diff --git a/group.tex b/group.tex index 99966df..b324f7c 100644 --- a/group.tex +++ b/group.tex @@ -876,7 +876,7 @@ \section{Abstract groups} \begin{xca} \label{xca:conj} - Let $\mathcal G\jdeq(S,e,\mu,\iota)$ be an abstract group and let $g:S$. If $s:S$, let $\conj^g(s)\defequi g\cdot s\cdot g^{-1}$. Show that the resulting function $c^g:S\to S$ preserves the group structure (for instance $g\cdot(s\cdot s')\cdot g^{-1}=(g\cdot s\cdot g^{-1} )\cdot(g\cdot s\cdot g^{-1})$) and is an equivalence. The resulting identification $\conj^g:\mathcal G\eqto\mathcal G$ is called \emph{conjugation} by $g$.\index{conjugation} + Let $\mathcal G\jdeq(S,e,\mu,\iota)$ be an abstract group and let $g:S$. If $s:S$, let $\conj^g(s)\defequi g\cdot s\cdot g^{-1}$. Show that the resulting function $\conj^g:S\to S$ preserves the group structure (for instance $g\cdot(s\cdot s')\cdot g^{-1}=(g\cdot s\cdot g^{-1} )\cdot(g\cdot s\cdot g^{-1})$) and is an equivalence. The resulting identification $\conj^g:\mathcal G\eqto\mathcal G$ is called \emph{conjugation} by $g$.\index{conjugation} \end{xca} \begin{remark} @@ -1407,40 +1407,35 @@ \section{Homomorphisms} \begin{example} \label{exa:conj-concrete} - In terms of concrete groups, one can think of conjugation as moving + In terms of concrete groups one can think of conjugation as moving the shape of the concrete group along a path in the classifying type. This path can in particular be a loop at the shape. More precisely, - let $G$ be a group, $y$ an element of $\BG$, and $p$ a path of type $\shape_G\eqto y$. - Then $(\id_\BG,\inv p)$ is a pointed equivalence of type $\BG \equivto_* (\BG_\div , y)$ - and hence induces an isomorphism from $G$ to $\mkgroup(\BG_\div , y)$. - By \cref{remark:groupsasunivalenttype} we then get an identification of these groups. + let $G$ be a group, $y$ an element of $\BG$, and $p$ a path of + type $\shape_G\eqto y$. + Then $(\id_\BG,\inv p)$ is a pointed equivalence of type + $\BG \equivto_* (\BG_\div , y)$ and hence induces an isomorphism + from $G$ to $\mkgroup(\BG_\div , y)$.\footnote{% + One may wonder why $\inv p$ in $(\id_\BG,\inv p)$. + The reason is our convention for the direction of the + pointing path of a pointed map in combination with the + convention that $\conj^p$ is transport along $p$.} + By \cref{remark:groupsasunivalenttype} we then get an + identification of these groups. Moreover, by path induction on $p$, the equivalence - $\USym(\mkgroup(\id_\BG,\inv p))\jdeq\loops(\id_\BG,\inv p)$ of type $(\sh_G\eqto\sh_G)\equivto(y\eqto y)$ - can be identified with the map $g \mapsto p g \inv p$. In the special case + $\USym(\mkgroup(\id_\BG,\inv p))\jdeq\loops(\id_\BG,\inv p)$ + of type $(\sh_G\eqto\sh_G)\equivto(y\eqto y)$\footnote{% + Note that $\USym(\mkgroup(\BG_\div,y)) \jdeq + \loops(BG_\div,y) \jdeq (y\eqto y)$.} + can be identified with the map $g \mapsto p g \inv p$. + In the special case of $y\jdeq\sh_G$ this map is precisely $\conj^p$ from \cref{xca:conj}. - The more general form introduced here is also called conjugation.\index{conjugation} - - \MB{OLD OBSOLTE?}In terms of concrete groups, it is not so easy to define the - classifying map of conjugation by $g$, as homomorphism (indeed, automorphism) - $\conj^g : \Hom(G,G)$, for any particular given $g:\USymG$.\footnote{% - We shall develop a general method of doing this in~\cref{sec:homabsisconcr} - below.