Resolvent set

In linear algebra and operator theory, the resolvent set of a linear operator is a set of complex numbers for which the operator is in some sense "well-behaved". The resolvent set plays an important role in the resolvent formalism.

Definitions

Let X be a Banach space and let $L\colon D(L)\rightarrow X$ be a linear operator with domain $D(L)\subseteq X$. Let id denote the identity operator on X. For any $\lambda \in \mathbb {C}$, let

$L_{\lambda }=L-\lambda \,\mathrm {id} .$

A complex number $\lambda$ is said to be a regular value if the following three statements are true:

1. $L_{\lambda }$ is injective, that is, the corestriction of $L_{\lambda }$ to its image has an inverse $R(\lambda ,L)=(L-\lambda \,\mathrm {id} )^{-1}$ called the resolvent;
2. $R(\lambda ,L)$ is a bounded linear operator;
3. $R(\lambda ,L)$ is defined on a dense subspace of X, that is, $L_{\lambda }$ has dense range.

The resolvent set of L is the set of all regular values of L:

$\rho (L)=\{\lambda \in \mathbb {C} \mid \lambda {\mbox{ is a regular value of }}L\}.$

The spectrum is the complement of the resolvent set

$\sigma (L)=\mathbb {C} \setminus \rho (L),$

and subject to a mutually singular spectral decomposition into the point spectrum (when condition 1 fails), the continuous spectrum (when condition 2 fails) and the residual spectrum (when condition 3 fails).

If $L$ is a closed operator, then so is each $L_{\lambda }$, and condition 3 may be replaced by requiring that $L_{\lambda }$ be surjective.

Properties

• The resolvent set $\rho (L)\subseteq \mathbb {C}$ of a bounded linear operator L is an open set.
• More generally, the resolvent set of a densely defined closed unbounded operator is an open set.

References

• Reed, M.; Simon, B. (1980). Methods of Modern Mathematical Physics: Vol 1: Functional analysis. Academic Press. ISBN 978-0-12-585050-6.
• Renardy, Michael; Rogers, Robert C. (2004). An introduction to partial differential equations. Texts in Applied Mathematics 13 (Second ed.). New York: Springer-Verlag. xiv+434. ISBN 0-387-00444-0. MR 2028503 (See section 8.3)

External links

• Voitsekhovskii, M.I. (2001) [1994], "Resolvent set", Encyclopedia of Mathematics, EMS Press

See also

• Resolvent formalism
• Spectrum (functional analysis)
• Decomposition of spectrum (functional analysis)