diff --git a/properties/P000182.md b/properties/P000182.md index ce106f24c3..ca1c62398d 100644 --- a/properties/P000182.md +++ b/properties/P000182.md @@ -16,3 +16,4 @@ Defined on page 127 of {{zb:0684.54001}}. ### Meta-properties - This property is hereditary. +- This property is preserved by countable products. diff --git a/properties/P000183.md b/properties/P000183.md index 7b97dc0ae2..be2f6283f7 100644 --- a/properties/P000183.md +++ b/properties/P000183.md @@ -6,16 +6,33 @@ refs: name: $\aleph_0$-spaces (E. Michael) - doi: 10.1016/j.topol.2015.05.085 name: $\mathfrak P$-spaces and related concepts (Gabriyelyan & Kakol) + - mo: 495213 + name: Product of spaces with countable $k$-networks + - mo: 506308 + name: Answer to "Product of spaces with countable $k$-networks" --- A space with a (finite or infinite) countable $k$-network. -Equivalently, a space with a countable pseudobase. +Equivalently: +1. A space with countable pseudobase. +2. A space with countable $cs$-network. +3. A space with countable $cs^\ast$-network. A family $\mathcal N$ of subsets of $X$ is called a *$k$-network* if for every compact set $K$ and open set $U$ in $X$ with $K\subseteq U$, there exists a finite $\mathcal{N}^* \subseteq \mathcal{N}$ with $K \subseteq \bigcup\mathcal{N}^* \subseteq U$. -And a family $\mathcal{N}$ of subsets of $X$ is called a *pseudobase* if for every compact set $K$ and open set $U$ in $X$ with $K\subseteq U$, there exists some $A\in\mathcal{N}$ with $K \subseteq A \subseteq U$. +A family $\mathcal{N}$ of subsets of $X$ is called a *pseudobase* if for every compact set $K$ and open set $U$ in $X$ with $K\subseteq U$, there exists some $A\in\mathcal{N}$ with $K \subseteq A \subseteq U$. -For the equivalence between the two characterizations, note that every pseudobase is a $k$-network. And conversely given a countable $k$-network $\mathcal N$, the collection of finite unions of elements of $\mathcal N$ is a countable pseudobase. +A family $\mathcal{N}$ of subsets of $X$ is called a *$cs$-network* if for every sequence $x_n$ convergent to $x$ and open set $U$ in $X$ with $x\in U$, there exists some $A\in\mathcal{N}$ with $x_n\in A$ for all but finitely many $n$. + +A family $\mathcal{N}$ of subsets of $X$ is called a *$cs^\ast$-network* if for every sequence $x_n$ convergent to $x$ and open set $U$ in $X$ with $x\in U$, there exists some $A\in\mathcal{N}$ with $x_n\in A$ for infinitely many $n$. + +To see the equivalence, first note that a pseudobase is a $k$-network, a $k$-network is a $cs$-network and a $cs$-network is a $cs^\ast$-network. In {{mo:506308}} it is shown that if $X$ has countable $cs^\ast$-network, then it has a countable $k$-network. Then finite unions of elements of that $k$-network form a countable pseudobase. See {{mr:206907}}, available at . + +--- +### Meta-properties + +- This property is preserved by countable products. (see {{mo:495213}}) + diff --git a/spaces/S000031/properties/P000183.md b/spaces/S000031/properties/P000183.md new file mode 100644 index 0000000000..098c809295 --- /dev/null +++ b/spaces/S000031/properties/P000183.md @@ -0,0 +1,7 @@ +--- +space: S000031 +property: P000183 +value: true +--- + +$X$ is a square of {S29} and {S29|P183}.