Lecture 22: The Spectral Theorem for a Compact SelfAdjoint Operator  Summary and Q&A
TL;DR
This content explains the spectral theory for selfadjoint compact operators, including the spectrum, eigenvalues, and eigenvectors.
Questions & Answers
Q: What is the spectrum of a bounded linear operator?
The spectrum of a bounded linear operator is the set of complex numbers for which the operator minus that number times the identity is not invertible. It consists of the numbers that make the operator noninvertible.
Q: Are the eigenvalues of a selfadjoint matrix real?
Yes, in the case of selfadjoint matrices, the eigenvalues are always real.
Q: Can selfadjoint compact operators be diagonalized?
Yes, similar to selfadjoint matrices, selfadjoint compact operators can be diagonalized in an orthonormal basis of eigenvectors.
Q: What does the Fredholm alternative theorem state for selfadjoint compact operators?
The Fredholm alternative for selfadjoint compact operators states that either the operator minus a given eigenvalue is bijective, or the null space of the operator corresponds to the eigenvalue and is nontrivial and finite dimensional.
Summary & Key Takeaways

Spectral theory is a generalization of eigenvalues for matrices to bounded linear operators. It involves the spectrum, which is the set of numbers for which the operator is not invertible.

In the case of selfadjoint operators, the spectrum consists of the eigenvalues, and the operator can be diagonalized in an orthonormal basis of eigenvectors.

Compact operators, which are limits of finite rank operators, also have similar properties, with countably infinite eigenvalues that converge to zero.