The spectrum of a linear operator that operates on a Banach space (a fundamental concept of functional analysis) consists of all scalars such that the operator does not have a bounded inverse on . The spectrum has a standard **decomposition** into three parts:

- a
**point spectrum**, consisting of the eigenvalues of ; - a
**continuous spectrum**, consisting of the scalars that are not eigenvalues but make the range of a proper dense subset of the space; - a
**residual spectrum**, consisting of all other scalars in the spectrum.

This decomposition is relevant to the study of differential equations, and has applications to many branches of science and engineering. A well-known example from quantum mechanics is the explanation for the discrete spectral lines and the continuous band in the light emitted by excited atoms of hydrogen.

Let *X* be a Banach space, *B*(*X*) the family of bounded operators on *X*, and *T* ∈ *B*(*X*). By definition, a complex number *λ* is in the **spectrum** of *T*, denoted *σ*(*T*), if *T* − *λ* does not have an inverse in *B*(*X*).

If *T* − *λ* is one-to-one and onto, then its inverse is bounded; this follows directly from the open mapping theorem of functional analysis. So, *λ* is in the spectrum of *T* if and only if *T* − *λ* is either not one-to-one or not onto. One distinguishes three separate cases:

*T*−*λ*is not injective. That is, there exist two distinct elements*x*,*y*in*X*such that (*T*−*λ*)(*x*) = (*T*−*λ*)(*y*). Then*z*=*x*−*y*is a non-zero vector such that*T*(*z*) =*λz*. In other words,*λ*is an eigenvalue of*T*in the sense of linear algebra. In this case,*λ*is said to be in the**point spectrum**of*T*, denoted*σ*_{p}(*T*).*T*−*λ*is injective, and its range is a dense subset*R*of*X*; but is not the whole of*X*. In other words, there exists some element*x*in*X*such that (*T*−*λ*)(*y*) can be as close to*x*as desired, with*y*in*X*; but is never equal to*x*. It can be proved that, in this case,*T*−*λ*is not bounded below (i.e. it sends far apart elements of*X*too close together). Equivalently, the inverse linear operator (*T*−*λ*)^{−1}, which is defined on the dense subset*R*, is not a bounded operator, and therefore cannot be extended to the whole of*X*. Then*λ*is said to be in the**continuous spectrum**,*σ*(_{c}*T*), of*T*.*T*−*λ*is injective but does not have dense range. That is, there is some element*x*in*X*and a neighborhood*N*of*x*such that (*T*−*λ*)(*y*) is never in*N*. In this case, the map (*T*−*λ*)^{−1}*x*→*x*may be bounded or unbounded, but in any case does not admit a unique extension to a bounded linear map on all of*X*. Then*λ*is said to be in the**residual spectrum**of*T*,*σ*(_{r}*T*).

So *σ*(*T*) is the disjoint union of these three sets,

The spectrum of an unbounded operator can be divided into three parts in the same way as in the bounded case, but because the operator is not defined everywhere, the definitions of domain, inverse, etc. are more involved.

Given a σ-finite measure space (*S*, *Σ*, *μ*), consider the Banach space *L ^{p}*(

The operator norm of *T* is the essential supremum of *h*. The essential range of *h* is defined in the following way: a complex number *λ* is in the essential range of *h* if for all *ε* > 0, the preimage of the open ball *B _{ε}*(

If *λ* is not in the essential range of *h*, take *ε* > 0 such that *h*^{−1}(*B _{ε}*(

This shows *T _{h}* −

If *λ* is such that *μ*( *h*^{−1}({*λ*})) > 0, then *λ* lies in the point spectrum of *T _{h}* as follows. Let

so λ is an eigenvalue of *T*_{h}.

Any *λ* in the essential range of *h* that does not have a positive measure preimage is in the continuous spectrum of *T _{h}*. To show this, we must show that

Direct calculation shows that *f _{n}* ∈

in the *L ^{p}*(

Therefore, multiplication operators have no residual spectrum. In particular, by the spectral theorem, normal operators on a Hilbert space have no residual spectrum.

