Two-axis extremizer conjecture for the radius-2\sqrt{2} reverse Littlewood–Offord problem

For each sufficiently large integer nn, let v1,,vnv_1,\ldots,v_n be arbitrary unit vectors in R2\mathbb{R}^2, and let ϵ1,,ϵn\epsilon_1,\ldots,\epsilon_n be independent Rademacher random variables. For an integer tt with tnt\leq n, define vi=(1,0)v_i'=(1,0) for iti\leq t and vi=(0,1)v_i'=(0,1) for i>ti>t. Two-axis extremizer conjecture. For all sufficiently large nn, there exists some tnt\leq n such that

P[ϵ1v1++ϵnvn22]P[ϵ1v1++ϵnvn22].\mathbb{P}[\\|\epsilon_1v_1+\cdots+\epsilon_nv_n\\|_2\leq\sqrt{2}]\geq\mathbb{P}[\\|\epsilon_1v_1'+\cdots+\epsilon_nv_n'\\|_2\leq\sqrt{2}].

This asks whether the smallest radius-2\sqrt{2} concentration probability is attained by a configuration consisting of copies of the two coordinate-axis vectors. The source presents it as an open stronger statement and notes that it would identify the relevant extremal configuration.

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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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