www.mathphysics.com

Anchor text: A Short History of Operator Theory by Evans M. Harrell II

Wikipedia: Spectral theorem

Anchor text: Derivation of the heat equation

Wikipedia: Heat equation

Anchor text: Evans M. Harrell II

Wikipedia: Spectral theory

Anchor text: A Short History of Operator Theory

Wikipedia: Erik Ivar Fredholm

Anchor text: History of Operator Theory

Wikipedia: Operator theory