# Nowhere-dense set

2020 Mathematics Subject Classification: *Primary:* 54A05 *Secondary:* 54C05 [MSN][ZBL]

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.

#### References

[AP] | A.V. Arkhangel'skii, V.I. Ponomarev, "Fundamentals of general topology: problems and exercises" , Reidel (1984) |

[Ox] | J.C. Oxtoby, "Measure and category" , Springer (1971) MR0393403 Zbl 0217.09201 |

[Ke] | J.L. Kelley, "General topology" , v. Nostrand (1955) MR0070144 Zbl 0066.1660 |

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Nowhere-dense set.

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