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pow:problem1s21

Problem 1 (due Monday, March 1)

We say that a vector in $\mathbb R^3$ is ${\rm positive}$ (${\rm negative}$) if all its coordinates are positive (resp. negative). Let $v_1,v_2,v_3,v_4$ be vectors in $\mathbb R^3$ such that the sum of any two of these vectors is either positive or negative. Prove that at least one of the vectors $v_1,v_2,v_3,v_4, v_1+v_2+v_3+v_4$ is either positive or negative.

The problem was solved by Chris Eppolito, Yuqiao Huang, Ashton Keith, Maxwell T Meyers, and Wei Yang. One solver provided an (almost complete) solution in which it was observed that for vectors in $\mathbb R^3$ actually one of the vectors $v_1,v_2,v_3,v_4$ is either positive or negative. The other solvers provided complete solutions which work for vectors in $\mathbb R^n$ for any $n$. Detailed solutions are discussed in the following link Solution.

pow/problem1s21.txt · Last modified: 2021/03/07 10:38 by mazur

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