Directed Poincar\'e Inequalities and $L^1$ Monotonicity Testing of Lipschitz Functions

TL;DR

This paper establishes a new connection between directed isoperimetric inequalities and monotonicity testing for Lipschitz functions on continuous domains, leading to novel $L^1$ monotonicity testers.

Contribution

It proves directed Poincaré inequalities for Lipschitz functions on [0,1]^n and develops an $L^1$ monotonicity testing framework based on these inequalities.

Findings

Proved $d^{ ext{mono}}_1(f) \,\lesssim\, \mathbb{E}[\| abla^- f\|_1]$ for Lipschitz functions.

Introduced a monotone rearrangement technique for continuous functions.

Developed an $L^1$ monotonicity tester for Lipschitz functions on [0,1]^n.

Abstract

We study the connection between directed isoperimetric inequalities and monotonicity testing. In recent years, this connection has unlocked breakthroughs for testing monotonicity of functions defined on discrete domains. Inspired the rich history of isoperimetric inequalities in continuous settings, we propose that studying the relationship between directed isoperimetry and monotonicity in such settings is essential for understanding the full scope of this connection. Hence, we ask whether directed isoperimetric inequalities hold for functions $f : [0,1]^n \to \mathbb{R}$, and whether this question has implications for monotonicity testing. We answer both questions affirmatively. For Lipschitz functions $f : [0,1]^n \to \mathbb{R}$, we show the inequality $d^{\mathsf{mono}}_1(f) \lesssim \mathbb{E}\left[\|\nabla^- f\|_1\right]$, which upper bounds the $L^1$ distance to monotonicity of $f$ by a measure of its "directed gradient". A key ingredient in our proof is the monotone rearrangement of $f$, which generalizes the classical "sorting operator" to continuous settings. We use this inequality to give an $L^1$ monotonicity tester for Lipschitz functions $f : [0,1]^n \to \mathbb{R}$, and this framework also implies similar results for testing real-valued functions on the hypergrid.