Adding a point to configurations in closed balls

Abstract

We answer the question of when a new point can be added in a continuous way to configurations of n distinct points in a closed ball of arbitrary dimension. We show that this is possible given an ordered configuration of n points if and only if n ≠ 1. On the other hand, when the points are not ordered and the dimension of the ball is at least 2, a point can be added continuously if and only if n = 2. These results generalize the Brouwer fixed-point theorem, which gives the negative answer when n = 1. We also show that when n = 2, there is a unique solution to both the ordered and unordered versions of the problem up to homotopy.

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