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\lecture{15}{2023-12-05}{}
Recall:
\begin{definition}+
Let $X$ be a set.
A \vocab{group action} of a group $G$ on $X$
is a function
$\alpha\colon G \times X \to X$
such that
\begin{itemize}
\item $\forall x \in X.~\alpha(1_G,x) = x$,
\item $\forall g,h \in G, x \in X.~\alpha(gh,x) = \alpha(g,\alpha(h,x))$.
\end{itemize}
Often we will abbreviate $\alpha(g,x)$ as $g\cdot x$.
\end{definition}
\begin{remark}+
Group actions of a group $G$ on a set $X$
correspond to group-homomorphisms
$G \to \Sym(X)$.
Indeed for a group action $\alpha\colon G \times X \to X$
consider
\begin{IEEEeqnarray*}{rCl}
G&\longrightarrow & \Sym(X) \\
g&\longmapsto & (x \mapsto g \cdot x).
\end{IEEEeqnarray*}
\end{remark}
\begin{theorem}[The Boundedness Theorem]
\yalabel{Boundedness Theorem}{Boundedness}{thm:boundedness}
@ -64,7 +38,35 @@ Recall:
\todo{TODO: Copy from official notes}
\end{proof}
\pagebreak
\section{Abstract Topological Dynamics}
Recall:
\begin{definition}+
Let $X$ be a set.
A \vocab{group action} of a group $G$ on $X$
is a function
$\alpha\colon G \times X \to X$
such that
\begin{itemize}
\item $\forall x \in X.~\alpha(1_G,x) = x$,
\item $\forall g,h \in G, x \in X.~\alpha(gh,x) = \alpha(g,\alpha(h,x))$.
\end{itemize}
Often we will abbreviate $\alpha(g,x)$ as $g\cdot x$.
\end{definition}
\begin{remark}+
Group actions of a group $G$ on a set $X$
correspond to group-homomorphisms
$G \to \Sym(X)$.
Indeed for a group action $\alpha\colon G \times X \to X$
consider
\begin{IEEEeqnarray*}{rCl}
G&\longrightarrow & \Sym(X) \\
g&\longmapsto & (x \mapsto g \cdot x).
\end{IEEEeqnarray*}
\end{remark}
\begin{definition}
Let $T$ be a topological group\footnote{usually $T = \Z$ with the discrete topology}