# Equivalence and invariance of the chi and Hoffman constants of a matrix

## TL;DR

This paper proves that the chi and Hoffman constants for a full column rank matrix are identical and explores their invariance and equivalence with related condition measures, extending to subspace-dependent variants.

## Contribution

It establishes the equality and invariance of the chi and Hoffman constants, and relates them to other condition measures, revealing fundamental connections.

## Key findings

- Chi and Hoffman constants are identical for full column rank matrices.
- Invariance of these constants under sign changes of matrix rows.
- Extensions to subspace-dependent variants and relations to other condition measures.

## Abstract

We show that the following two condition measures of a full column rank matrix $A \in \mathbb{R}^{m\times n}$ are identical: the chi constant and a signed Hoffman constant. This identity is naturally suggested by the evident invariance of the chi constant under sign changes of the rows of $A$. We also show that similar equivalence and invariance properties extend to variants of the chi and Hoffman constants that depend only on the linear subspace $A(\mathbb{R}^n):=\{Ax: x\in\mathbb{R}^n\} \subseteq \mathbb{R}^m$. Finally, we show similar identities between the chi constants and signed versions of Renegar's and Grassmannian condition measures.