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Adjusting a conjecture of Erdős

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Author(s): Walter Carnielli | Pietro K. Carolino

Journal: Contributions to Discrete Mathematics
ISSN 1715-0868

Volume: 6;
Issue: 1;
Date: 2011;
Original page

ABSTRACT
We investigate a conjecture of Paul Erdős, the last unsolved problem among those proposed in his landmark paper [2]. The conjecture states that there exists an absolute constant $C > 0$ such that, if $v_1, dots, v_n$ are unit vectors in a Hilbert space, then at least $C frac{2n}{n}$ of all $epsilon in {-1,1}^n$ are such that $|sum_{i=1}^n epsilon_i v_i| leq 1$. We disprove the conjecture. For Hilbert spaces of dimension $d > 2,$ the counterexample is quite strong, and implies that a substantial weakening of the conjecture is necessary. However, for $d = 2,$ only a minor modification is necessary, and it seems to us that it remains a hard problem, worthy of Erdős. We prove some weaker related results that shed some light on the hardness of the problem.
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