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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*{topologically enriched category} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{enriched_category_theory}{}\paragraph*{{Enriched category theory}}\label{enriched_category_theory} [[!include enriched category theory contents]] \hypertarget{topology}{}\paragraph*{{Topology}}\label{topology} [[!include topology - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{topologically_enriched_presheaves}{Topologically enriched presheaves}\dotfill \pageref*{topologically_enriched_presheaves} \linebreak \noindent\hyperlink{related_concepts}{Related concepts}\dotfill \pageref*{related_concepts} \linebreak \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} In the following we say \emph{[[Top]]-[[enriched category]]} and \emph{[[Top]]-[[enriched functor]]} etc. for what often is referred to as ``[[topological category]]'' and ``[[topological functor]]'' etc. As discussed there, these latter terms are ambiguous. \begin{defn} \label{kTop}\hypertarget{kTop}{} Write \begin{displaymath} Top_{cg} \hookrightarrow Top \end{displaymath} for the [[full subcategory]] of [[Top]] on the [[compactly generated topological spaces]]. Under forming [[Cartesian product]] \begin{displaymath} (-)\times (-) \;\colon\; Top_{cg} \times Top_{cg} \longrightarrow Top_{cg} \end{displaymath} and compactly generated [[mapping spaces]] \begin{displaymath} (-)^{(-)} \;\colon\; Top_{cg}^{op}\times Top_{cg} \longrightarrow Top_{cg} \end{displaymath} this is a [[cartesian closed category]] (see at \emph{[[convenient category of topological spaces]]}). \end{defn} \begin{defn} \label{TopEnrichedCategory}\hypertarget{TopEnrichedCategory}{} A \textbf{[[topologically enriched category]]} $\mathcal{C}$ is a $Top_{cg}$-[[enriched category]], hence: \begin{enumerate}% \item a [[class]] $Obj(\mathcal{C})$, called the \textbf{class of [[objects]]}; \item for each $a,b\in Obj(\mathcal{C})$ a [[compactly generated topological space]] \begin{displaymath} \mathcal{C}(a,b)\in Top_{cg} \,, \end{displaymath} called the \textbf{space of [[morphisms]]} or the \textbf{[[hom-space]]} between $a$ and $b$; \item for each $a,b,c\in Obj(\mathcal{C})$ a [[continuous function]] \begin{displaymath} \circ_{a,b,c} \;\colon\; \mathcal{C}(a,b)\times \mathcal{C}(b,c) \longrightarrow \mathcal{C}(a,c) \end{displaymath} out of the cartesian product, called the \emph{[[composition]]} operation \item for each $a \in Obj(\mathcal{C})$ a point $id_a\in \mathcal{C}(a,a)$, called the \emph{[[identity]]} morphism on $a$ \end{enumerate} such that the composition is [[associativity|associative]] and [[unitality|unital]]. \end{defn} \begin{remark} \label{UnderlyingCategoryOfTopEnrichedCategory}\hypertarget{UnderlyingCategoryOfTopEnrichedCategory}{} Given a [[topologically enriched category]] as in def. \ref{TopEnrichedCategory}, then forgetting the topology on the [[hom-spaces]] (along the [[forgetful functor]] $U \colon Top_k \to Set$) yields an ordinary [[locally small category]] with \begin{displaymath} Hom_{\mathcal{C}}(a,b) = U(\mathcal{C}(a,b)) \,. \end{displaymath} It is in this sense that $\mathcal{C}$ is a category with [[extra structure]], and hence ``[[enriched category|enriched]]''. \end{remark} The archetypical example is the following: \begin{example} \label{TopkAsATopologicallyEnrichedCategory}\hypertarget{TopkAsATopologicallyEnrichedCategory}{} The category $Top_{cg}$ from def. \ref{kTop} itself, being a [[cartesian closed category]], canonically obtains the structure of a [[topologically enriched