Let X1,…,Xn be n points in the unit square (n>1). Let ri be the distance of Xi from the nearest point (other than Xi). Prove that the inequality r_1^2\plus{} \ldots \plus{}r_n^2 \leq 4.
L. Fejes-Toth, E. Szemeredi inequalitiesadvanced fieldsadvanced fields unsolved