diff --git a/etale-cohomology.tex b/etale-cohomology.tex index adda2e378..25886a977 100644 --- a/etale-cohomology.tex +++ b/etale-cohomology.tex @@ -7645,91 +7645,6 @@ \section{Vanishing of finite higher direct images} \end{proof} -\section{Schemes \'etale over a point} -\label{section-schemes-etale-point} - -\noindent -In this section we describe schemes \'etale over the spectrum of -a field. Before we state the result we introduce -the category of $G$-sets for a topological group $G$. - -\begin{definition} -\label{definition-G-set-continuous} -Let $G$ be a topological group. -A {\it $G$-set}, sometime called a {\it discrete $G$-set}, -is a set $X$ endowed with a left action $a : G \times X \to X$ -such that $a$ is continuous when $X$ is given the discrete topology and -$G \times X$ the product topology. -A {\it morphism of $G$-sets} $f : X \to Y$ is simply any $G$-equivariant -map from $X$ to $Y$. -The category of $G$-sets is denoted {\it $G\textit{-Sets}$}. -\end{definition} - -\noindent -The condition that $a : G \times X \to X$ is continuous signifies -simply that the stabilizer of any $x \in X$ is open in $G$. -If $G$ is an abstract group $G$ (i.e., a group but not a topological group) -then this agrees with our preceding definition (see for example -Sites, Example \ref{sites-example-site-on-group}) -provided we endow $G$ with the discrete topology. - -\medskip\noindent -Recall that if $K \subset L$ is an infinite Galois extension the -Galois group $G = \text{Gal}(L/K)$ comes endowed with a canonical -topology. Namely the open subgroups are the subgroups of the form -$\text{Gal}(L/K') \subset G$ where $K'/K$ is a finite subextension of $L/K$. -The index of an open subgroup is always finite. -We say that $G$ is a profinite (topological) group. - -\begin{lemma} -\label{lemma-sheaves-point} -Let $K$ be a field. -Let $K^{sep}$ a separable closure of $K$. -Consider the profinite group -$$ -G = \text{Aut}_{\Spec(K)}(\Spec(K^{sep}))^{opp} = -\text{Gal}(K^{sep}/K) -$$ -The functor -$$ -\begin{matrix} -\text{schemes \'etale over }K & -\longrightarrow & -G\textit{-Sets} \\ -X/K & \longmapsto & -\Mor_{\Spec(K)}(\Spec(K^{sep}), X) -\end{matrix} -$$ -is an equivalence of categories. -\end{lemma} - -\begin{proof} -A scheme $X$ over $K$ is \'etale over $K$ if and only if -$X \cong \coprod_{i\in I} \Spec(K_i)$ with -each $K_i$ a finite separable extension of $K$. -The functor of the lemma associates to $X$ the $G$-set -$$ -\coprod\nolimits_i \Hom_K(K_i, K^{sep}) -$$ -with its natural left $G$-action. Each element has an open stabilizer -by definition of the topology on $G$. Conversely, any $G$-set $S$ -is a disjoint union of its orbits. Say $S = \coprod S_i$. Pick $s_i \in S_i$ -and denote $G_i \subset G$ its open stabilizer. By Galois theory the fields -$(K^{sep})^{G_i}$ are finite separable field extensions of $K$, and -hence the scheme -$$ -\coprod\nolimits_i \Spec((K^{sep})^{G_i}) -$$ -is \'etale over $K$. This gives an inverse to the functor of the lemma. -Some details omitted. -\end{proof} - -\begin{remark} -\label{remark-covering-surjective} -Under the correspondence of the lemma, the coverings in the small \'etale site -$\Spec(K)_\etale$ of $K$ correspond to surjective families of -maps in $G\textit{-Sets}$. -\end{remark} @@ -7838,7 +7753,7 @@ \section{Galois action on stalks} \end{enumerate} Altogether we see that $\mathcal{F}_{\overline{s}}$ becomes a $\text{Gal}_{\kappa(s)}$-set (see -Definition \ref{definition-G-set-continuous}). +\'Etale Morphisms, Definition \ref{etale-definition-G-set-continuous}). Hence we may think of the stalk functor as a functor $$ \Sh(S_\etale) \longrightarrow @@ -7863,7 +7778,7 @@ \section{Galois action on stalks} \begin{proof} Let us construct the inverse to this