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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*{CartSp} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{differential_geometry}{}\paragraph*{{Differential geometry}}\label{differential_geometry} [[!include synthetic differential geometry - contents]] \hypertarget{higher_geometry}{}\paragraph*{{Higher geometry}}\label{higher_geometry} [[!include higher geometry - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{properties}{Properties}\dotfill \pageref*{properties} \linebreak \noindent\hyperlink{as_a_small_category_of_objects_with_a_basis}{As a small category of objects with a basis}\dotfill \pageref*{as_a_small_category_of_objects_with_a_basis} \linebreak \noindent\hyperlink{as_a_site}{As a site}\dotfill \pageref*{as_a_site} \linebreak \noindent\hyperlink{as_a_category_with_open_maps}{As a category with open maps}\dotfill \pageref*{as_a_category_with_open_maps} \linebreak \noindent\hyperlink{as_an_algebraic_theory}{As an algebraic theory}\dotfill \pageref*{as_an_algebraic_theory} \linebreak \noindent\hyperlink{as_a_pregeometry}{As a pre-geometry}\dotfill \pageref*{as_a_pregeometry} \linebreak \noindent\hyperlink{related_concepts}{Related concepts}\dotfill \pageref*{related_concepts} \linebreak \noindent\hyperlink{References}{References}\dotfill \pageref*{References} \linebreak \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} \begin{defn} \label{}\hypertarget{}{} Write $CartSp$ for the [[category]] whose \begin{itemize}% \item [[object]]s are [[Cartesian space]]s $\mathbb{R}^n$ for $n \in \mathbb{N}$; \item [[morphisms]] are suitable structure-preserving [[function]]s between these spaces. \end{itemize} For definiteness we write $CartSp_{lin}$ for the category whose objects are [[Cartesian space]]s regarded as [[real number|real]] [[vector space]]s and whose morphisms are [[linear function]]s between these; \begin{itemize}% \item $CartSp_{top}$ for the category whose objects are Cartesian spaces regarded as [[topological spaces]] equipped with their [[Euclidean topology]] and morphisms are [[continuous maps]] between them. \item $CartSp_{smooth}$ for the category whose objects are Cartesian spaces regarded as [[smooth manifolds]] with their standard [[smooth structure]] and morphisms are [[smooth function]]s. \end{itemize} \end{defn} \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \hypertarget{as_a_small_category_of_objects_with_a_basis}{}\subsubsection*{{As a small category of objects with a basis}}\label{as_a_small_category_of_objects_with_a_basis} A [[Cartesian space]] carries a lot of structure, for instance [[CartSp]] may be naturally regarded as a [[full subcategory]] of the category $C$, for $C$ (any one of) the category of \begin{itemize}% \item [[vector spaces]], \item [[affine spaces]], \item [[normed vector spaces]], \item [[inner product spaces]], \item [[Euclidean spaces]]. \end{itemize} In all these cases, the inclusion $CartSp \hookrightarrow C$ is an [[equivalence of categories]]: choosing an [[isomorphism]] from any of these objects to a [[Cartesian space]] amounts to choosing a [[basis]] of a [[vector space]], a [[coordinate system]]. \hypertarget{as_a_site}{}\subsubsection*{{As a site}}\label{as_a_site} \begin{defn} \label{}\hypertarget{}{} Write \begin{itemize}% \item [[CartSp]]${}_{top}$ for the category whose [[object]]s are Cartesian spaces and whose [[morphism]]s are all [[continuous maps]] between these. \item [[CartSp]]${}_{smooth}$ for the category whose [[object]]s are Cartesian spaces and whose [[morphism]]s are all [[smooth functions]] between these. \item [[CartSp]]${}_{synthdiff}$ for the [[full subcategory]] of the category of [[smooth loci]] on those of the form $\mathbb{R}^n \times D$ for $D$ an [[infinitesimal space]] (the formal dual of a Weil algebra). \end{itemize} \end{defn} \begin{prop} \label{}\hypertarget{}{} In all three cases there is the [[good open cover]] [[coverage]] that makes [[CartSp]] a [[site]]. \end{prop} \begin{proof} For [[CartSp]]${}_{top}$ this is obvious. For [[CartSp]]${}_{smooth}$ this is somewhat more subtle. It is a folk theorem (see the references at [[open ball]]). A detailed proof is at [[good open cover]]. This directly carries over to $CartSp_{synthdiff}$. \end{proof} \begin{prop} \label{AsDenseSubsite}\hypertarget{AsDenseSubsite}{} \begin{itemize}% \item The site $CartSp_{top}$ is a [[dense subsite]] of the site of [[paracompact topological space|paracompact]] [[topological manifold]]s with the [[open cover]] [[coverage]]. \item The site $CartSp_{smooth}$ is a [[dense subsite]] of the [[site]] [[Diff]] of [[paracompact topological space|paracompact]] [[smooth manifold]]s equipped with the [[open cover]] [[coverage]]. \end{itemize} \end{prop} \begin{prop} \label{}\hypertarget{}{} Equipped with this structure of a site, [[CartSp]] is an [[∞-cohesive site]]. \end{prop} The