diff --git a/spaces/S000195/README.md b/spaces/S000195/README.md index 4043cf15fa..8aa2060547 100644 --- a/spaces/S000195/README.md +++ b/spaces/S000195/README.md @@ -8,4 +8,4 @@ refs: name: Answer to "Is there a first countable, $T_1$, weakly Lindelof, sequentially compact space which is not also compact?" --- -The set $\omega_1$ with topology equal to the join of the cofinite topology and left ray topology in the [lattice of topologies](https://en.wikipedia.org/wiki/Lattice_of_topologies) on $\omega_1$. This topology is generated by the sets of the form $\alpha\setminus F=[0,\alpha)\setminus F$ for $\alpha<\omega_1$ and $F\subseteq\omega_1$ finite. +The set $\omega_1$ with topology equal to the join of the cofinite topology and left ray topology in the [lattice of topologies](https://en.wikipedia.org/wiki/Lattice_of_topologies) on $\omega_1$. This topology is the coarsest topology finer than both the cofinite topology and the left ray topology. It is generated by the sets of the form $\alpha\setminus F=[0,\alpha)\setminus F$ for $\alpha<\omega_1$ and $F\subseteq\omega_1$ finite. diff --git a/spaces/S000195/properties/P000002.md b/spaces/S000195/properties/P000002.md deleted file mode 100644 index 5966cebd50..0000000000 --- a/spaces/S000195/properties/P000002.md +++ /dev/null @@ -1,10 +0,0 @@ ---- -space: S000195 -property: P000002 -value: true -refs: - - mathse: 4874321 - name: Answer to "Is there a first countable, $T_1$, weakly Lindelof, sequentially compact space which is not also compact?" ---- - -By construction finite sets are closed. diff --git a/spaces/S000195/properties/P000016.md b/spaces/S000195/properties/P000016.md deleted file mode 100644 index be4e15d2e7..0000000000 --- a/spaces/S000195/properties/P000016.md +++ /dev/null @@ -1,10 +0,0 @@ ---- -space: S000195 -property: P000016 -value: false -refs: - - mathse: 4874321 - name: Answer to "Is there a first countable, $T_1$, weakly Lindelof, sequentially compact space which is not also compact?" ---- - -$\{\alpha:\alpha<\omega_1\}$ is an open cover with no finite subcover. diff --git a/spaces/S000195/properties/P000024.md b/spaces/S000195/properties/P000024.md new file mode 100644 index 0000000000..f347bdade0 --- /dev/null +++ b/spaces/S000195/properties/P000024.md @@ -0,0 +1,14 @@ +--- +space: S000195 +property: P000024 +value: false +--- + +Let $U$ be a closed neighborhood of $\omega$, and let $\alpha \ge \omega$. We have that $U$ is cofinite in $[0, \omega)$ (meaning that $[0, \omega) \setminus U$ is finite). + +Then let $V$ be a neighborhood of $\alpha$. Then $V$ is also cofinite in $[0, \omega)$. +So $U \cap V$ is cofinite in $[0, \omega)$, and therefore nonempty. +This argument works for any neighborhood $V$, so every neighborhood of $\alpha$ intersects $U$. Since $U$ is closed, $\alpha \in U$. + +Since this holds for any $\alpha \ge \omega$, we have that $[\omega, \omega_1) \subseteq U$. +Then $\{ [0, \alpha) : \alpha \in X \}$ is an open cover of $U$ without finite subcover, so $U$ is not compact. diff --git a/spaces/S000195/properties/P000026.md b/spaces/S000195/properties/P000026.md deleted file mode 100644 index fd2d35018d..0000000000 --- a/spaces/S000195/properties/P000026.md +++ /dev/null @@ -1,10 +0,0 @@ ---- -space: S000195 -property: P000026 -value: true -refs: - - mathse: 4874321 - name: Answer to "Is there a first countable, $T_1$, weakly Lindelof, sequentially compact space which is not also compact?" ---- - -$\omega$ is a countable dense subset. diff --git a/spaces/S000195/properties/P000084.md b/spaces/S000195/properties/P000084.md new file mode 100644 index 0000000000..35e7772ff1 --- /dev/null +++ b/spaces/S000195/properties/P000084.md @@ -0,0 +1,7 @@ +--- +space: S000195 +property: P000084 +value: false +--- + +The point $\omega + \omega$ has no Hausdorff neighborhood. diff --git a/spaces/S000195/properties/P000085.md