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---
title: Test
parent: Test
grand_parent: Spectral Theory
nav_order: 2
---
# {{ page.title }}
{: .definition-title }
> Definition (resolvent operator, regular value, resolvent set, spectrum, spectral value)
>
> Let $T : \dom{T} \to X$ be an operator in a complex normed space $X$.
> We write
>
> $$
> T_{\lambda} = T - \lambda = T - \lambda I,
> $$
>
> where $\lambda$ is a complex number and
> $I$ is the identical operator on the domain of $T$.
> If the operator $T_{\lambda}$ is injective,
> that is, it has an inverse $T_{\lambda}^{-1}$
> (with domain $\ran{T_{\lambda}}$),
> then we call
>
> $$
> R_{\lambda}(T) = T_{\lambda}^{-1} = (T - \lambda)^{-1} = (T - \lambda I)^{-1}
> $$
>
> the *resolvent operator* of $T$ for $\lambda$.
> A *regular value* of $T$ is a complex number $\lambda$ for which the resolvent $R_{\lambda}(T)$ exists,
> has dense domain and is bounded.
> The set of all regular values of $T$ is called the *resolvent set* of $T$ and denoted $\rho(T)$.
> The complement of the resolvent set in the complex plane
> is called the *spectrum* of $T$ and denoted $\sigma(T)$.
> The elements of the spectrum of $T$ are called the *spectral values* of $T$.
{% definition Point Spectrum, Residual Spectrum, Continuous Spectrum %}
Let $T : \dom{T} \to X$ be an operator in a complex normed space $X$.
The *point spectrum* $\pspec{T}$ is the set of all $\lambda \in \CC$
for which the resolvent $R_\lambda(T)$ does not exist.
The *residual spectrum* $\rspec{T}$ is the set of all $\lambda \in \CC$
for which the resolvent $R_\lambda(T)$ exists, but is not densely defined.
The *continuous spectrum* $\cspec{T}$ is the set of all $\lambda \in \CC$
for which the resolvent $R_\lambda(T)$ exists and is densely defined, but unbounded.
{% enddefinition %}
By definition, the sets $\pspec{T}$, $\rspec{T}$, $\cspec{T}$ and $\rho(T)$
form a partition of the complex plane.
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