Quasi-orthogonal extension of skew-symmetric matrices

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.

Theorems & Definitions (17)

• Theorem 2.1
• Theorem 2.2
• proof : Proof of Theorem \ref{['existence']}
• Theorem 3.1
• Proposition 3.2
• proof : Proof of Theorem \ref{['exact_value']}
• Remark 3.3
• Example 3.4
• Theorem 4.1
• Proposition 4.2: McCarthy1996