I read at the top of the page $580$ of [this][1] paper that the imbedding $W^{1,2}(\Gamma,d\mu) \hookrightarrow L^2(\Gamma,d\mu)$ is compact for a bounded domain $\Gamma \subset \mathbb{R}^n$ and a measure $\mu$ as defined in the second paragraph of the section $2$. I would like a reference for this result.

Thanks in advance!

  [1]: https://core.ac.uk/download/pdf/82623799.pdf