w23-logic-3/inputs/tutorial_01.tex

69 lines
1.9 KiB
TeX
Raw Normal View History

2023-12-05 17:14:51 +01:00
\tutorial{01}{2023-10-17}{}
2023-10-19 16:59:53 +02:00
\begin{fact}
A countable product of separable spaces $(X_n)_{n \in \N}$ is separable.
\end{fact}
\begin{proof}
Choose a countable dense subset $D_n \subseteq X_n$
Fix some point $(a_1,a_2,\ldots) \in \prod_n X_n$
and consider $\bigcup_{i \in \N} \prod_{n \le i} D_n \times \prod_{n > i} \{a_n\}$.
\end{proof}
\begin{fact}
\begin{itemize}
\item Let $X$ be a topological space.
2024-01-09 20:23:57 +01:00
Then $X$ \nth{2} countable $\implies$ X separable.
2023-10-19 16:59:53 +02:00
\item If $X$ is a metric space and separable,
2024-01-09 20:23:57 +01:00
then $X$ is \nth{2} countable.
2023-10-19 16:59:53 +02:00
\end{itemize}
\end{fact}
\begin{proof}
For the first point, choose some point from every basic open set.
For the second point consider balls of rational radius
around the points of a countable dense subset.
\end{proof}
\begin{definition}
A topological space is \vocab{Lindelöf}
if every open cover has a countable subcover.
\end{definition}
\begin{fact}
Let $X$ be a metric space.
If $X$ is Lindelöf,
2024-01-09 20:23:57 +01:00
then it is \nth{2} countable.
2023-10-19 16:59:53 +02:00
\end{fact}
\begin{proof}
For all $q \in \Q$
2024-01-09 20:23:57 +01:00
consider the cover $B_q(x), x \in X$
2023-10-19 16:59:53 +02:00
and choose a countable subcover.
The union of these subcovers is
a countable base.
\end{proof}
\begin{fact}
Let $X$ be a topological space.
2024-01-09 20:23:57 +01:00
If $X$ is \nth{2} countable,
2023-10-19 16:59:53 +02:00
then it is Lindelöff.
\end{fact}
\begin{proof}
Let $A_0, A_1,\ldots$
be a countable base.
Let $\{U_i\}_{i \in I}$
be a cover.
Consider $J \coloneqq \{j : \exists i \in I.~A_j \in U_i\}$.
For every $j \in J$ choose a $U_i$ such that
$A_j \subseteq U_j$.
Let $I' \subseteq I$ be the subset of chosen indices.
Then $\{U_i\}_{i \in I'}$ is a countable subcover.
\end{proof}
\begin{remark}
For metric spaces the notions
2024-01-09 20:23:57 +01:00
of being \nth{2} countable, separable
2023-10-19 16:59:53 +02:00
and Lindelöf coincide.
In arbitrary topological spaces,
Lindelöf is the strongest of these notions.
\end{remark}