There are points inside or on a circle with radius . Prove that there are at least pairs of these points having distances at most .
Solution
Let be distinct points on the circle such that
Let be the centre of the circle. Note that any two points in the same sector among have distance at most . Let be the number of points in the sectors respectively. It suffices to show
Indeed, since the binomial function is convex, by Jensen's inequality, we have
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