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Let $x\in\mathbb{R}^n$ and $y\in\mathbb{R}^n$ be two unit vectors such that $\sum_{i}{x_i}=\sum_{i}{y_i}=0$

$$ x_{1}y_{1}+x_{2}y_{2}+\cdots +x_{n}y_{n} \gt 1-\frac{1}{n} .$$

Can we prove that $$x_{1}y_{\sigma(1)}+x_{2}y_{\sigma(2)}+\cdots +x_{n}y_{\sigma(n)} \neq 0$$

for all permutations $\sigma$?

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    $\begingroup$ Why do you ask? $\endgroup$
    – Igor Rivin
    Commented Jun 15, 2017 at 0:50

1 Answer 1

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No. Let $$\vec{x} = \vec{y} = \frac{1}{\sqrt{12+6 \sqrt{3}}} (-2-\sqrt{3},1,1+\sqrt{3}).$$ So $\vec{x} \cdot \vec{y} = 1$, but $x_1 y_2+x_2 y_1 + x_3 y_3=0$.

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  • $\begingroup$ very nice, I was trying more complicated constructions. $\endgroup$
    – kodlu
    Commented Jun 15, 2017 at 4:40

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