In the special case when *S* is the set of natural numbers and *μ* is the counting measure, the corresponding *L ^{p}*(

For 1 < *p* < ∞, *l ^{p}* is reflexive. Define the left shift

*T* is a partial isometry with operator norm 1. So *σ*(*T*) lies in the closed unit disk of the complex plane.

*T** is the right shift (or unilateral shift), which is an isometry on *l ^{q}*, where 1/

For *λ* ∈ **C** with |*λ*| < 1,

and *T x* = *λ x*. Consequently, the point spectrum of *T* contains the open unit disk. Now, *T** has no eigenvalues, i.e. *σ _{p}*(

The spectrum of a bounded operator is closed, which implies the unit circle, { |*λ*| = 1 } ⊂ **C**, is in *σ*(*T*). Again by reflexivity of *l ^{p}* and the theorem given above (this time, that

then

which cannot be in *l ^{p}*, a contradiction. This means the unit circle must lie in the continuous spectrum of

So for the left shift *T*, *σ _{p}*(

For *p* = 1, one can perform a similar analysis. The results will not be exactly the same, since reflexivity no longer holds.

Hilbert spaces are Banach spaces, so the above discussion applies to bounded operators on Hilbert spaces as well. A subtle point concerns the spectrum of *T**. For a Banach space, *T** denotes the transpose and *σ*(*T**) = *σ*(*T*). For a Hilbert space, *T** normally denotes the adjoint of an operator *T* ∈ *B*(*H*), not the transpose, and *σ*(*T**) is not *σ*(*T*) but rather its image under complex conjugation.

For a self adjoint *T* ∈ *B*(*H*), the Borel functional calculus gives additional ways to break up the spectrum naturally.

This subsection briefly sketches the development of this calculus. The idea is to first establish the continuous functional calculus then pass to measurable functions via the Riesz-Markov representation theorem. For the continuous functional calculus, the key ingredients are the following:

- 1. If
*T*is self adjoint, then for any polynomial*P*, the operator norm satisfies

- 2. The Stone-Weierstrass theorem, which implies that the family of polynomials (with complex coefficients), is dense in
*C*(*σ*(*T*)), the continuous functions on*σ*(*T*).

The family *C*(*σ*(*T*)) is a Banach algebra when endowed with the uniform norm. So the mapping

is an isometric homomorphism from a dense subset of *C*(*σ*(*T*)) to *B*(*H*). Extending the mapping by continuity gives *f*(*T*) for *f* ∈ C(*σ*(*T*)): let *P _{n}* be polynomials such that

For a fixed *h* ∈ *H*, we notice that

is a positive linear functional on *C*(*σ*(*T*)). According to the Riesz-Markov representation theorem that there exists a unique measure *μ _{h}* on

This measure is sometimes called the **spectral measure associated to h**. The spectral measures can be used to extend the continuous functional calculus to bounded Borel functions. For a bounded function *g* that is Borel measurable, define, for a proposed *g*(*T*)

Via the polarization identity, one can recover (since *H* is assumed to be complex)

and therefore *g*(*T*) *h* for arbitrary *h*.

In the present context, the spectral measures, combined with a result from measure theory, give a decomposition of *σ*(*T*).

Let *h* ∈ *H* and *μ _{h}* be its corresponding spectral measure on

where *μ*_{ac} is absolutely continuous with respect to the Lebesgue measure, *μ*_{sc} is singular with respect to the Lebesgue measure and atomless,
and *μ*_{pp} is a pure point measure.^{[1]}

All three types of measures are invariant under linear operations. Let *H*_{ac} be the subspace consisting of vectors whose spectral measures are absolutely continuous with respect to the Lebesgue measure. Define *H*_{pp} and *H*_{sc} in analogous fashion. These subspaces are invariant under *T*. For example, if *h* ∈ *H*_{ac} and *k* = *T h*. Let *χ* be the characteristic function of some Borel set in *σ*(*T*), then

So

and *k* ∈ *H*_{ac}. Furthermore, applying the spectral theorem gives

This leads to the following definitions:

- The spectrum of
*T*restricted to*H*_{ac}is called the**absolutely continuous spectrum**of*T*,*σ*_{ac}(*T*). - The spectrum of
*T*restricted to*H*_{sc}is called its**singular spectrum**,*σ*_{sc}(*T*). - The set of eigenvalues of
*T*are called the**pure point spectrum**of*T*,*σ*_{pp}(*T*).

