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\newtheorem{prop}{Proposition} \newtheorem{cor}{Corollary} \newtheorem*{utheorem}{Theorem} \newtheorem*{ulemma}{Lemma} \newtheorem*{uprop}{Proposition} \newtheorem*{ucor}{Corollary} \theoremstyle{definition} \newtheorem{defn}{Definition} \newtheorem{example}{Example} \newtheorem*{udefn}{Definition} \newtheorem*{uexample}{Example} \theoremstyle{remark} \newtheorem{remark}{Remark} \newtheorem{note}{Note} \newtheorem*{uremark}{Remark} \newtheorem*{unote}{Note} %------------------------------------------------------------------- \begin{document} %------------------------------------------------------------------- \section*{pro-manifold} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{differential_geometry}{}\paragraph*{{Differential geometry}}\label{differential_geometry} [[!include synthetic differential geometry - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{procartesian_spaces}{Pro-Cartesian spaces}\dotfill \pageref*{procartesian_spaces} \linebreak \noindent\hyperlink{embedding_into_smooth_loci}{Embedding into smooth loci}\dotfill \pageref*{embedding_into_smooth_loci} \linebreak \noindent\hyperlink{SiteOfTowersOfCartesianSpaces}{The site of towers of Cartesian spaces and pro-morphisms}\dotfill \pageref*{SiteOfTowersOfCartesianSpaces} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} A \emph{pro-manifold} is a [[pro-object]] in a [[category]] of [[manifolds]], i.e. a formal [[projective limit]] of [[manifolds]]. Details depend on what exactly is understood by ``[[manifold]]'', i.e. whether [[topological manifolds]] or [[smooth manifold]], etc. Typically one wants to mean [[pro-objects]] in manifolds of [[finite number|finite]] [[dimensions]], the point being then that a pro-manifold is like an [[infinite-dimensional manifold]] but with ``mild'' infinite dimensionality, expressed by the very fact that it may be presented as a formal projective limit of finite dimensional manifolds. To amplify this specification, one should properly speak of ``pro-(finite dimensional smooth manifolds)'', but beware that people often abbreviate to ``pro-manifold'' regardless. Also ``pro-finite manifold'' is in use, which however, strictly speaking, is a misnomer since a ``finite manifold'' is one with a [[finite number]] of points. An important example of pro-objects in finite-dimensional smooth manifolds are infinite [[jet bundles]]. These are the formal projective limits of the underlying finite-order jet bundles. \hypertarget{procartesian_spaces}{}\subsection*{{Pro-Cartesian spaces}}\label{procartesian_spaces} \hypertarget{embedding_into_smooth_loci}{}\subsubsection*{{Embedding into smooth loci}}\label{embedding_into_smooth_loci} \begin{defn} \label{proCartSp}\hypertarget{proCartSp}{} Write [[CartSp]] for the [[full subcategory]] of that of [[smooth manifolds]] on the [[Cartesian spaces]], i.e. on those of the form $\mathbb{R}^n$, for $n \in \mathbb{N}$. Write \begin{displaymath} ProCartSp \coloneqq Pro(CartSp) \end{displaymath} for its category of [[pro-objects]], the \textbf{pro-Cartesian spaces}. \end{defn} \begin{prop} \label{ProCartSpFullyFaithfulInSmoothLoc}\hypertarget{ProCartSpFullyFaithfulInSmoothLoc}{} The functor which sends a formal cofiltered limit of Cartesian spaces to its actual [[cofiltered limit]] of [[smooth loci]] is a [[fully faithful functor]], hence constitutes a [[full subcategory]] inclusion of [[pro-Cartesian spaces]] (def. \ref{proCartSp}) into [[smooth loci]]: \begin{displaymath} Pro(CartSp) \hookrightarrow SmthLoc \,. \end{displaymath} \end{prop} \begin{proof} Since $Pro(\mathcal{C}) \simeq (Ind(\mathcal{C}^{op}))^{op}$ (\href{pro-object#ProObjectAsFormalDualOfIndObject}{remark}) it is sufficient to show that the functor in question is on opposite categories a fully faithful functor of the form \begin{displaymath} Ind(CartSp^{op}) \hookrightarrow SmthLoc^{op} = SmthAlg_{\mathbb{R}} \,, \end{displaymath} where $SmothAlg_{\mathbb{R}}$ is the category of [[smooth algebras]]. Now, there is the [[fully faithful functor]] \begin{displaymath} i \;\colon\; CartSp \hookrightarrow SmthLoc \end{displaymath} (\href{smooth+locus#SmoothManifoldsFullSubcategoryOfSmoothLoci}{prop.}) hence a fully faithful functor \begin{displaymath} i^{op} \colon CartSp^{op} \hookrightarrow SmthAlg_{\mathbb{R}} \,. \end{displaymath} Moreover, the image of the latter is in [[compact objects]] $i^{op} \colon CartSp^{op} \hookrightarrow (SmthAlg_{\mathbb{R}})_{cpt} \hookrightarrow SmthAlg$, because \begin{displaymath} C^\infty(\mathbb{R}^n) \simeq y(\mathbb{R}^n) \in SmthAlg_{\mathbb{R}} \simeq Func_\times(CartSp,Set) \end{displaymath} is [[representable functor|co-representable]], hence [[compact object|compact]] (by the [[Yoneda lemma]] and since colimits are computed objectwise \href{limits+and+colimits+by+example#LimitsOfPresheaves}{prop.}). This implies that the composite \begin{displaymath} Ind(CartSp^{op}) \overset{Ind(i^{op})}{\hookrightarrow} Ind(SmthAlg_{\mathbb{R}}) \overset{L}{\longrightarrow} SmthAlg_{\mathbb{R}} \end{displaymath} is also fully faithful (\href{ind-object#JFIsFullyFaithful}{prop.}). Here $Ind(i^{op})$ takes formal filtered colimits in $CartSp^{op}$ to the corresponding formal colimits in $SmthAlg_{\mathbb{R}}$ (\href{ind-object#FunctorialityOfInd}{prop.}), while $L$ takes formal filtered colimits to actual [[filtered colimits]] (\href{ind-object#ReflectionToYonedaEmbedding}{prop.}). Hence this is indeed the functor in question. \end{proof} \hypertarget{SiteOfTowersOfCartesianSpaces}{}\subsubsection*{{The site of towers of Cartesian spaces and pro-morphisms}}\label{SiteOfTowersOfCartesianSpaces} \begin{quote}% under construction \end{quote} \begin{defn} \label{TowersOfCartesianSpaces}\hypertarget{TowersOfCartesianSpaces}{} Write \begin{displaymath} TowCartSp \hookrightarrow ProCartSp \end{displaymath} for the [[full subcategory]] of the category of [[pro-Cartesian spaces]] (def. \ref{proCartSp}) on those [[pro-objects]] in [[CartSp]] which are presented as formal [[sequential limits]] of [[tower]] diagrams, i.e. where the indexing category $\mathcal{K} = \mathbb{N}_{\geq}$. \end{defn} \begin{defn} \label{ProGoodOpenCoverOnACartesianSpace}\hypertarget{ProGoodOpenCoverOnACartesianSpace}{} For $U \in TowCartSp$ a [[tower of Cartesian spaces]] (def. \ref{TowersOfCartesianSpaces}), say that a \textbf{tower of [[good open covers]]} of $U$ is a sequence of morphisms $\{U_i \overset{\phi_i}{\to} U\}$ in $TowCartSp$ such that these are the formal [[sequential limit]] of a cofiltered diagram of [[good open covers]] $\{U_i^k \overset{\phi_i^k}{\to} U^k\}$. \begin{displaymath} \itexarray{ U_i^{k} &\overset{\underset{\longleftarrow}{\lim}^f}{\mapsto}& U_i \\ {}^{\mathllap{\phi_i^k}}\downarrow && \downarrow^{\mathrlap{\phi_i}} \\ U^k &\overset{\underset{\longleftarrow}{\lim}^f}{\mapsto}& U } \end{displaymath} \end{defn} \begin{defn} \label{}\hypertarget{}{} The collection of towers of [[good open covers]] on $TowCartSp$, according to def. \ref{ProGoodOpenCoverOnACartesianSpace}, constitutes a [[coverage]]. \end{defn} \begin{proof} By the definition of [[coverage]] (\href{coverage#ConditionsOnACoverage}{def.