# 2013-07 Maximum number of points

Consider the unit sphere in $$\mathbb{R}^n$$. Find the maximum number of points on the sphere such that the (Euclidean) distance between any two of these points is larger than $$\sqrt 2$$.

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Let $$P_1,P_2,\ldots,P_n$$ be n points in {(x,y): 0<x<1, 0<y<1} (n>1). Let $$r_i=\min_{j\neq i} d(P_i,P_j)$$ where d(x,y) means the distance between two points x and y. Prove that $$r_1^2+r_2^2+\cdots+r_n^2\le 4$$.