The closure of the eigenvalues is the spectrum of *T* restricted to *H*_{pp}. So

A bounded self adjoint operator on Hilbert space is, a fortiori, a bounded operator on a Banach space. Therefore, one can also apply to *T* the decomposition of the spectrum that was achieved above for bounded operators on a Banach space. Unlike the Banach space formulation,^{[clarification needed]} the union

need not be disjoint. It is disjoint when the operator *T* is of uniform multiplicity, say *m*, i.e. if *T* is unitarily equivalent to multiplication by *λ* on the direct sum

for some Borel measures . When more than one measure appears in the above expression, we see that it is possible for the union of the three types of spectra to not be disjoint. If *λ* ∈ *σ _{ac}*(

When *T* is unitarily equivalent to multiplication by *λ* on

the decomposition of *σ*(*T*) from Borel functional calculus is a refinement of the Banach space case.

The preceding comments can be extended to the unbounded self-adjoint operators since Riesz-Markov holds for locally compact Hausdorff spaces.

In quantum mechanics, observables are self adjoint operators, often not bounded, and their spectra are the possible outcomes of measurements. Absolutely continuous spectrum of a physical observable corresponds to free states of a system, while the pure point spectrum corresponds to bound states. The singular spectrum correspond to physically impossible outcomes. An example of a quantum mechanical observable which has purely continuous spectrum is the position operator of a free particle moving on a line. Its spectrum is the entire real line. Also, since the momentum operator is unitarily equivalent to the position operator, via the Fourier transform, they have the same spectrum.

Intuition may induce one to say that the discreteness of the spectrum is intimately related to the corresponding states being "localized". However, a careful mathematical analysis shows that this is not true. The following is an element of and increasing as .

However, the phenomena of Anderson localization and dynamical localization describe, when the eigenfunctions are localized in a physical sense. Anderson Localization means that eigenfunctions decay exponentially as . Dynamical localization is more subtle to define.

Sometimes, when performing physical quantum mechanical calculations, one encounters "eigenvectors" that do not lie in *L*^{2}(**R**), i.e. wave functions that are not localized. These are the free states of the system. As stated above, in the mathematical formulation, the free states correspond to the absolutely continuous spectrum. Alternatively, if it is insisted that the notion of eigenvectors and eigenvalues survive the passage to the rigorous, one can consider operators on rigged Hilbert spaces.

It was believed for some time that singular spectrum is something artificial. However, examples as the almost Mathieu operator and random Schrödinger operators have shown, that all types of spectra arise naturally in physics.

Let be a closed operator defined on the domain which is dense in *X*. Then there is a decomposition of the spectrum of *A* into a disjoint union,

where

- is the fifth type of the essential spectrum of
*A*(if*A*is a self-adjoint operator, then for all ); - is the discrete spectrum of
*A*, which consists of normal eigenvalues, or, equivalently, of isolated points of such that the corresponding- Point spectrum, the set of eigenvalues.
- Essential spectrum, spectrum of an operator modulo compact perturbations.
- Discrete spectrum (mathematics), the set of normal eigenvalues.
**^**Bogachev, Vladimir (2007).*Measure Theory volume 1*. Springer. p. 344.

- N. Dunford and J.T. Schwartz,
*Linear Operators, Part I: General Theory*, Interscience, 1958. - M. Reed and B. Simon,
*Methods of Modern Mathematical Physics I: Functional Analysis*, Academic Press, 1972.