One- and two-particle spectral gap identities for the symmetric inclusion process and related models

TL;DR

This paper investigates spectral gap identities in the symmetric inclusion process, revealing conditions under which these identities hold or fail, and providing bounds and limits that enhance understanding of particle diffusion models.

Contribution

It establishes that the spectral gap identity holds for non-conservative SIP regardless of interaction strength and derives bounds and limits when the identity fails.

Findings

Spectral gap identity holds for non-conservative SIP regardless of interaction.

Sharp bounds for spectral gap when the identity breaks down.

Two-particle spectral gap identity in the vanishing diffusivity limit.

Abstract

The symmetric inclusion process (SIP) models particles diffusing on a graph with mutual attraction. We recently showed that, in the log-concave regime (where diffusivity dominates interaction), the spectral gap of the conservative SIP matches that of a single particle. In this paper, our main result demonstrates that this identity generally fails outside this regime, but always holds for the non-conservative SIP, regardless of the interaction strength. When this one-particle spectral gap identity breaks down, we derive sharp bounds for the gap in terms of diffusivity, and reveal a two-particle spectral gap identity in the vanishing diffusivity limit. Our approach leverages the rigid eigenstructure of SIP, refined comparisons of Dirichlet forms for arbitrary diffusivity and particle numbers, and techniques from slow-fast system analysis. These findings extend to the dual interacting diffusion known as Brownian energy process, and shed some light on the spectral gap behavior for related Dirichlet-reversible systems on general, non-mean-field, geometries.