diff --git a/actions.tex b/actions.tex index 426179b..19a334b 100644 --- a/actions.tex +++ b/actions.tex @@ -1518,14 +1518,12 @@ \subsection{The Orbit-stabilizer theorem and Lagrange's theorem} \end{construction} \begin{implementation}{thm:orbitstab} The desired equivalence is the composition of elementary equivalences - for sums and products, followed by contracting away the variable $z$:% -\footnote{\MB{New:} To understand the proof better, it may help to look -at elements $(\sh_G,x,!,g)$ and $(\sh_G,x,!,g')$ of $(\tilde G_x)_{hG_x}$. -To have and identification of these means having a stabilizer $s$ of $x$ -that transports $g$ to $g'$. There can be at most -one such stabilizer, namely $s=\inv g g'$. -Hence $(\sh_G,x,!,g)\eqto(\sh_G,x,!,g')$ is a proposition, and -equivalent to $g\cdot x = g'\cdot x$.} + for sums and products, followed by contracting away the variable $z$: +\footnote{\MB{New:} Note that \cref{xca:stab-action-on-G} already +implies that $(\tilde G_x)_{hG_x}$ is a set: +given $(\sh_G,x,!,g)$ and $(z,y,!,g')$, there can be at most +one $s : \sh_G \eqto z$ such that $s\cdot_{\tilde G_x} g = sg = g'$. +} \begin{align*} (\tilde G_x)_{hG_x} &\jdeq \sum_{u : \BG_x}\tilde G_x(u) \\ @@ -1535,10 +1533,11 @@ \subsection{The Orbit-stabilizer theorem and Lagrange's theorem} \end{align*} \end{implementation} -The above theorem has some interesting consequences: -Since $(\tilde G_x)_{hG_x}$ is a set, all its components are contractible. +The above theorem has some interesting consequences. +One is that $(\tilde G_x)_{hG_x}$ is a set, +and hence all its components are contractible. This means that for any $g:\USymG$, the stabilizer group $(G_x)_g$ -is free, \ie $\tilde G_x$ is free. +is trivial, \ie $\tilde G_x$ is free. \cref{lem:free-pt-char} below will then allow us to conclude that all $(G_x \cdot_{\tilde G_x} g)$ are equivalent to $\USymG_x$. @@ -1552,25 +1551,25 @@ \subsection{The Orbit-stabilizer theorem and Lagrange's theorem} 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.} + Hence the map $(\blank \cdot x)$ is injective iff $\USymG_x$ is contractible. + Now use \cref{xca:connected-trivia} yielding that that + $G_x$ is trivial iff $\USymG_x$ is contractible. \end{proof} The above results culminate in the following theorem. \begin{theorem}[Lagrange's Theorem]\label{thm:lagrange} -\footnote{\MB{Replaces \cref{old:lagrange}. Beware AC!}} Let $G$ be a group. +\footnote{\MB{Replaces \cref{old:lagrange}. Transitivity of $X$ not needed. +See \cref{con:lagrange} below, where transitivity is used.}} + Let $G$ be a group. %\cref{def:set-of-subgroups} For all subgroups $(X,x,!):\Sub_G$, the $G_x$-set $\tilde G_x$, which has underlying set $\USymG$, is free. As a consequence, - all orbits under this action are merely equivalent to $\USymG_x$ via - $(\blank \cdot_{\tilde G_x} g): \USymG_x \equivto (G_x\cdot_{\tilde G_x} g)$ - by \cref{lem:free-pt-char}. + all orbits under this action are merely equivalent to $\USymG_x$. \end{theorem} \MB{Question: -How close can we get to $\USymG \eqto X(\sh_G) \times \USymG_x$? -(Finite case? Only merely? AC?)}. +How close can we get to $\USymG \eqto (X(\sh_G) \times \USymG_x)$?}. The subgroup in \cref{thm:lagrange} is given as $(X,x,!):\Sub_G$ with $X$ transitive, @@ -1602,7 +1601,7 @@ \subsection{The Orbit-stabilizer theorem and Lagrange's theorem} \USymG \equivto \sum_{O:{\tilde G_x}/G_x} \inv{[O]} \equivto \sum_{y:X(\sh_G)} \inv{[[y]_x]} -\equivto \sum_{y:X(\sh_G)}\sum_{g:\USymG} ([y]_x = [g]) . +\equivto \sum_{y:X(\sh_G)}\sum_{g:\USymG} [y]_x = [g] . \] Clearly, $[y]_x = [g]$ iff $(\sh_G,y,!,\refl{\sh_G})=(\sh_G,x,!,g)$ iff @@ -1612,7 +1611,30 @@ \subsection{The Orbit-stabilizer theorem and Lagrange's theorem} For $y=x$, the inner sum type on the right is the underlying set $\USymG_x$ of the stabilizer group of $x$. +Sufficient for having an equivalence, for all $y:X(\sh_G)$, between +$\USymG_x$ and $\sum_{g:\USymG} (g\cdot_X y = x)$ is to have +a function $f: \prod_{y:X(\sh_G)}\sum_{g:\USymG} g\cdot_X y = x$, +which provides such an equivalence by precomposition with $\fst(f(y))$. +Thus we have implemented the following construction. + + +\begin{construction}[\MB{Lagrange, alternative?}]\label{con:lagrange}\footnote{% +This formulation has the following advantages: +(1) no $\tilde G_x$ in statement (only used in the proof); +(2) transitivity of $X$ is actually used; +(3) works for the finite case, but also, \eg for the $\ZZ$ acting on $\bn3$. +\MB{Remove after discussion.}} +Let $G$ be a group. + For all subgroups $(X,x,!):\Sub_G$, we have a function of type +\[ +\bigl(\prod_{y:X(\sh_G)}\sum_{g:\USymG} g\cdot_X y = x \,\bigr) \to +\bigl(\USymG \eqto (X(\sh_G) \times \USymG_x)\bigr). +\] +\end{construction} + + \subsection{Not used} + Thus, both the underlying set $X(\sh_G)$ and the action type $X_{hG}$ have equivalence relations with quotient set $X/G$.\footnote{% This also justifies the notation $X/G$.