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

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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 t≤nt\leq n, define vi′=(1,0)v_i'=(1,0) for i≤ti\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 t≤nt\leq n such that

P[∣ϵ1v1+⋯+ϵnvn∣2≤2]≥P[∣ϵ1v1′+⋯+ϵnvn′∣2≤2].\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.

References

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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