A subset $A$ of topological space $X$ is nowhere dense if, for every nonempty open $U\subset X$, the intersection $U\cap A$ is not dense in $U$. Common equivalent definitions are:
- For every nonempty open set $U\subset X$, the interior of $U\setminus A$ is not empty.
- The closure of $A$ has empty interior.
- The complement of the closure of $A$ is dense.
In an infinite-dimensional Hilbert space, every compact subset is nowhere dense. The same holds for infinite-dimensional Banach spaces, non-locally-compact Hausdorff topological groups, and products of infinitely many non-compact Hausdorff topological spaces.
The Baire Category theorem asserts that if $X$ is a complete metric space or a locally compact Hausdorff space, then the complement of a countable union of nowhere dense sets is always nonempty.
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|[Ke]||J.L. Kelley, "General topology" , v. Nostrand (1955) MR0070144 Zbl 0066.1660|
Nowhere-dense set. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Nowhere-dense_set&oldid=28115