There is considerable divergence in the literature concerning the definition of the essential spectrum of a densely defined closed operator $A$ on a Banach space $X$. I want to find a direct definition of the essential spectrum such that it is the complement of the discrete spectrum and is stable under compact perturbations. It seems to me that I have succeeded, but I still have one problem that I really what to know the proof.
It seems that we always define the discrete spectrum $\sigma_{\text{d}}(A)$ of $A$ by $$\sigma_{\text{d}}(A):=\{\lambda\in\mathbb C: \lambda \text{ is an isolated eigenvalue of }\ A \text{ and has finite algebraic multiplicity}\}.\tag 1$$ Here the algebraic multiplicity of an isolated eigenvalue $\lambda$ is defined as the dimension of the range of the Riesz projection $P_\lambda$.
This is a relatively long post. I wrote almost all things that I have done relating to this topic. In the end of the post, I will write a self-contained statement of my problem. If you want to save time, you can skip the heavy body of the post.
For a self-adjiont operator $A$ on a Hilbert space $X$, we can define the essential spectrum $\sigma_{\text{ess}}(A)$ by $\sigma_{\text{ess}}(A):=\sigma(A)\setminus \sigma_{\text{d}}(A)$. Then we can use Weyl's criterion to prove that the essential spectrum of self-adjiont operators is invariant under symmetric compact perturbations.
However, when it comes to general densely defined closed operators on Banach spaces, things become complicated. Kato defined in Chapter 4 of his book Perturbation of Linear Operators that $$\sigma_{\text{ess}}(A):=\sigma(A)\setminus \{\lambda\in \mathbb C: \lambda I-A \text{ is semi-Fredholm}\}.$$ Then Theorem 5.35 in Chapter 4 proves the stability of $\sigma_{\text{ess}}(A)$ under compact perturbations. But as Therorem 5.33 implies, the relation $\sigma_{\text{ess}}(A)=\sigma(A)\setminus \sigma_{\text{d}}(A)$ is not always right.
I searched for some materials and finally find out that the definition given by F.Wolf should be satisfying. He wrote: The spectrum $\sigma(A)$ of an operator $A$ can be divided into two parts: The essential spectrum $\sigma_e(A)$ consisting of the points $\lambda$ at which $\Re(\lambda I-A)$, the range of $\lambda I-A$ is not closed and of eigenvalues of infinite multipilicity. The second part may be called the Fredholm part of $\sigma(A)$ consists beside others of $\rho(A)$, the resolvent set of $A$ and of isolated eigenvalues of finite multiplicity. But I also find that in another paper the author rewrote Wolf's definition of essential spectrum as the subset of $\sigma(A)$ consisting those $\lambda$ such that $\lambda I-A$ is not Fredholm. These two definitions seem different at the first glance, and I wonder why they are the same.
My Problem. Let $A$ be a densely defined closed operator $A$ on a Banach space $X$ and define $\sigma_\text{d}(A)$ as in (1). Prove that $$\sigma_\text{d}(A)=\{\lambda\in\sigma(A): \lambda I-A \text{ is Fredholm}\}.$$ That is to say, $\sigma_\text{d}(A)$ consists of those complex $\lambda\in\sigma(A)$ for which
- $\Re(\lambda I-A)$, the range of $\lambda I-A$, is closed,
- $\text{dim}\ N(\lambda I-A)$, the geometric multiplicity of $\lambda$, is finite,
- $\text{dim}\ \left(X/\Re(\lambda I-A)\right)$, the codimension of $\Re(\lambda I-A)$, is finite.
Any hints or useful references are welcome!