Difference between revisions of "2011 IMO Problems/Problem 2"
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Revision as of 01:20, 19 November 2023
Let be a finite set of at least two points in the plane. Assume that no three points of
are collinear. A windmill is 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.
Solution
Choose a coordinate system so that all points in have distinct x-coordinates. Number the points
of
by increasing x-coordinates:
.
In order to divide the set into two halves, define
so that
where
for an odd number of points and
for an even number of points.
Start the "windmill" process with the line going vertically through the point
. Attach a down-up direction to this line so that we can color all points as follows: Points to the left of
(with lower x-coordinates) are blue, the pivot point on
is white and point to the right of
(with higher x-coordinates) are red. We have now
blue points, one white point and
red points.
After processing the "windmill" by 180 degrees, the line goes vertically up-down. Now, points with lower x-coordinates are to the right of
and colored red; points with higher x-coordinates are to the left of
and colored blue.
Note that at each pivot exchange, the old pivot point enters the same side of where the new pivot point came from. This means that throughout the "windmill" process, the number of blue points and the number of red points stay constant, respectively: We still have
blue points, one white point and
red points. This means that the current pivot point is
.
Note that all blue and all red points changed their color from the start of the "windmill" process. This implies that every point was a pivot at some stage of the rotation.
For every 180 degrees of "windmill" rotation, the same argument applies: all colored points must change their color and hence be a pivot at some stage. Infinitely many rotations imply infinitely many color changes. This completes the proof.
See Also
2011 IMO (Problems) • Resources | ||
Preceded by Problem 1 |
1 • 2 • 3 • 4 • 5 • 6 | Followed by Problem 3 |
All IMO Problems and Solutions |