diff --git a/properties/P000130.md b/properties/P000130.md index a108331cb..76d661a49 100644 --- a/properties/P000130.md +++ b/properties/P000130.md @@ -12,4 +12,8 @@ Given as condition (3) in {{wikipedia:Locally_compact_space}}. See also the art ---- #### Meta-properties +- This property is hereditary with respect to open sets. +- This property is hereditary with respect to closed sets. +- This property is hereditary with respect to locally closed sets (equivalent to previous two meta-properties). +A set $A \subseteq X$ is called [*locally closed*](https://en.wikipedia.org/wiki/Locally_closed_subset) if every $x \in A$ has neighbourhood $U$ with $U \cap A$ closed in $U$ (equivalently, $A$ is the intersection of an open set and a closed set). - This property is preserved by arbitrary disjoint unions. diff --git a/properties/P000170.md b/properties/P000170.md index f23e4aaa7..8ed05a52e 100644 --- a/properties/P000170.md +++ b/properties/P000170.md @@ -23,3 +23,8 @@ Equivalently, every {P16} subspace is closed in $X$ and {P130}. The property is defined in section 2 of {{doi:10.1007/BF02194829}}, where Theorem 2.1 shows the equivalences above. Note: this property is not to be confused with other variants of k-Hausdorff (e.g. {P171}), where k-closed is defined in terms of different notions of compactly generated space or k-space. + +---- +#### Meta-properties + +- This property is hereditary. diff --git a/theorems/T000813.md b/theorems/T000813.md new file mode 100644 index 000000000..443ae64cc --- /dev/null +++ b/theorems/T000813.md @@ -0,0 +1,32 @@ +--- +uid: T000813 +if: + and: + - P000167: true + - P000051: true + - P000170: true +then: + P000136: true +--- + +Let $K$ be a {P16} subset of $X$. Then +$K$ is {P3} (since $X$ is {P170}), +and $K$ also {P167} and {P51}. +To show that $K$ is finite, without loss of generality we can assume that $X$ itself +is {P16}, {P3}, {P167} and {P51} +and show that $X$ is finite. + +If every point of $X$ is isolated, $X$ is finite by compactness. +Otherwise, because $X$ is {P51}, choose $x \in X$ with Cantor–Bendixson rank $1$ (that is, $x\in X'\setminus X''$). +So $x$ is not isolated in $X$ and there is a neighbourhood $L$ of $x$ +such that all point in $L\setminus\{x\}$ are isolated in $X$. +Since $X$ is {P11}, we can assume $L$ is closed in $X$. +The neighbourhood $L$ must be infinite, +because otherwise $x$ would be isolated. + +Take a countably infinite set $M\subseteq L$ containing $x$. +Since $L$ is {P203}, $M$ is closed in $L$, hence closed in $X$ and compact. +And since $M$ is {P167} and {P16}, +it is not {P181} +[(Explore)](https://topology.pi-base.org/spaces?q=167+%2B+16+%2B+181), +which is a contradiction.