} - However, conjugation defines uniformly a homomorphism - $\conj : \Hom(G,\Aut(G))$. - And this has a very pretty classifying map, - $\Bconj : \BG \ptdto \BAut(G)$, defined by $t \mapsto \mkgroup(\BG_\div,t)$, - that is, if we have a shape $t$ for $G$, - then we get a new group in the component of $G$ - by taking $t$ as the new designed shape. - - To see that this really captures conjugation as defined above on underlying - symmetries, we generalize, and prove for all $t:\BG$ and all $g : \shape_G \eqto t$, - that $\ap{\Bconj}(g)$ applied to any $s : \USymG$ equals - $gsg^{-1} : t \eqto t$.\footnote{% - Note that $\USym(\Bconj(t)) \jdeq \USym(\mkgroup(\BG_\div,t)) \jdeq \loops(BG_\div,t) - \jdeq (t\eqto t)$.} - And this follows by induction on $g$. -\MB{ENDOLD} + \footnote{% + One can also start from $\conj^p$ and ask for a pointed map + $f: \BG_\div \ptdto \BG_\div$ such that $\loops(f) = \conj^p$. + We shall develop a general method for constructing + such a map $f$ in~\cref{sec:homabsisconcr} below.} + The more general form introduced here is also called conjugation. + \index{conjugation} \end{example} The above example motivates and justifies the following definition of @@ -3520,17 +3515,17 @@ \section{$G$-sets vs $\abstr(G)$-sets} What abstract $\abstr(G)$-set does this correspond to? In particular, under the equivalence $\abstr:\Hom(H,G)\to\Hom^\abstr(\abstr(H),\abstr(G))$, what is the corresponding action of $\abstr(G)$ on the abstract homomorphisms? -The answer is that $g:\USym G$ acts on $\Hom^\abstr(\abstr(H),\abstr(G))$ by postcomposing with conjugation $c^g$ by $g$ as defined in \cref{ex:conjhomo}. +The answer is that $g:\USym G$ acts on $\Hom^\abstr(\abstr(H),\abstr(G))$ by postcomposing with conjugation $\conj^g$ by $g$ as defined in \cref{ex:conjhomo}. Let us spell this out in some detail: If $(F,p):\Hom(H,G)(\shape_G)\defequi - \sum_{F:\BH_\div\to \BG_\div}(\shape_G=F(\shape_H))$ and $g:\USym G$, then $g\cdot(F,p)\defequi(F,p\,g^{-1})$. If we show that the action of $g$ sends $\abstr(F,p)$ to $c^g\circ\abstr(F,p)$ we are done. + \sum_{F:\BH_\div\to \BG_\div}(\shape_G=F(\shape_H))$ and $g:\USym G$, then $g\cdot(F,p)\defequi(F,p\,g^{-1})$. If we show that the action of $g$ sends $\abstr(F,p)$ to $\conj^g\circ\abstr(F,p)$ we are done. Recall that $\abstr(F,p)$ consists of the composite $$\xymatrix{\USym H\ar[r]^-{F^=}&(F(\shape_H)=F(\shape_G))\ar[rr]^-{t\mapsto p^{-1}t\,p}&&\USym G},$$ (\ie $\abstr(F,p)$ applied to $q:\USym H $ is $p^{-1}F^=(q)\,p$) together with the proof that this is an abstract group homomorphism. We see that $\abstr(F,p\,g^{-1})$ is given by conjugation: -$q\mapsto(p\,g^{-1})^{-1}F^=(q)\,(p\,g^{-1})=g\,(p^{-1}F^=(q)\,p)\,g^{-1}$, or in other words $c^g\circ\abstr(F,p)$. +$q\mapsto(p\,g^{-1})^{-1}F^=(q)\,(p\,g^{-1})=g\,(p^{-1}F^=(q)\,p)\,g^{-1}$, or in other words $\conj^g\circ\abstr(F,p)$. \end{example} For reference we list the conclusion of this example as a lemma: \begin{lemma}\label{lem:abstrandconj} diff --git a/macros.tex b/macros.tex index 68189cd..dbd0235 100644 --- a/macros.tex +++ b/macros.tex @@ -627,7 +627,7 @@ \newBcommand{Hom} \newBcommand{Iso} \newBcommand{sgn} % sign homomorphism -\newBcommand[c]{conj} % conjugation homomorphism +\newBcommand{conj} % conjugation homomorphism MB c := conj 040124 \newBcommand{Z} % center \newBcommand[F]{FG} % free group \newBcommand[V]{VG} % Vierer group