 Spectrum (functional analysis)

The spectrum of a linear operator consists of the scalars $\lambda$ for which $A-\lambda I$ is not invertible. In finite dimensions it is the set of eigenvalues.

In functional analysis, the notion of the spectrum of an operator is a fundamental concept that extends the idea of eigenvalues from finite-dimensional linear algebra to more general settings, particularly in the study of bounded linear operators on Banach spaces and Hilbert spaces.

Set of eigenvalues of a linear operator.