diff --git a/actions.tex b/actions.tex index 86a7ede..19f5445 100644 --- a/actions.tex +++ b/actions.tex @@ -1467,7 +1467,7 @@ \subsection{Homomorphisms and torsors} f^*\,\pathsp{\blank}^H$, all represented by the following diagram:\footnote{% \MB{(MB) Ugly: $f_*$ has no $B$, while $\Bf$ has. - I would like to define $\text{Tors}_G \defeq \Aut_{G-\Set}(\princ G)$ + I would like to define $\text{Tors}_G \defeq \Aut_{\GSet}(\princ G)$ so that we can use $\text{BTors}_G$. Further, all pointed maps without $B$, and apply $\mkgroup$ to lift the diagram to groups and iso/homomorphisms. @@ -1536,23 +1536,39 @@ \section{Any symmetry is a symmetry in $\Set$} \begin{theorem}[Cayley] \label{lem:allgpsarepermutationgps} - \marginnote{By \cref{lem:eq-mono-cover} ``$\rho_G$ is a monomorphism'' means that the induced map $\rho_G^\abstr$ from the symmetries of $\shape_G$ in $\BG_\div$ to the symmetries of $\USymG$ in $\Set$ is an injection, \ie ``any symmetry is a symmetry in $\Set$''.}Let $G$ be a group. Then - $\rho_G:\Hom(G,\SG_{\USymG}) $ is a monomorphism. + \marginnote{By \cref{def:typeofmono}, $\rho_G$ is a monomorphism means + that the induced map $\USym\rho_G$ from the symmetries of $\shape_G$ in + $\BG_\div$ to the symmetries of $\USymG$ in $\Set$ is an injection, + \ie ``any symmetry is a symmetry in $\Set$''.} + Let $G$ be a group. Then + $(G,\rho_G,!)$ is a monomorphism into $\SG_{\USymG}$. + In other words, $G$ is a subgroup of $\SG_{\USymG}$. \end{theorem} \begin{proof} - In view of \cref{lem:eq-mono-cover} we need to show that $\rho_G:\BG\to \conncomp \Set {\USymG}$ is a - \covering. - Under the identity - $$\bar{\pathsp{}}:\BG=(\typetorsor_G,\princ G)$$ of - \cref{lem:BGbytorsor}, $\rho_G$ translates to the + In view of \cref{def:typeofmono} we need to show that + $\B\rho_G\jdeq\princ G :\BG \to \conncomp \Set {\USymG}$ is a \covering.\footnote{\MB{Confusing, since families of sets are also called \coverings! Better: The fibers of $\princ G$ are sets.}} + Under the pointed equivalence + $$\pathsp{\blank}:\BG\ptdto (\typetorsor_G,\princ G)$$ of + \cref{lem:BGbytorsor}, $\princ G$ translates\footnote{ + \MB{Exercise? $\princ G(\inv{\pathsp{\blank}}(E)\eqto\ev_{\sh_G}(E)$ for all $E$. Maps pointed, OK for $\sh_G$.}} to the evaluation map - $$\mathrm{ev}_{\shape_G}:\conncomp{(\BG\to\Set)}{\princ G}\to\conncomp{\Set}{\USymG},\qquad - \mathrm{ev}_{\shape_G}(E)=E(\shape_G).$$ + $$\mathrm{ev}_{\shape_G}:\conncomp{(\BG\to\Set)}{\princ G}\ptdto + \conncomp{\Set}{\USymG},\qquad + \mathrm{ev}_{\shape_G}(E)\defeq E(\shape_G).$$ We must show that the preimages $\inv{\ev_{\shape_G}}(X)$ for $X:\Sigma_{\USymG}$ are sets. This fiber is equivalent to - $\sum_{E:\conncomp{(\BG\to\Set)}{\uc{\sh_G}}}(X=E(\shape_G))$ which is a - set precisely when $\sum_{E:\BG\to\Set}(X=E(\shape_G))$ is a set. We + $\sum_{E:\conncomp{(\BG\to\Set)}{\uc{\sh_G}}}(X\eqto E(\shape_G))$ which, + \MB{being a subtype}, is a + set precisely when $\sum_{E:\BG\to\Set}(X\eqto E(\shape_G))$ is a set. + \MB{The latter is the type of pointed maps from $\BG$ to $(\Set,X)$ + and hence a set by \cref{lem:hom-is-set}, in particular + \cref{ft:ptd-decr-h-lev}.}\footnote{% + \MB{Rest of the proof not needed. It is not a coincidence that + the type is $\Hom(G,\Aut_\Set(X))$, but why exactly? Worth a footnote!}} + + + We must then show that if $(E,p),(F,q):\sum_{E:\BG\to\Set}(X=E(\shape_G))$, then the type $(E,p)=(F,q)$, which is equivalent \marginnote{Note that if $r:\shape_G=x$, then @@ -1575,13 +1591,13 @@ \section{Any symmetry is a symmetry in $\Set$} composite of the identities $\phi(x)=F(r)qp^{-1}E(r)^{-1}=\psi(x)$ given above. Since $\BG$ is connected, and $\phi(x)=\psi(x)$ is a proposition, one can consider that $\shape_G=x$ is not empty, and we - are done. + are done.\footnote{\MB{5.6 would become half a page now, a bit short.}} \end{proof} \begin{xca} Given a group $G$ we defined in \cref{sec:groupssubperm} a monomorphism from $G$ to the permutation group $\Aut_{\USymG}(\Set)$. Write out the corresponding subgroup of $\Aut_{\USymG}(\Set)$. \end{xca} - + \section{The orbit-stabilizer theorem} diff --git a/group.tex b/group.tex index ab7a247..9819e30 100644 --- a/group.tex +++ b/group.tex @@ -1090,6 +1090,7 @@ \section{Homomorphisms} \end{displaymath} This concludes the proof that $f\eqto f'$ is a proposition, or in other words that $\Hom(G,H)$ is a set.\footnote{% + \label{ft:ptd-decr-h-lev} The same argument shows that the type $X\ptdto Y$ is a set whenever $X$ is connected and $Y$ is a groupoid. A more general fact is that $X \ptdto Y$ is an $n$-type