What does $K \Subset U$ mean? I think that $K$ is a compact set with, $K \subset \subset U$ (precompact), where $U$ is an open set. Am I right? Thank you.
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I have seen $K\subset\subset G$ if $K$ is compact, $G$ is open and $K\subseteq G$. I have seen it used in the context of locally compact topological spaces.