nLab: Spectral Theory
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Short nLab entry defining spectral theory as the study of spectra of linear operators on infinite-dimensional spaces, linking to the Gelfand–Naimark theorem, quantum mechanics and spectral flow, followed by an extensive topic-sorted bibliography useful for finding primary references.
More resources on Spectral Theory
Wolfram MathWorld
MathWorld is an online mathematics encyclopedia from Wolfram Research offering detailed, browsable articles on topics across the math spectrum, including algebra, geometry, calculus, and number theory. Each entry includes definitions, theorems, formulas, diagrams, worked examples, and links to further reading.
Spectral Theory and Its Applications
Graduate text by Bernard Helffer on spectral theory of linear operators on Hilbert space, covering unbounded operators, the spectral theorem, the max–min principle, Schrödinger operators and the Dirichlet Laplacian. Prepares readers to analyse eigenvalue problems arising in mathematical physics.
A Short Course on Spectral Theory
Compact graduate text (Springer GTM 209) developing spectral theory from Banach algebras and the Gelfand transform through the spectral theorem for normal operators, compact and Fredholm operators, and index theory. Readers finish able to follow modern operator-theory literature.