Erdős's odd-dimensionality-adjusted reverse Littlewood–Offord conjecture

Let v1,,vnv_1,\ldots,v_n be unit vectors in R2\mathbb{R}^2, where nn is odd, and let ϵ1,,ϵn\epsilon_1,\ldots,\epsilon_n be independent Rademacher random variables. Odd-nn Erdős conjecture. There exists an absolute constant c>0c>0 such that

P[ϵ1v1++ϵnvn21]cn.\mathbb{P}[\\|\epsilon_1v_1+\cdots+\epsilon_nv_n\\|_2\leq 1]\geq \frac{c}{n}.

The paper proves the corresponding bound with radius 2\sqrt{2} and explains that its method gives some radius strictly below 2\sqrt{2} for odd nn, but reaching radius 11 appears to require new ideas. Thus this strengthened odd-nn statement remains open.

Sources & referencesView supporting material

Primary source

Xiaoyu He, Tomas Juskevicius, Bhargav Narayanan and Sam Spiro, “The Reverse Littlewood–Offord problem of Erdős”, arXiv:2408.11034 (2024).

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