Super-identical pseudospectra
Maxime Fortier Bourque, Thomas Ransford · Journal of the London Mathematical Society · 2009
The complex N × N matrices A and B are said to have super-identical pseudospectra if, for each z ∈ ℂ, the singular values of A − zI are the same as those of B − zI. We explore this condition and its consequences. On the positive side, drawing on ideas from invariant theory, we prove that there exists an integer m = m(N) such that ‘almost every’ m-tuple of N × N matrices with super-identical pseudospectra contains a pair that are unitarily equivalent. On the negative side, we present an example of a pair of non-derogatory 4 × 4 matrices A and B with super-identical pseudospectra such that ||A2|| ≠ ||B2||.