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Consider a compact operator $T$ acting on a Banach space $X$. Then, the Riesz-Schauder theorem gives us the following properties of the spectrum $\sigma(T)$ of $T$

  1. Every non-zero $\lambda \in \sigma(T)$ is an eigenvalue of $T$ with finite multiplicity.
  2. The eigenvalues can only accumulate at $0$.
  3. $\sigma(T)$ is at most countable.

My question is this - suppose there exists an operator $T$ acting on a Banach space $X$ such that its spectrum has the above three properties. Then, must $T$ be compact? If not, are there any extra assumptions that $T$ should have for it to be compact?

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  • $\begingroup$ It should be that $\sigma (T)$ is "at most countable", not "at most countably finite". The spectrum of a compact operator on a Banach space can be an infinite set. $\endgroup$ Commented Mar 29 at 2:54
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    $\begingroup$ Ah, that was a typo. You're correct. $\endgroup$
    – Ishan Deo
    Commented Mar 29 at 2:56
  • $\begingroup$ What about a projection? math.stackexchange.com/questions/253897/…. Fourier transform also provides a counterxample $\endgroup$ Commented Mar 29 at 8:22
  • $\begingroup$ @EvangelopoulosFoivos Do those examples satisfy the first property? $\endgroup$ Commented Mar 29 at 9:52
  • $\begingroup$ @DeanMiller ah... I misread the first bullet $\endgroup$ Commented Mar 29 at 11:30

1 Answer 1

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It is difficult to impose conditions only on the spectrum of a bounded linear operator that imply the operator in question is compact. There are none that I am aware of and the following example shows that it is not reasonable to expect that there are such conditions.

Define $\tau \colon L^{2}(\mathbb{R}) \to L^{2}(\mathbb{R})$ by $(\tau f)(x) := f(x-1)$. Then $\tau$ is a well-defined isometric isomorphism on $L^{2}(\mathbb{R})$. Define $\phi \colon \mathbb{R} \to \mathbb{R}$ by $\phi (x) := \exp (-x^{2})$. As $\phi \in L^{\infty}(\mathbb{R})$ and $\phi$ is not equal to zero almost everywhere, we have that $M_{\phi}\colon L^{2}(\mathbb{R}) \to L^{2}(\mathbb{R})$ defined by $M_{\phi}f := \phi f$ is a bounded linear operator on $L^{2}(\mathbb{R})$ that is not compact. Now define $A := M_{\phi} \circ \tau$. Then $A$ is a bounded linear operator on $L^{2}(\mathbb{R})$ that is not compact. Some simple calculations show that \begin{equation} \|A^{n}\| = \sup_{x\in\mathbb{R}} \Big|\exp \Big(-\sum_{k=0}^{n-1} (x - k)^{2} \Big)\Big| \leq \exp (- \tfrac{(n-1)^{2}}{4}) \end{equation} for each $n\in\mathbb{N}$ and consequently that $\lim_{n\to\infty} \|A^{n}\|^{\tfrac{1}{n}} = 0$. Hence we have $\sigma (A) = \{0\}$.

This provides an example of a bounded linear operator on a separable Hilbert space that is not compact and yet has spectrum equal to $\{0\}$. In particular, the spectrum of this non-compact operator satisfies the three properties from your question.

However, if, in addition to the assumptions on the spectrum, the operator is a normal (or self-adjoint if the Hilbert space is real) operator on a Hilbert space, then it follows that the operator is compact. See here for a proof.

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    $\begingroup$ Nice example! (+) $\endgroup$ Commented Mar 29 at 6:14

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