b/spaces/S000195/properties/P000085.md new file mode 100644 index 0000000000..98a1d761d4 --- /dev/null +++ b/spaces/S000195/properties/P000085.md @@ -0,0 +1,16 @@ +--- +space: S000195 +property: P000085 +value: false +--- + +Define a function $f : X \to \mathbb{R}$ by +$$f(x)= +\begin{cases} +1/x &\text{if }x < \omega \text{ odd},\\ +0 &\text{if }x < \omega \text{ even},\\ +0 & \text{if }x \ge \omega +\end{cases} +$$ + +Then $f$ is continuous, and $U = \{ x : f(x) \ne 0 \}$ and $V = \{ x : f(1 + x) \ne 0 \}$ are disjoint cozero sets whose closures both contain the point $\omega$. diff --git a/spaces/S000195/properties/P000093.md b/spaces/S000195/properties/P000093.md new file mode 100644 index 0000000000..d14279401c --- /dev/null +++ b/spaces/S000195/properties/P000093.md @@ -0,0 +1,7 @@ +--- +space: S000195 +property: P000093 +value: true +--- + +For $\alpha < \omega_1$ and $F \subseteq \omega_1$ finite, let $U_{\alpha, F} = [0, \alpha) \setminus F$. Then the $U_{\alpha, F}$ are all countable and form a basis for the topology. diff --git a/spaces/S000195/properties/P000099.md b/spaces/S000195/properties/P000099.md deleted file mode 100644 index 0919b6be9a..0000000000 --- a/spaces/S000195/properties/P000099.md +++ /dev/null @@ -1,7 +0,0 @@ ---- -space: S000195 -property: P000099 -value: false ---- - -The sequence $a_n=n$ converges to every $\alpha\in\omega_1\setminus\omega$. diff --git a/spaces/S000195/properties/P000101.md b/spaces/S000195/properties/P000101.md new file mode 100644 index 0000000000..90e233d229 --- /dev/null +++ b/spaces/S000195/properties/P000101.md @@ -0,0 +1,15 @@ +--- +space: S000195 +property: P000101 +value: false +--- + +Define $p : X \to \mathbb{N}$ by $p(\alpha) = n$ if $\alpha = \beta + n$ but $\alpha \ne \gamma + n + 1$ for any ordinal $\gamma$. +Define a function $f : X \to X$ by +$$f(x)= +\begin{cases} +x &\text{if }p(x) \text{ even},\\ +x + 1 &\text{if }p(x) \text{ odd},\\ +\end{cases} +$$ +Then $f$ is a retract onto $\{ 2 * x : x \in X \}$, which is not closed. diff --git a/spaces/S000195/properties/P000115.md b/spaces/S000195/properties/P000115.md new file mode 100644 index 0000000000..b69a8198b5 --- /dev/null +++ b/spaces/S000195/properties/P000115.md @@ -0,0 +1,16 @@ +--- +space: S000195 +property: P000115 +value: false +--- + +Consider the open cover $\mathcal{U} = \{ [0, \alpha) : \alpha < \omega_1 \}$. Suppose that $\mathcal{V}$ is a $\sigma$-locally finite closed refinement of $\mathcal{U}$. +Since infinite closed subset of $X$ are unbounded, they are not subordinate to any of the $[0, \alpha) \in \mathcal{U}$, so $\mathcal{V}$ consists of finite subsets of $X$. +Write $\mathcal{V} = \bigcup_n \mathcal{V}_n$, where $\mathcal{V}_n$ are locally finite. + +Suppose for sake of contradiction that $S = \bigcup\mathcal{V}_n$ is infinite. Since {S195|P21}, $S$ has a limit point. +Call this limit point $x$, so $x$ is a limit point of $S$. Then $x$ has an open neighborhood $U$ intersecting only finitely many members of $\mathcal{V}_n$. +All the members of $\mathcal{V}_n$ are finite, so $U \cap S$ is also finite. But {S195|P2} means that $U \cap S$ then has no limit points, +so in particular $x$ is not a limit point of $U \cap S$. + +Then all the $\bigcup\mathcal{V}_n$ are finite, and so $X = \bigcup_n \bigcup\mathcal{V}_n$ is countable, which is impossible since $X$ is uncountable. diff --git a/spaces/S000195/properties/P000130.md b/spaces/S000195/properties/P000130.md new file mode 100644 index 0000000000..196c63eeb2 --- /dev/null +++ b/spaces/S000195/properties/P000130.md @@ -0,0 +1,8 @@ +--- +space: S000195 +property: P000130 +value: true +--- + +For $\alpha < \omega_1$ and $F \subseteq \omega_1$ finite, let $U_{\alpha, F} = [0, \alpha) \setminus F$. +For a point $\alpha \in X$, sets of the form $U_{\alpha, F}$ for $\alpha \notin F$ are compact and form a