diff --git a/actions.tex b/actions.tex index 4dc769e..61ac624 100644 --- a/actions.tex +++ b/actions.tex @@ -953,7 +953,7 @@ \section{Fixed points and orbits} $G$, we'll mostly work in that generality. \begin{definition} \label{def:orbittype} - If $X : G\to\UU$, then the \emph{orbit type}\index{orbit type} + If $X : \BG\to\UU$, then the \emph{orbit type}\index{orbit type} of the action is\footnote{% The superscripts and subscripts are decorated with ``$hG$'', following a convention in homotopy theory. @@ -1071,14 +1071,14 @@ \section{Fixed points and orbits} Surjectivity follows from the connectivity of $\BG$. By~\cref{rem:set-trunc-as-quotient}, $X/G \jdeq \setTrunc{X_{hG}}$ is itself the - set quotient of $X_{hg} \jdeq \sum_{z:\BG}X(z)$ by + set quotient of $X_{hG} \jdeq \sum_{z:\BG}X(z)$ by the relation $\sim$ defined by letting $(z,x)\sim(w,y)$ if and only if $\exists_{g:z\eqto w}(g\cdot x=y)$. Thus,~\cref{thm:quotient-property} gives the desired conclusion. \end{proof} Thus, both the underlying set $X(\shape_G)$ and the orbit type -$X_{hg}$ have equivalence relations with quotient set $X/G$.\footnote{% +$X_{hG}$ have equivalence relations with quotient set $X/G$.\footnote{% This also justifies the notation $X/G$. We have a diagram of surjective maps: \[ @@ -1093,7 +1093,7 @@ \section{Fixed points and orbits} If $X : \BG \to \Set$ is a $G$-set, and $x : X(\shape_G)$ is an element of the underlying set, then we let \begin{enumerate} - \item $G_x \defeq \Aut_{X_{hg}}(\shape_G,x)$ + \item $G_x \defeq \Aut_{X_{hG}}(\shape_G,x)$ be the \emph{stabilizer group}\index{stabilizer}% \index{group!stabilizer} at $x$, and \item $G\cdot x \defeq \setof{y : X(\shape_G)}{[x] =_{X/G} [y]}$ @@ -1101,13 +1101,14 @@ \section{Fixed points and orbits} \end{enumerate} \end{definition} Note that the classifying type $\BG_x$ of $G_x$ -is identified with the fiber of $\settrunc{\blank} : X_{hg} \to X/G$, +is identified with the fiber of $\settrunc{\blank} : X_{hG} \to X/G$, and $G\cdot x$ (pointed at $x$) is identified with the fiber of $[\blank] : X(\shape_G) \to X/G$, both taken at $[x]$, the orbit containing $x$. Also, the base point of $\BG_x$ depends on the choice of $x$, -but the underlying type $(\BG_x)_\div$ only depends on $[x]:X/G$. +but the underlying type $(\BG_x)_\div$, \MB{being a connected component,} +only depends on $[x]:X/G$. Thus, we can decompose our diagram by writing $X(\shape_G)$ and $X_{hG}$ as sums of the respective fibers.\footnote{% Yielding the diagram @@ -1118,14 +1119,16 @@ \section{Fixed points and orbits} \& X/G, \& \end{tikzcd} \] - where we use $b$ to denote a \emph{bane}/orbit. (Too cute?)} + where we use $b$ to denote a \emph{bane}/orbit. (Too cute? + \MB{Unclear because of "orbit set". Orbit is pointed bane? Bane is unpointed + orbit? What about saying in 5.4.1 that $X/G$ is the set of (unpointed) orbits?})} The stabilizer group $G_x$ comes equipped with a homomorphism $i_x : \Hom(G_x,G)$, classified by the projection $\fst:X_{hG} \to \BG$.\footnote{% Since the projection is still a \covering, $\iota_x$ is a monomorphism - (\cref{lem:eq-mono-cover}), so $G_x$ together with $i_x$ - becomes a \emph{subgroup} of $G$.} + (\cref{def:typeofmono}), so $G_x$ together with $i_x$ + becomes a \emph{subgroup} of $G$.\MB{Promote to lemma?