diff --git a/actions.tex b/actions.tex index 4afbe70..51feb45 100644 --- a/actions.tex +++ b/actions.tex @@ -451,8 +451,8 @@ \subsection{Transitive $G$-sets} \begin{marginfigure} \noindent\begin{tikzpicture} \pgfmathsetmacro{\len}{1} - \node[vertex,label=above:$x$] (n1) at (0:\len) {}; - \node[vertex] (n2) at (120:\len) {}; + \node[vertex] (n1) at (0:\len) {}; + \node[vertex,label=above:$x$] (n2) at (120:\len) {}; \node[vertex] (n3) at (240:\len) {}; \begin{scope}[every to/.style={bend right=22}] % generator a @@ -682,12 +682,12 @@ \subsection{Subgroups through $G$-sets} As an example, recall from \cref{def:RmtoS1} the $\Sc$-set $R_m : \Sc\to\Set$ defined by $R_m(\base) \defeq \bn m$ and $R_m(\Sloop) \defis \etop\zs$. Here $m>0$ so that we can point -$R_m$ by $0: R_m(\base)$.\footnote{Any element of $\bn m$ would do.} +$R_m$ by $0: R_m(\base)$.\footnote{Any element of $\bn m$ would do.} Transitivity of $R_m$ is obvious. Which symmetries $p: \base\eqto\base$ are picked out by $R_m$? Those that keep the point $0: R_m(\base)$ in place, that is, those that satisfy $R_m(p)(0)=0$, \ie $p=\Sloop^{mk}$ for some integer $k$. -Given $\alpha_m$ in \cref{con:psi-alpha-m}, it should not come as a +Given $\alpha_m$ in \cref{con:psi-alpha-m}, it should not come as a surprise that these are precisely the symmetries picked out by $\dg{m}$. The case of $m=0$ connects to another old friend, the $\Sc$-set @@ -720,27 +720,29 @@ \subsection{Subgroups through $G$-sets} \begin{example} \label{exa:fix1subSGn}% -Consider the group $\SG_n$ (\cref{ex:groups}\ref{ex:permgroup}) -for given $n>0$. For any $k:\bn n$, define -the $\SG_n$-set $X_k : \BSG_n \to\Set$ by $X_k(A,!)\defeq A$ for any -$A:\FinSet_n$. Then $X_k$ is obviously transitive. We point $X_k$ by -$k: X_k(\sh_{\SG_n}) \jdeq \bn n$.\footnote{Here the choice of the point +Consider the symmetric group $\SG_n$ from~\cref{ex:groups}\ref{ex:permgroup}, +for some $n>0$. It has a canonical action, +the $\SG_n$-set $X : \BSG_n \to\Set$ given by $X(A,!)\defeq A$ +for any $A:\FinSet_n$, which is obviously transitive. +For any $k:\bn n$, we can point $X$ by +$k: X(\sh_{\SG_n}) \jdeq \bn n$.\footnote{The choice of the point does matter for the symmetries that are picked out.} -Thus we have $(X_k,k,!):\Sub_{\SG_n}$. +Thus we have $(X,k):\Sub(\SG_n)$. The symmetries that are picked out are those $\pi : \bn n \eqto \bn n$ -that satisfy $(\pi \cdot_{X_k}k) = k$.\footnote{% -This uses the alternative notation for the group action of $X_k$ +that satisfy $(\pi \cdot_X k) = k$.\footnote{% +This uses the alternative notation for the group action of $X$ introduced in \cref{def:Gset}.} In other words, $\pi$ keeps $k$ in place and can be any permutation -of the other elements of $\bn n$. From the next \cref{xca:n-is-ptd-n+1} -we get that the underlying group of each $(X_k,k,!)$ +of the other elements of $\bn n$. +From the next~\cref{xca:n-is-ptd-n+1} +we get that the underlying group of each $(X,k)$ is isomorphic to $\SG_{n-1}$. \end{example} -\begin{xca} \label{xca:n-is-ptd-n+1} -Give an equivalence from the type of -$n$-element sets to the type of pointed $(n{+}1)$-element sets. -Hint: use \cref{xca:finsets-decidable}. +\begin{xca}\label{xca:n-is-ptd-n+1} + Give an equivalence from the type of + $n$-element sets to the type of pointed $(n{+}1)$-element sets. + Hint: use~\cref{xca:finsets-decidable}. \end{xca} \begin{xca} \label{xca:A-is-A-1+1} @@ -751,6 +753,7 @@ \subsection{Subgroups through $G$-sets} For yet another example, consider the cyclic group $\CG_6$ of order $6$; perhaps visualized as the rotational symmetries of a regular hexagon, \ie the rotations by $2\pi\cdot m /6$, where $m=0,1,2,3,4,5$. The symmetries of the regular triangle (rotations by $2\pi\cdot m/3$, where $m=0,1,2$) can also be viewed as symmetries of the hexagon. Thus there is a subgroup of $\CG_6$ which, as a group, is isomorphic to $\CG_3$.\marginnote{Make a TikZ drawing of the hexagon and triangle inscribe in it.} +%LINK TO CH 3 AND SQUARE ROOT OF 6 BUNDLE \begin{example} \label{exa:C3subC6}% diff --git a/macros.tex b/macros.tex index 4048e3d..083ff07 100644 --- a/macros.tex +++ b/macros.tex @@ -859,9 +859,9 @@ % sep=0, outer sep=0, circle]% % {$\bullet$};}}% \newcommand*{\base}{{\sbt}}%point in circle -\newcommand*{\Cloop}{\circlearrowleft}% loop in circle \InfCycSet -\newcommand*{\Sloop}{\circlearrowleft}% loop in circle \Sc -\newcommand*{\qedge}{\curvearrowright}% edge in graph quotient +\newcommand*{\Cloop}{\mathop\circlearrowleft}% loop in circle \InfCycSet +\newcommand*{\Sloop}{\mathop\circlearrowleft}% loop in circle \Sc +\newcommand*{\qedge}{\mathop\curvearrowright}% edge in graph quotient \newcommand*{\conncomp}[2]{{{#1}_{\left(#2\right)}}}% \newcommand*{\univcover}[2]{{{#1}^0_{\left(#2\right)}}}%