corresponding [[cohesive topos]] [[sheaf topos|of sheaves]] is \begin{itemize}% \item $Sh_{(1,1)}(CartSp_{smooth})$, discussed at [[diffeological space]]. \item $Sh_{(1,1)}(CartSp_{synthdiff})$, discussed at [[Cahiers topos]]. \end{itemize} The corresponding [[cohesive (∞,1)-topos]] [[(∞,1)-category of (∞,1)-sheaves|of (∞,1)-sheaves]] is \begin{itemize}% \item $Sh_{(\infty,1)}(CartSp_{top}) =$ [[ETop∞Grpd]]; \item $Sh_{(\infty,1)}(CartSp_{smooth}) =$ [[Smooth∞Grpd]]; \item $Sh_{(\infty,1)}(CartSp_{synthdiff}) =$ [[SynthDiff∞Grpd]]; \end{itemize} \begin{cor} \label{}\hypertarget{}{} We have [[equivalences of categories]] \begin{itemize}% \item $Sh(CartSp_{top}) \simeq Sh(TopMfd)$ \item $Sh(CartSp_{smooth}) \simeq Sh(Diff)$ \end{itemize} and [[equivalences of (∞,1)-categories]] \begin{itemize}% \item $Sh_{(\infty,1)}(CartSp_{top}) \simeq Sh_{(\infty,1)}(TopMfd)$; \item $Sh_{(\infty,1)}(CartSp_{smooth}) \simeq Sh_{(\infty,1)}(Diff)$. \end{itemize} \end{cor} \begin{proof} The first two statements follow by the \hyperlink{AsDenseSubsite}{above proposition} with the \emph{comparison lemma} discussed at [[dense sub-site]]. For the second condition notice that since an [[∞-cohesive site]] is in particular an [[∞-local site]] we have that $Sh_{(\infty,1)}(CartSp)$ is a [[local (∞,1)-topos]]. As discussed there, this implies that it is a [[hypercomplete (∞,1)-topos]]. By the discussion at [[model structure on simplicial presheaves]] this means that it is [[presentable (∞,1)-category|presented]] by the Joyal-Jardine-[[model structure on simplicial sheaves]] $Sh(CartSp)^{\Delta^{op}}_{loc}$. The claim then follows with the first two statements. \end{proof} \hypertarget{as_a_category_with_open_maps}{}\subsubsection*{{As a category with open maps}}\label{as_a_category_with_open_maps} There is a canonical structure of a category with [[open map]]s on $CartSp$ (\ldots{}) \hypertarget{as_an_algebraic_theory}{}\subsubsection*{{As an algebraic theory}}\label{as_an_algebraic_theory} The category $CartSp$ is (the [[syntactic category]] of ) a [[Lawvere theory]]: the theory for [[smooth algebra]]s. \hypertarget{as_a_pregeometry}{}\subsubsection*{{As a pre-geometry}}\label{as_a_pregeometry} Equipped with the above [[coverage]]-structure, [[open map]]-structure and [[Lawvere theory]]-property, $CartSp$ is essentially a [[pregeometry (for structured (∞,1)-toposes)]]. (Except that the pullback stability of the open maps holds only in the weaker sense of [[coverage]]s). (\ldots{}) $\,$ \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} [[!include geometries of physics -- table]] $\,$ \hypertarget{References}{}\subsection*{{References}}\label{References} In secton 2 of \begin{itemize}% \item [[R. Blute]], [[J.R.B. Cockett]], [[Robert Seely]], \emph{Cartesian differential categories}, Theory and Applications of Categories, Vol. 22, 2009, No. 23, pp 622-672. (\href{http://www.tac.mta.ca/tac/volumes/22/23/22-23abs.html}{journal}, \href{http://www.tac.mta.ca/tac/volumes/22/23/22-23.pdf}{pdf}) \end{itemize} $CartSp$ is discussed as an example of a ``cartesian differential category''. There are various slight variations of the category $CartSp$ (many of them [[equivalence of categories|equivalent]]) that one can consider without changing its basic properties as a category of test spaces for [[generalized smooth spaces]]. A different choice that enjoys some popularity in the literature is the category of open (contractible) subsets of Euclidean spaces. For more references on this see [[diffeological space]]. The [[site]] $CartSp_{synthdiff}$ of [[formal smooth manifold|infinitesimally thickened]] Cartesian spaces is known as the site for the [[Cahiers topos]]. It is considered in detail in section 5 of \begin{itemize}% \item [[Anders Kock]], \emph{Convenient vector spaces embed into the Cahiers topos} (\href{http://www.numdam.org/item?id=CTGDC_1986__27_1_3_0}{numdam}) \end{itemize} and briefly mentioned in example 2) on p. 191 of \begin{itemize}% \item [[Anders Kock]], \emph{Synthetic differential geometry} (\href{http://home.imf.au.dk/kock/sdg99.pdf}{pdf}) \end{itemize} following the original article \begin{itemize}% \item [[Eduardo Dubuc]], \emph{Sur les modeles de la geometrie differentielle synthetique} (\href{http://www.numdam.org/item?id=CTGDC_1979__20_3_231_0}{numdam}). \end{itemize} With an eye towards [[Frölicher space]]s the site is also considered in section 5 of \begin{itemize}% \item Hirokazu Nishimura, \emph{Beyond the Regnant Philosophy of Manifolds} (\href{http://arxiv.org/abs/0912.0827}{arXiv:0912.0827}) \end{itemize} category: category [[!redirects CartSp]] [[!redirects Cart Sp]] [[!redirects CartesianSpace]] [[!redirects Cartesian Space]] [[!redirects CartesianSpaces]] [[!redirects Cartesian Spaces]] [[!redirects SmoothCartSp]] [[!redirects smooth Cartesian space]] [[!redirects smooth Cartesian spaces]] \end{document}