Definition:Residual Spectrum of Densely-Defined Linear Operator

Definition
Let $\struct {\HH, \innerprod \cdot \cdot}$ be a Hilbert space over $\C$.

Let $\struct {\map D T, T}$ be a densely-defined linear operator.

We define the residual spectrum $\map {\sigma_r} T$ as the set of $\lambda \in \C$ such that:


 * $T - \lambda I$ is injective but $\map {\paren {T - \lambda I} } {\HH}$ is not everywhere dense in $\HH$.