In
mathematics, the pseudospectrum of an
operator is a
set containing the
spectrum of the operator and the numbers that are "almost"
eigenvalues. Knowledge of the pseudospectrum can be particularly useful for understanding
non-normal operators and their eigenfunctions.
The ε-pseudospectrum of a matrix A consists of all eigenvalues of matrices which are ε-close to A:[1]
Lloyd N. Trefethen and Mark Embree: "Spectra And Pseudospectra: The Behavior of Nonnormal Matrices And Operators", Princeton Univ. Press,
ISBN978-0691119465 (2005).