neighborhood basis around $\alpha$. diff --git a/spaces/S000195/properties/P000132.md b/spaces/S000195/properties/P000132.md new file mode 100644 index 0000000000..2940d3c746 --- /dev/null +++ b/spaces/S000195/properties/P000132.md @@ -0,0 +1,9 @@ +--- +space: S000195 +property: P000132 +value: true +--- + +Every closed set is either finite or cocountable. Cocountable sets are $G_\delta$ because they are the countable intersection of complements of singletons. Since a finite union of $G_\delta$ sets is $G_\delta$, it suffices to check that singletons are $G_\delta$. + +Let $x \in X$, we need to show that $\{x\}$ is a countable intersection of open sets. For each $y < x$, the set $U_y = [0, x] \setminus \{y\}$ is open. There are only countably many $y < x$, so $\bigcap_{y < x} U_y = \{x\}$ is a $G_\delta$ set. diff --git a/spaces/S000195/properties/P000142.md b/spaces/S000195/properties/P000142.md new file mode 100644 index 0000000000..d50920bbd6 --- /dev/null +++ b/spaces/S000195/properties/P000142.md @@ -0,0 +1,18 @@ +--- +space: S000195 +property: P000142 +value: true +--- + +Let $A \subseteq X$ such that for every compact Hausdorff subspace $K \subseteq X$, the set $A \cap K$ is relatively closed in $K$. +If $A$ is finite, then $X \setminus A = \bigcup_{\alpha < \omega_1} [0, \alpha) \setminus A$ is open. +If $A$ is infinite, then being a subset of an ordinal, it is well-ordered, so it has an initial segment order-isomorphic to $\omega$. +Let $U \subseteq A$ be this initial segment, and let $u$ be its supremum in $X$. + +Consider an arbitrary $v \in [u, \omega_1)$. +Set $U_v = U \cup \{v\}$. Then $U_v$ with the subspace topology from $X$ is homeomorphic to {S20}. +Then since {S20|P16} and {S20|P3}, the intersection $A \cap U_v$ is closed in $U_v$. +Therefore $v \in A$. Since this works for any $v \in [u, \omega_1)$, we must have $[u, \omega_1) \subseteq A$. + +But also $A$ is disjoint from $[0, u) \setminus U$, since $U$ is an initial segment in $A$. +We conclude that $A = U \cup [u, \omega_1)$. Then $X \setminus A = \bigcup_{\alpha < u} [0, \alpha) \setminus U$ is open (since each of the $[0, \alpha) \cap U$ is finite). diff --git a/spaces/S000195/properties/P000169.md b/spaces/S000195/properties/P000169.md new file mode 100644 index 0000000000..cd09fbcf17 --- /dev/null +++ b/spaces/S000195/properties/P000169.md @@ -0,0 +1,17 @@ +--- +space: S000195 +property: P000169 +value: false +--- + +Let $U$ be a regular open neighborhood of $\omega$, and let $\alpha \ge \omega$. We have that $U$ is cofinite in $[0, \omega)$ (meaning that $[0, \omega) \setminus U$ is finite). + +Then let $V$ be a neighborhood of $\alpha$. Then $V$ is also cofinite in $[0, \omega)$. +So $U \cap V$ is cofinite in $[0, \omega)$, and therefore nonempty. +This argument works for any neighborhood $V$, so every neighborhood of $\alpha$ intersects $U$. Therefore, $\alpha \in \operatorname{cl}(U)$. + +Since this holds for any $\alpha \ge \omega$, we have that $[\omega, \omega_1) \subseteq \operatorname{cl}(U)$. +Then $X \setminus \operatorname{cl}(U)$ is a finite subset of $[0, \omega)$, so $\operatorname{cl}(U)$ is open. +Then $\operatorname{int}(\operatorname{cl}(U)) = \operatorname{cl}(U)$. +Since $U$ is regular we also have that $\operatorname{int}(\operatorname{cl}(U)) = U$, so in fact $\operatorname{cl}(U) = U$, and $[\omega, \omega_1) \subseteq U$. +So no regular open neighborhood of $U$ misses any $\alpha \in [\omega, \omega_1)$. diff --git a/spaces/S000195/properties/P000180.md b/spaces/S000195/properties/P000180.md new file mode 100644 index 0000000000..132667ed67 --- /dev/null +++ b/spaces/S000195/properties/P000180.md @@ -0,0 +1,7 @@ +--- +space: S000195 +property: P000180 +value: true +--- + +Let $U \subseteq X$. Let $S$ be the set of $x \in U$ such that $[0, x) \cap U$ is finite. Then $S$ is a countable dense subset of $U$.