Boolean functions on $S_n$ which are nearly linear
Discrete analysis (2021)
We show that if $f\colon S_n \to \{0,1\}$ is $ε$-close to linear in $L_2$ and $\mathbb{E}[f] \leq 1/2$ then $f$ is $O(ε)$-close to a union of "mostly disjoint" cosets, and moreover this is sharp: any such union is close to linear. This constitutes a sharp Friedgut-Kalai-Naor theorem for the symmetric group.
Using similar techniques, we show that if $f\colon S_n \to \mathbb{R}$ is linear, $\Pr[f \notin \{0,1\}] \leq ε$, and $\Pr[f = 1] \leq 1/2$, then $f$ is $O(ε)$-close to a union of mostly disjoint cosets, and this is also sharp; and that if $f\colon S_n \to \mathbb{R}$ is linear and $ε$-close to $\{0,1\}$ in $L_\infty$ then $f$ is $O(ε)$-close in $L_\infty$ to a union of disjoint cosets.
@article{DAS_2021_a1,
author = {Yuval Filmus},
title = {Boolean functions on $S_n$ which are nearly linear},
journal = {Discrete analysis},
year = {2021},
language = {en},
url = {http://geodesic.mathdoc.fr/item/DAS_2021_a1/}
}
Yuval Filmus. Boolean functions on $S_n$ which are nearly linear. Discrete analysis (2021). http://geodesic.mathdoc.fr/item/DAS_2021_a1/