}) we need to check that for every tower of [[good open covers]] $\{U_i \overset{\phi_i}{\to} U\}$ and for every morphism $V \overset{g}{\longrightarrow} U$ in $TowCartSp$, there exists a tower of [[good open covers]] $\{V_j \overset{\psi_j}{\longrightarrow} V\}$ of $V$ such that for each index $j$ we may find an index $i$ and a morphism $V_j \overset{}{\to} U_i$ such as to make a [[commuting diagram]] of the form \begin{displaymath} \itexarray{ V_j &\overset{}{\longrightarrow}& U_i \\ \downarrow && \downarrow^{\mathrlap{\phi_i}} \\ V &\underset{g}{\longrightarrow}& U } \,. \end{displaymath} Now by \href{tower#ProMorphismsBetweenTowerDiagrams}{this prop.} the bottom morphism is represented by a sequence of component morphisms \begin{displaymath} V^{n(k)} \overset{}{\longrightarrow} U^k \,. \end{displaymath} Since ordinary [[good open covers]] do form a [[coverage]] on [[CartSp]] (\href{good+open+cover#GoodOpenCoversFormACoverageOnParacompactSmooothManifolds}{prop.}) each of these component diagrams may be completed \begin{displaymath} \itexarray{ \tilde V^{n(k)}_j &\overset{}{\longrightarrow}& U^k_i \\ \downarrow && \downarrow^{\mathrlap{\phi^k_i}} \\ V^{n(k)} &\underset{g^k}{\longrightarrow}& U^k } \end{displaymath} by first forming the [[pullback]] [[open cover]] $(g^k)^\ast U^k_i \to V^{n(k)}$ and then refining this to a [[good open cover]] $\tilde V^{n(k)}_j \to V^{n(k)}$. By the [[universal property]] of the [[pullback]], there are morphisms \begin{displaymath} \tilde V^{n(k+1)} \longrightarrow (g^k)^\ast U^k_i \end{displaymath} that make the evident cube commute \begin{displaymath} \itexarray{ \tilde V^{n(k+1)}_j &\overset{}{\longrightarrow}& U^{k+1}_i \\ \downarrow && \downarrow^{\mathrlap{\phi^{k+1}_i}} \\ V^{n(k+1)} &\underset{g^{k+1}}{\longrightarrow}& U^{k+1} } \;\;\;\;\;\;\;\;\;\;\;\; \Rightarrow \;\;\;\;\;\;\;\;\;\;\;\; \itexarray{ (g^{k})^\ast U_i^k &\overset{}{\longrightarrow}& U^k_i \\ \downarrow && \downarrow^{\mathrlap{\phi^k_i}} \\ V^{n(k)} &\underset{g^k}{\longrightarrow}& U^k } \end{displaymath} Take \begin{displaymath} V^{n(0)}_j \coloneqq \tilde V^{n(0)}_j \end{displaymath} and then inductively define \begin{displaymath} V^{n(k+1)}_j \end{displaymath} to be a refinement by a good open cover of the joint refinement of $\{\tilde V^{n(k+1)}_j\}$ with the pullback of $\{V^{n(k)}_j\}$ to $V^{n(k+1)}$. This refines the above commuting cubes to \begin{displaymath} \itexarray{ V_j^{n(k+1)} &\overset{}{\longrightarrow}& U^{k+1}_i \\ \downarrow && \downarrow^{\mathrlap{\phi^{k+1}_i}} \\ V^{n(k+1)} &\underset{g^{k+1}}{\longrightarrow}& U^{k+1} } \;\;\;\;\;\;\;\;\;\;\;\; \Rightarrow \;\;\;\;\;\;\;\;\;\;\;\; \itexarray{ V_j^{n(k)} &\overset{}{\longrightarrow}& U^k_i \\ \downarrow && \downarrow^{\mathrlap{\phi^k_i}} \\ V^{n(k)} &\underset{g^k}{\longrightarrow}& U^k } \end{displaymath} and hence provides components for the required diagram in $TowCartSp$. \end{proof} [[!redirects pro-manifolds]] [[!redirects promanifold]] [[!redirects promanifolds]] [[!redirects pro-Cartesian space]] [[!redirects pro-Cartesian spaces]] [[!redirects tower of Cartesian spaces]] [[!redirects towers of Cartesian spaces]] [[!redirects tower of cartesian spaces]] [[!redirects towers of cartesian spaces]] \end{document}