has reached at least one solution on its own.**Update the wiki.**Once the number of comments here becomes too large to easily digest at once, participants are encouraged to work on the wiki page to summarise the progress made so far, to help others get up to speed on the status of the project.**Metacomments go in the discussion thread.**Any non-research discussions regarding the project (e.g. organisational suggestions, or commentary on the current progress) should be made at the discussion thread.**Be polite and constructive, and make your comments as easy to understand as possible.**Bear in mind that the mathematical level and background of participants may vary widely.

Have fun!