functor. In -Lemma \ref{lemma-sheaves-point} +\'Etale Morphisms of Schemes, Lemma \ref{etale-lemma-sheaves-point} we have seen that given a $G$-set $M$ there exists an \'etale morphism $X \to \Spec(K)$ such that $\Mor_K(\Spec(K^{sep}), X)$ is @@ -7878,9 +7793,9 @@ \section{Galois action on stalks} \begin{remark} \label{remark-every-sheaf-representable} -Another way to state the conclusions of -Lemmas \ref{lemma-sheaves-point} and -Theorem \ref{theorem-equivalence-sheaves-point} +Another way to state the conclusion of +Theorem \ref{theorem-equivalence-sheaves-point} and +\'Etale Morphisms of Schemes, Lemma \ref{etale-lemma-sheaves-point} is to say that every sheaf on $\Spec(K)_\etale$ is representable by a scheme $X$ \'etale over $\Spec(K)$. This does not mean that every sheaf is representable in the sense of diff --git a/etale.tex b/etale.tex index 034a1766d..9bf4ae7db 100644 --- a/etale.tex +++ b/etale.tex @@ -2184,6 +2184,94 @@ \section{Relative morphisms} +\section{Schemes \'etale over a point} +\label{section-schemes-etale-point} + +\noindent +In this section we describe schemes \'etale over the spectrum of a field. +Before we state the result we introduce the category of $G$-sets for a +topological group $G$. + +\begin{definition} +\label{definition-G-set-continuous} +Let $G$ be a topological group. +A {\it $G$-set}, sometime called a {\it discrete $G$-set}, +is a set $X$ endowed with a left action $a : G \times X \to X$ +such that $a$ is continuous when $X$ is given the discrete topology and +$G \times X$ the product topology. +A {\it morphism of $G$-sets} $f : X \to Y$ is simply any $G$-equivariant +map from $X$ to $Y$. +The category of $G$-sets is denoted {\it $G\textit{-Sets}$}. +\end{definition} + +\noindent +The condition that $a : G \times X \to X$ is continuous signifies +simply that the stabilizer of any $x \in X$ is open in $G$. +If $G$ is an abstract group $G$ (i.e., a group but not a topological group) +then this agrees with our preceding definition (see for example +Sites, Example \ref{sites-example-site-on-group}) +provided we endow $G$ with the discrete topology. + +\medskip\noindent +Recall that if $K \subset L$ is an infinite Galois extension then the +Galois group $G = \text{Gal}(L/K)$ comes endowed with a canonical +topology, see Fields, Section \ref{fields-section-infinite-galois}. + +\begin{lemma} +\label{lemma-sheaves-point} +Let $K$ be a field. Let $K^{sep}$ a separable closure of $K$. +Consider the profinite group $G = \text{Gal}(K^{sep}/K)$. +The functor +$$ +\begin{matrix} +\text{schemes \'etale over }K & +\longrightarrow & +G\textit{-Sets} \\ +X/K & \longmapsto & +\Mor_{\Spec(K)}(\Spec(K^{sep}), X) +\end{matrix} +$$ +is an equivalence of categories. +\end{lemma} + +\begin{proof} +A scheme $X$ over $K$ is \'etale over $K$ if and only if +$X \cong \coprod_{i\in I} \Spec(K_i)$ with +each $K_i$ a finite separable extension of $K$ +(Morphisms, Lemma \ref{morphisms-lemma-etale-over-field}). +The functor of the lemma associates to $X$ the $G$-set +$$ +\coprod\nolimits_i \Hom_K(K_i, K^{sep}) +$$ +with its natural left $G$-action. Each element has an open stabilizer +by definition of the topology on $G$. Conversely, any $G$-set $S$ +is a disjoint union of its orbits. Say $S = \coprod S_i$. Pick $s_i \in S_i$ +and denote $G_i \subset G$ its open stabilizer. By Galois theory +(Fields, Theorem \ref{fields-theorem-inifinite-galois-theory}) +the fields $(K^{sep})^{G_i}$ are finite separable field extensions of $K$, and +hence the scheme +$$ +\coprod\nolimits_i \Spec((K^{sep})^{G_i}) +$$ +is \'etale over $K$. This gives an inverse to the functor of the lemma. +Some details omitted. +\end{proof} + +\begin{remark} +\label{remark-covering-surjective} +Under the correspondence of Lemma \ref{lemma-sheaves-point}, +the coverings in the small \'etale site +$\Spec(K)_\etale$ of $K$ correspond to surjective families of +maps in $G\textit{-Sets}$. +\end{remark} + + + + + + + + \section{Galois categories} \label{section-galois} @@ -2215,20 +2303,16 @@ \section{Galois categories} \begin{example} \label{example-galois-category-G-sets} -Let $G$ be a topological group. A $G$-set, sometimes called a discrete -$G$-set is a set $X$ endowed with a left action $a : G \times X \to X$ -such that $a$ is continuous when $X$ is given the discrete topology -and $G \times X$ the product topology. A morphism of $G$-sets $f : X \to Y$ -is a $G$-equivariant map from $X$ to $Y$. The category of $G$-sets -is denoted $G\textit{-Sets}$. -An important example will be the forgetful functor +Let $G$ be a topological group. An important example will be the +forgetful functor \begin{equation} \label{equation-forgetful} \textit{Finite-}G\textit{-Sets} \longrightarrow \textit{Sets} \end{equation} -where $\textit{Finite-}G\textit{-Sets}$ -is the full subcategory of $G\textit{-Sets}$ -whose objects are the finite $G$-sets. +where $\textit{Finite-}G\textit{-Sets}$ is the full subcategory of +$G\textit{-Sets}$ whose objects are the finite $G$-sets. +The category $G\textit{-Sets}$ of $G$-sets is defined in +Definition \ref{definition-G-set-continuous}. \end{example} \noindent @@ -3116,6 +3200,64 @@ \section{Finite \'etale morphisms} transformation of functors to get (3). \end{proof} +\begin{lemma} +\label{lemma-fundamental-group-Galois-group} +Let $k$ be a field and let $\overline{k}$ be an algebraic closure. +Set $X = \Spec(k)$ and denote $\overline{x} : \Spec(\overline{k}) \to X$ +be the geometric point corresponding to our chose algebraic closure. +Let $k \subset k^{sep} \subset \overline{k}$ be the separable +algebraic closure. There is a canonical isomorphism +$$ +\text{Gal}(k^{sep}/k) \longrightarrow \pi_1(X, \overline{x}) +$$ +of profinite topological groups. +\end{lemma} + +\begin{proof} +We first carefully construct the map. Observe that +$\text{Gal}(k^{sep}/k) = \text{Aut}(\overline{k}/k)$ +as $\overline{k}$ is the perfection of $k^{sep}$. +Then recall that $\pi_1(X, \overline{x}) = \text{Aut}(F_{\overline{x}})$ +where $F_{\overline{x}}$ is the functor +$$ +Y \longmapsto F_{\overline{x}}(Y) = \Mor_X(\Spec(\overline{k}), Y) +$$ +Consider the map +$$ +\text{Aut}(\overline{k}/k) \times F_{\overline{x}}(Y) +\to F_{\overline{x}}(Y),\quad +(\sigma, \overline{y}) \mapsto +\sigma \cdot \overline{y} = \overline{y} \circ \Spec(\sigma) +$$ +This is an action because +$$ +\sigma\tau \cdot \overline{y} = +\overline{y} \circ \Spec(\sigma\tau) = +\overline{y} \circ \Spec(\tau) \circ \Spec(\sigma) = +\sigma \cdot (\tau \cdot \overline{y}) +$$ +The action is functorial in $Y \in \textit{F\'Et}_X$ and we +obtain the canonical map. + +\medskip\noindent +Using our map above for every object $Y$ in $\textit{F\'Et}_X$ +the finite set $F_{\overline{x}}(Y)$ gets a canonical +$\text{Gal}(k^{sep}/k)$-action. To finish the proof it suffices +to show that each $F_{\overline{x}}(Y)$ is an object of +$\textit{Finite-}\text{Gal}(k^{sep}/k)\textit{-Sets}$ +and that in this way we obtain an equivalence of categories +$\textit{F\'Et}_X \to \textit{Finite-}\text{Gal}(k^{sep}/k)\textit{-Sets}$. +This is sufficient by the recognition results in +Proposition \ref{proposition-galois} and +Lemma \ref{lemma-single-out-profinite}. +To see this one shows that the construction given here +is the same as the construction in the equivalence +Lemma \ref{lemma-sheaves-point} +and that the equivalence of that lemma induces an equivalence between +the category of finite \'etale schemes over $\Spec(K)$ +and finite $G$-sets. We omit the details. +\end{proof} +