}} There are two possible extreme cases that are important: \begin{definition}\label{def:invariant-free} Let $X$ be a $G$-set and $x:X(\shape_G)$ an element of the underlying set. @@ -1140,29 +1143,31 @@ \section{Fixed points and orbits} \end{definition} \begin{lemma}\label{lem:invariant-char} - Given a $G$-set $X$, an element $x:(\shape_G)$ is + Given a $G$-set $X$, an element $x:X(\shape_G)$ is invariant if and only if the orbit $G\cdot x$ is contractible, \ie $x = g\cdot x$ for all $g:\USymG$. \end{lemma} \begin{proof} The orbit $G\cdot x$ is the fiber of $\Bi_x : \BG_x \ptdto \BG$ at $\shape_G$. Since $\BG$ is connected, - this is contractible if and only if all fiber of $\Bi$ are contractible, + this is contractible if and only if all fibers of $\Bi$ are contractible, \ie $\Bi_x$ is an equivalence, which in turn is equivalent to $i_x$ being an isomorphism. \end{proof} \begin{lemma}\label{lem:free-pt-char} - Given a $G$-set $X$, an element $x:(\shape_G)$ is\marginnote{% + Given a $G$-set $X$, an element $x:X(\shape_G)$ is\marginnote{% Doesn't fit well here; move to where?} free if and only if the (surjective) map - $\blank \cdot x : G \to G\cdot x$ is injective + $\blank \cdot x : \USymG \to G\cdot x$ is injective (and hence a bijection). \end{lemma} \begin{proof} Consider two elements of the orbit, say $g\cdot x,g'\cdot x$ for $g,g':\USymG$. We have $g\cdot x=g' \cdot x$ if and only if $x = \inv{g} g'\cdot x$ if and only if $\inv{g} g'$ lies in $\USymG_x$. + \MB{Now apply \cref{xca:connected-trivia} yielding that that + $G_x$ is trivial iff $\USymG_x$ is contractible.} \end{proof} When $X : \BG \to \Set$ is a $G$-set for an ordinary group $G$, @@ -1190,12 +1195,22 @@ \section{Fixed points and orbits} \begin{proof} Fix an invariant $x : X(\shape_G)$, so $g\cdot x = x$ for all $g: \USymG$. - We need to show that the type + We need to show that the type\footnote{% + \MB{Isn't this the Yoneda lemma? Is the following argument OK? + Call the type $T_x$. It is a proposition since $\BG$ is connected + (for all $x$, useful to point out). So it suffices to construct an + element $f:T_x$. Since $x$ is invariant, $\Bi_x\jdeq\fst$ is an equivalence. + Hence all $\inv\Bi_x(z)$ are contractible, and they are equivalent with + $\sum_{y:X(z)}\Trunc{(z,y)\eqto(\sh_G,x)}$ (by contracting out, \cref{lem}). + This gives $f$, and we get the proposition $x=f(\sh_G)$ from + $\Trunc{(\sh_G,f(\sh_G))\eqto(\sh_G,x)}$, again by the invariance of $x$. + }} \[ - \sum_{f : \prod_{z:\BG}X(z)}f(\shape_G)=x + \sum_{f : \prod_{z:\BG}X(z)} x=f(\shape_G) \] is contractible. - This is equivalent to the type of pointed sections + This is equivalent to the type of pointed sections\footnote{% + \MB{See ft 52 on p. 40 which had to be corrected: a section is a pair.}} of the projection $\fst : (X_{hG},x) \ptdto \BG$. Since $\BG$ is connected, this is in turn equivalent to the type of pointed sections of $\Bi_x : \BG_x \ptdto \BG$, @@ -1203,6 +1218,17 @@ \section{Fixed points and orbits} This is a proposition, and it's true if and only if $i_x$ is an isomorphism. \end{proof} +\begin{xca}\label{xca:Gset-A->B} +\MB{New, w.i.p.} +Let $G$ be the group $\SG_2\times\SG_2$ and $X$ the $G$-set mapping +any pair $(A,B)$ of $2$-element sets to the set $A\to B$. +Elaborate the action of $G$ on $X(\sh_G)$ and determine the +set of orbits and the set of fixed points. +%Using that any fixed point $f$ of $X$ is completely determined by its +%value $f(\sh) : X(\sh_G)$, determine all fixed points. +\end{xca} + + \section{The classifying type is the type of torsors} \label{sec:torsors} This section can be seen as a motivation for the use of torsors. diff --git a/intro-uf.tex b/intro-uf.tex index 9c913c8..1025d81 100644 --- a/intro-uf.tex +++ b/intro-uf.tex @@ -2710,7 +2710,7 @@ \section{More on equivalences; surjections and injections} in other words, we have a function of type $\prod_{b:B} \sum_{a:A} (b \eqto f(a))$. This is equivalent to saying we have a function - $g:B\to A$ such that $f\circ g \eqto \id_B$ + $g:B\to A$ and an identification $p: f\circ g \eqto \id_B$ (such a $g$ is called a \emph{section} of $f$).} \end{definition}