Propagation of excess normal defect for cyclic matrices

From papers

Let ACn×nA \in \mathbb{C}^{n\times n} and A~C(n+1)×(n+1)\tilde{A}\in \mathbb{C}^{(n+1)\times (n+1)} with n4n \geq 4 be the matrices defined by

A=(0a10a2an1an0),A~=(0a10a2an1an00).A = \begin{pmatrix} 0 & a_1 & & \cdots & 0 \\ & & a_2 & & \\ \vdots & & & \ddots & \vdots \\ & & & & a_{n-1} \\ a_n & & & \cdots & 0 \end{pmatrix},\qquad \tilde{A} = \begin{pmatrix} 0 & a_1 & & \cdots & & 0 \\ & & a_2 & & \\ \vdots & & & \ddots & & \vdots \\ & & & & a_{n-1} & \\ & & & & & a_n \\ 0 & & & \cdots & & 0 \end{pmatrix}.

Here nd\operatorname{nd} is the normal defect and ε\varepsilon is the positive index of inertia of the commutator used in the paper. Propagation conjecture. If

nd(A)=ε(A)+1,\operatorname{nd}(A)=\varepsilon(A)+1,

then

nd(A~)=ε(A~)+1.\operatorname{nd}(\tilde{A})=\varepsilon(\tilde{A})+1.

The claim is motivated by an explicit example and by the apparent structural similarity between normal completions of AA and A~\tilde{A}; its general validity is not established.

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Sources & referencesView supporting material

Primary source

Ryan D. Wasson and Hugo J. Woerdeman, “The Normal Defect of Some Classes of Matrices”, arXiv:1211.4145 (2012).

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