Borcea's normal-matrix invertibility conjecture

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Let AA be a normal matrix, let v(0),…,v(n−1)\mathbf v^{(0)},\dots,\mathbf v^{(n-1)} be an orthonormal basis, and let PℓP_\ell denote the orthogonal projection onto v(ℓ)\mathbf v^{(\ell)}. Write

Bℓ=PℓAPℓ∗B_\ell=P_\ell A P_\ell^*

for the corresponding compressions.

Borcea's normal-matrix invertibility conjecture. If every eigenvalue of every BℓB_\ell lies outside the unit disk and

∥(I−Pℓ)APℓ∥≤1\|(I-P_\ell)AP_\ell\|\leq1

for all ℓ\ell, then AA is invertible.

The statement is identified as the matrix-theoretic reduction of Statement (i), and hence as equivalent to the 2-variance conjecture in the surrounding discussion. The source provides no proof or disproof.

References

Primary source

Dmitry Khavinson, Rajesh Pereira, Mihai Putinar, Edward B. Saff and Serguei Shimorin, “Borcea's variance conjectures on the critical points of polynomials”, arXiv:1010.5167 (2011).

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