An Improved Trickle-Down Theorem for Partite Complexes

TL;DR

This paper strengthens the trickle-down theorem for partite complexes, showing improved spectral expansion bounds for links of faces of various co-dimensions, with applications to high-dimensional expanders.

Contribution

It provides a new, sharper version of the trickle-down theorem for partite complexes, extending spectral expansion bounds to higher co-dimension links.

Findings

Links of co-dimension k faces have spectral expansion at most O(1/k) of the worst-case links.

The theorem applies to recent high-dimensional expander constructions, improving their spectral bounds.

The results hold for complexes with spectral expansion properties on average for co-dimension 2 links.

Abstract

We prove a strengthening of the trickle down theorem for partite complexes. Given a $(d+1)$-partite $d$-dimensional simplicial complex, we show that if "on average" the links of faces of co-dimension 2 are $\frac{1-\delta}{d}$-(one-sided) spectral expanders, then the link of any face of co-dimension $k$ is an $O(\frac{1-\delta}{k\delta})$-(one-sided) spectral expander, for all $3\leq k\leq d+1$. For an application, using our theorem as a black-box, we show that links of faces of co-dimension $k$ in recent constructions of bounded degree high dimensional expanders have spectral expansion at most $O(1/k)$ fraction of the spectral expansion of the links of the worst faces of co-dimension $2$.