Quasi-orthogonal extension of skew-symmetric matrices

TL;DR

This paper introduces the concept of quasi-orthogonal matrices, proves that skew-symmetric matrices can be extended to such matrices, and characterizes their spectral properties in certain graph structures.

Contribution

It establishes the existence of quasi-orthogonal extensions for skew-symmetric matrices and determines the minimal extension size, introducing the quasi-orthogonality index.

Findings

Any skew-symmetric matrix is a principal sub-matrix of a quasi-orthogonal matrix.

The minimal extension size (quasi-orthogonality index) is determined for skew-symmetric matrices.

Spectral characterization of skew-adjacency matrices with low quasi-orthogonality index.

Abstract

A real matrix $Q$ is quasi-orthogonal if $Q^{\top}Q=qI$, for some positive real number $q$. We prove that any $n\times n$ skew-symmetric matrix $S$ is a principal sub-matrix of a skew-symmetric quasi-orthogonal matrix $Q$, called a quasi-orthogonal extension of $S$. Moreover, we determine the least integer $d$ such that $S$ has a quasi-orthogonal extension of order $n+d$. This integer is called the quasi-orthogonality index of $S$. Lastly, we give a spectral characterization of skew-adjacency matrices of tournaments with quasi-orthogonality index at most three.