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Omni-MATH / Find all positive integers n such that the following statement holds: Suppose real numbers a₁, a₂, …, a_(n), b₁, b₂, …, b_(n) satisfy |a_(k)|+|b_(k)|=1 for all k=1,…,n. Then there …

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problem

Find all positive integers nn such that the following statement holds: Suppose real numbers a1a_1, a2a_2, …\dots, ana_n, b1b_1, b2b_2, …\dots, bnb_n satisfy |ak|+|bk|=1|a_k|+|b_k|=1 for all k=1,…,nk=1,\dots,n. Then there exists ε1\varepsilon_1, ε2\varepsilon_2, …\dots, εn\varepsilon_n, each of which is either −1-1 or 11, such that |∑i=1nεiai|+|∑i=1nεibi|≤1. \left| \sum_{i=1}^n \varepsilon_i a_i \right| + \left| \sum_{i=1}^n \varepsilon_i b_i \right| \le 1.
Plain-text mathematical notation (without MathML)
Find all positive integers n such that the following statement holds: Suppose real numbers a₁, a₂, …, a_(n), b₁, b₂, …, b_(n) satisfy |a_(k)|+|b_(k)|=1 for all k=1,…,n. Then there exists ε₁, ε₂, …, ε_(n), each of which is either −1 or 1, such that
    |∑_(i=1)^(n)ε_(i)a_(i)|+|∑_(i=1)^(n)ε_(i)b_(i)|≤1.
Original LaTeX notation
Find all positive integers $n$ such that the following statement holds: Suppose real numbers $a_1$, $a_2$, $\dots$, $a_n$, $b_1$, $b_2$, $\dots$, $b_n$ satisfy $|a_k|+|b_k|=1$ for all $k=1,\dots,n$. Then there exists $\varepsilon_1$, $\varepsilon_2$, $\dots$, $\varepsilon_n$, each of which is either $-1$ or $1$, such that
    \[ \left| \sum_{i=1}^n \varepsilon_i a_i \right| + \left| \sum_{i=1}^n \varepsilon_i b_i \right| \le 1. \]

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