➗Abstract Linear Algebra II Unit 5 – Spectral Theory

Key Concepts and Definitions

• Spectrum of an operator $T$ denoted as $\sigma(T)$ consists of all scalars $\lambda$ for which the operator $T - \lambda I$ is not invertible
• Point spectrum $\sigma_p(T)$ includes all eigenvalues of the operator $T$
• Continuous spectrum $\sigma_c(T)$ contains all $\lambda$ for which $T - \lambda I$ is injective, has a dense range, but the range is not the entire space
• Residual spectrum $\sigma_r(T)$ comprises all $\lambda$ for which $T - \lambda I$ is injective, but its range is not dense
  • The union of these three disjoint sets forms the entire spectrum: $\sigma(T) = \sigma_p(T) \cup \sigma_c(T) \cup \sigma_r(T)$
• Resolvent set $\rho(T)$ is the complement of the spectrum in the complex plane: $\rho(T) = \mathbb{C} \setminus \sigma(T)$
• Resolvent operator $R_\lambda(T) = (T - \lambda I)^{-1}$ is a bounded operator defined for all $\lambda$ in the resolvent set
• Spectral radius $r(T)$ is the supremum of the absolute values of the elements in the spectrum: $r(T) = \sup\{|\lambda| : \lambda \in \sigma(T)\}$

Spectral Theorem for Finite-Dimensional Spaces

• States that a normal operator $T$ on a finite-dimensional complex vector space $V$ is diagonalizable
  • A normal operator satisfies $T^*T = TT^*$, where $T^*$ is the adjoint of $T$
• Implies the existence of an orthonormal basis of eigenvectors for $T$
• Allows the representation of $T$ as a diagonal matrix with respect to this basis
  • The diagonal entries are the eigenvalues of $T$
• Provides a canonical form for normal operators in finite dimensions
• Enables the decomposition of the space into invariant subspaces corresponding to the eigenvalues
• Facilitates the analysis and understanding of the operator's behavior
• Finds applications in various fields, such as quantum mechanics and matrix analysis

Spectral Theory for Compact Operators

• Compact operators are a class of linear operators that map bounded sets to relatively compact sets
• The spectrum of a compact operator consists only of eigenvalues and possibly 0
  • The eigenvalues are countable and can only accumulate at 0
• Eigenvectors corresponding to distinct eigenvalues are orthogonal
• For a self-adjoint compact operator $T$, the spectral theorem provides a decomposition: $T = \sum_{n=1}^\infty \lambda_n \langle \cdot, e_n \rangle e_n$
  • $\lambda_n$ are the eigenvalues, and $e_n$ are the corresponding orthonormal eigenvectors
• The spectral theorem for compact operators extends to infinite-dimensional spaces
• Compact operators have a well-defined trace, given by the sum of their eigenvalues: $\text{tr}(T) = \sum_{n=1}^\infty \lambda_n$
• The Fredholm alternative characterizes the solvability of equations involving compact operators

Spectral Theory for Unbounded Operators

• Unbounded operators are linear operators that are not necessarily bounded or defined on the entire space
• The spectrum of an unbounded operator can be more complex and may include continuous and residual parts
• Self-adjoint unbounded operators admit a spectral decomposition using the spectral measure
  • The spectral measure assigns a projection-valued measure to Borel subsets of the real line
• The spectral theorem for unbounded self-adjoint operators provides a representation: $T = \int_{\mathbb{R}} \lambda dE(\lambda)$
  • $E(\lambda)$ is the spectral measure, and the integral is understood in the sense of strong operator convergence
• Unbounded operators often arise in quantum mechanics, describing observables with unbounded spectra
• The domain of an unbounded operator plays a crucial role in its properties and spectral analysis
• Techniques such as the Friedrichs extension and the theory of quadratic forms are used to study unbounded operators

Applications in Quantum Mechanics

• Quantum mechanics heavily relies on the mathematical framework of Hilbert spaces and linear operators
• Observables in quantum mechanics are represented by self-adjoint operators
  • The spectrum of an observable corresponds to the possible measurement outcomes
• The spectral theorem allows the decomposition of a quantum state into eigenstates of an observable
  • The eigenvalues represent the possible measurement results, and the eigenstates are the corresponding states
• The time evolution of a quantum system is described by unitary operators, which have a spectral decomposition
• The spectral theory of unbounded operators is essential for dealing with observables with continuous spectra (position, momentum)
• Entanglement and quantum correlations can be studied using the spectral properties of density operators
• The spectral analysis of Hamiltonians provides insights into the energy levels and dynamics of quantum systems

Computational Methods and Examples

• Numerical algorithms for computing eigenvalues and eigenvectors are crucial for practical applications
  • Examples include the power method, QR algorithm, and Lanczos algorithm
• The singular value decomposition (SVD) is a generalization of the spectral theorem for non-square matrices
  • SVD has applications in data compression, dimensionality reduction, and matrix approximation
• The fast Fourier transform (FFT) is an efficient algorithm for computing the discrete Fourier transform, which is related to the spectral decomposition of the shift operator
• Spectral methods in numerical analysis use the eigenvalues and eigenfunctions of differential operators to solve partial differential equations
• The spectral theory of graphs studies the eigenvalues and eigenvectors of the adjacency matrix or Laplacian matrix associated with a graph
  • It has applications in network analysis, data clustering, and graph partitioning
• Spectral clustering is a technique that uses the eigenvectors of a similarity matrix to partition data into clusters

Advanced Topics and Extensions

• The functional calculus allows the definition of functions of operators based on their spectral decomposition
  • It provides a way to extend scalar functions to operators while preserving algebraic properties
• The spectral theory of random matrices studies the statistical properties of eigenvalues and eigenvectors of matrices with random entries
  • It has applications in physics, statistics, and number theory
• The spectral theory of differential operators investigates the eigenvalues and eigenfunctions of linear differential operators
  • It plays a central role in the study of partial differential equations and quantum mechanics
• The spectral theory of Banach algebras extends the notion of spectrum to elements of a Banach algebra
  • It provides a framework for studying the invertibility and properties of operators in a more general setting
• Noncommutative spectral theory deals with the spectral properties of operators in noncommutative algebras, such as von Neumann algebras and C*-algebras
• The spectral theory of semigroups studies the spectral properties of one-parameter families of operators, which describe the evolution of a system over time

Common Pitfalls and Misconceptions

• Confusing the spectrum with the set of eigenvalues
  • The spectrum includes eigenvalues but may also contain continuous and residual parts
• Assuming that every operator is diagonalizable or has a complete set of eigenvectors
  • Not all operators, even in finite dimensions, are diagonalizable (consider the nilpotent matrix)
• Misinterpreting the meaning of the continuous spectrum
  • The continuous spectrum does not imply a continuous set of eigenvalues but rather the absence of eigenvalues in that part of the spectrum
• Overlooking the importance of the domain when dealing with unbounded operators
  • Unbounded operators are not defined on the entire space, and their properties depend on the choice of domain
• Misapplying the spectral theorem to non-normal or non-self-adjoint operators
  • The spectral theorem holds for normal operators in finite dimensions and self-adjoint operators in general
• Confusing the algebraic and geometric multiplicities of eigenvalues
  • The algebraic multiplicity is the multiplicity of an eigenvalue as a root of the characteristic polynomial, while the geometric multiplicity is the dimension of the corresponding eigenspace
• Misunderstanding the convergence of spectral expansions
  • Spectral expansions, such as the eigenfunction expansion for compact operators, may converge in different senses (pointwise, uniform, or in norm)