The general principles of quantum theory are expressed in terms of observables (measurable things), which are represented by linear operators on a Hilbert space. Hilbert spaces are introduced in article 90771. Each closed subspace of a Hilbert space has an associated projection operator, a special kind of linear operator. In quantum theory, projection operators are used to represent possible outcomes of a measurement. This article reviews some basic definitions and constructions associated with linear operators, with emphasis on projection operators.
