diff --git a/properties/P000024.md b/properties/P000024.md index 6fe677d55..5d6c92386 100644 --- a/properties/P000024.md +++ b/properties/P000024.md @@ -15,4 +15,13 @@ Each point has a local base of neighborhoods with compact closure. Equivalently (see Condition 2 of {{wikipedia:Locally_compact_space}}), every point in $X$ has a closed and compact neighborhood. -Defined on page 20 of {{zb:0386.54001}} as "strongly locally compact". Contrast with {P130}. +Defined on page 20 of {{zb:0386.54001}} as "strongly locally compact". + +Compare with {P130}. + +---- +#### Meta-properties + +- $X$ satisfies this property iff its Kolmogorov quotient $\text{Kol}(X)$ does. +- This property is hereditary with respect to closed sets. +- This property is preserved by finite products. diff --git a/properties/P000090.md b/properties/P000090.md index 391805ba9..2da66ae62 100644 --- a/properties/P000090.md +++ b/properties/P000090.md @@ -28,6 +28,7 @@ See also {{zb:0944.54018}}. ---- #### Meta-properties +- $X$ satisfies this property iff its Kolmogorov quotient $\text{Kol}(X)$ does. - This property is hereditary. - This property is preserved by arbitrary disjoint unions. - This property is preserved by finite products. diff --git a/theorems/T000927.md b/theorems/T000927.md new file mode 100644 index 000000000..38e158bb0 --- /dev/null +++ b/theorems/T000927.md @@ -0,0 +1,31 @@ +--- +uid: T000927 +if: + and: + - P000246: true + - P000024: true + - P000090: true + - P000001: true +then: + P000013: true +--- + +We assume $X$ is {P24}, {P90}, {P1} +and not {P13} and show it is not {P246}. + +*Notation*: Write $x\le y$ when $x\in\overline{\{y\}}$ (specialization order). +The smallest open neighborhoods of a point $a\in X$ and of a set $A\subseteq X$ are the upper sets +${\uparrow}a:=\{x\in X:a\le x\}$ and ${\uparrow}A:=\bigcup_{a\in A}{\uparrow}a$. +And let ${\downarrow}a:=\{x\in X:x\le a\}$. + +Let $A$ and $B$ be disjoint closed sets that don't have disjoint open neighborhoods. +So ${\uparrow}A\cap{\uparrow}B\ne\emptyset$ and ${\uparrow}a\cap{\uparrow}b\ne\emptyset$ +for some $a\in A$ and $b\in B$. +The set ${\downarrow}a$ is closed in $X$, hence {P24}. +Since {T641}, there is a point $a_1\le a$ with $\{a_1\}$ closed in ${\downarrow}a$, hence closed in $X$. +Similarly there is a point $b_1\le b$ with $\{b_1\}$ closed in $X$. +The points $a_1$ and $b_1$ are distinct, because $a_1\in A$ and $b_1\in B$. +The set $\{a_1,b_1\}$ is discrete and closed in $X$ and +its points cannot be separated by disjoint open sets as +${\uparrow}a_1\cap{\uparrow}b_1 \supseteq {\uparrow}a\cap{\uparrow}b\ne\emptyset$. +This shows that $X$ is not {P246}.