Let $a\in \mathcal{L}(L^2([0, 1])$ be a multiplication operator. I wonder whether there is any work on calculating its essential spectrum. Is there any way to explicitly compute its essential growth bound and elements of its discrete spectrum? What about the $n$-dimensional case, i.e., $a\in \mathcal{L}(L^2([0, 1], \mathbb{R}^{n\times n})$?