diff --git a/inputs/lecture_15.tex b/inputs/lecture_15.tex index 0d0fd8e..87d5d4e 100644 --- a/inputs/lecture_15.tex +++ b/inputs/lecture_15.tex @@ -1,31 +1,5 @@ \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}