Status Updates From Spectra and Pseudospectra: ...

"Quantum mechanics helped make the idea of group velocity famous. By contrast, quantum mechanics helped keep the idea of pseudospectra hidden, for quantum mechanical operators are Hermitian, and their Hermitian behavior shaped scientists' conceptions of eigenvalues and eigenvectors for generations." (p. 497)

Way to go quantum mechanics.

Hessenberg is quite different from Heisenberg

Contours integrals of the matrix resolvent in the complex plane around eigenvalues give you their spectral projector. Just sayin.

Distances to normality, singularity, and instability are all strikingly straightforward while not without deeply undermining counterexamples. Skimming pages 84 to 420 to catch on my return trip through the book...

"Those who find it worthy of remark that the word 'eigenvalue' is a blend of two languages may take pleasure in noting that 'pseudoeigenvalue', for better or worse, is a combination of three."

"In the highly non-normal case, vivid though the image may be, the location of the eigenvalues may be as fragile an indicator of underlying character as the hair color of a Hollywood actor." p. 11