Oppositely transformed matrices can be made hollow and almost hollow

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Let A,B∈Rn×nA,B\in\mathbb{R}^{n\times n} satisfy

trace⁡A=trace⁡B=0.\operatorname{trace} A=\operatorname{trace} B=0.

A matrix is hollow if all its diagonal entries are zero, and almost hollow if it has the corresponding property defined earlier in the paper. The transformations considered below act oppositely on the two matrices.

Hollowisation conjecture. There exists an orthogonal matrix V∈Rn×nV\in\mathbb{R}^{n\times n} such that VTAVV^TAV is hollow and VBVTVBV^T is almost hollow.

Here the transformation applied to AA is the inverse, equivalently the adjoint, of the transformation applied to BB, unlike in the preceding simultaneous-transformation proposition. The paper states this variant because it appears to hold numerically, but does not provide a proof.

References

Primary source

Tobias Damm and Heike Fassbender, “Simultaneous hollowisation, joint numerical range, and stabilization by noise”, arXiv:1910.08813 (2020).

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