# Diagonalizable Operator Compact iff Value Set Converges to Zero

## Theorem

Let $H$ be a Hilbert space of countable dimension.

Let $A: H \to H$ be a diagonalizable operator.

Let $\sequence {\alpha_n}_{n \mathop \in \N}$ be the value set of $A$, with respect to a suitable basis $E = \sequence {e_n}_{n \mathop \in \N}$ for $H$.

Then $A$ is compact if and only if:

$\ds \lim_{n \mathop \to \infty} \alpha_n = 0$