category]], def. \ref{TopEnrichedCategory}, with [[hom-spaces]] given by compactly generated [[mapping spaces]] \begin{displaymath} Top_{cg}(X,Y) \coloneqq Y^X \end{displaymath} and with [[composition]] \begin{displaymath} Y^X \times Z^Y \longrightarrow Z^X \end{displaymath} given by the (product$\dashv$ mapping-space)-[[adjunct]] of the [[evaluation morphism]] \begin{displaymath} X \times Y^X \times Z^Y \overset{(ev, id)}{\longrightarrow} Y \times Z^Y \overset{ev}{\longrightarrow} Z \,. \end{displaymath} \end{example} \begin{defn} \label{TopologicallyEnrichedFunctor}\hypertarget{TopologicallyEnrichedFunctor}{} A [[topologically enriched functor]] between two [[topologically enriched categories]] \begin{displaymath} F \;\colon\; \mathcal{C} \longrightarrow \mathcal{D} \end{displaymath} is a $Top_{cg}$-[[enriched functor]], hence: \begin{enumerate}% \item a [[function]] \begin{displaymath} F_0 \colon Obj(\mathcal{C}) \longrightarrow Obj(\mathcal{D}) \end{displaymath} of [[objects]]; \item for each $a,b \in Obj(\mathcal{C})$ a [[continuous function]] \begin{displaymath} F_{a,b} \;\colon\; \mathcal{C}(a,b) \longrightarrow \mathcal{D}(F_0(a), F_0(b)) \end{displaymath} of [[hom-spaces]] \end{enumerate} such that this preserves [[composition]] and [[identity]] morphisms in the evident sense. A [[homomorphism]] of topologically enriched functors \begin{displaymath} \eta \;\colon\; F \Rightarrow G \end{displaymath} is a $Top_{cg}$-[[enriched natural transformation]]: for each $c \in Obj(\mathcal{C})$ a choice of morphism $\eta_c \in \mathcal{D}(F(c),G(c))$ such that for each pair of objects $c,d \in \mathcal{C}$ the two continuous functions \begin{displaymath} \eta_d \circ F(-) \;\colon\; \mathcal{C}(c,d) \longrightarrow \mathcal{D}(F(c), G(d)) \end{displaymath} and \begin{displaymath} G(-) \circ \eta_c \;\colon\; \mathcal{C}(c,d) \longrightarrow \mathcal{D}(F(c), G(d)) \end{displaymath} agree. We write $[\mathcal{C}, \mathcal{D}]$ for the resulting category of topologically enriched functors. This itself naturally obtains the structure of [[topologically enriched category]], see at \emph{[[enriched functor category]]}. \end{defn} \hypertarget{topologically_enriched_presheaves}{}\subsection*{{Topologically enriched presheaves}}\label{topologically_enriched_presheaves} \begin{example} \label{TopologicallyEnrichedFunctorsToTopk}\hypertarget{TopologicallyEnrichedFunctorsToTopk}{} For $\mathcal{C}$ any topologically enriched category, def. \ref{TopEnrichedCategory} then a topologically enriched functor \begin{displaymath} F \;\colon\; \mathcal{C} \longrightarrow Top_{cg} \end{displaymath} to the archetical topologically enriched category from example \ref{TopkAsATopologicallyEnrichedCategory} may be thought of as a topologically [[enriched presheaf|enriched]] [[copresheaf]], at least if $\mathcal{C}$ is [[small category|small]] (in that its [[class]] of objects is a proper [[set]]). Hence the category of topologically enriched functors \begin{displaymath} [\mathcal{C}, Top_{cg}] \end{displaymath} according to def. \ref{TopologicallyEnrichedFunctor} may be thought of as the ([[copresheaf|co-]])[[presheaf category]] over $\mathcal{C}$ in the realm of topological enriched categories. A functor $F \in [\mathcal{C}, Top_{cg}]$ is equivalently \begin{itemize}% \item a [[compactly generated topological space]] $F_a\in Top_{cg}$ for each object $a \in Obj(\mathcal{C})$; \item a [[continuous function]] \begin{displaymath} F_a \times \mathcal{C}(a,b) \longrightarrow F_b \end{displaymath} for all pairs of objects $a,b \in Obj(\mathcal{C})$ \end{itemize} such that composition is respected, in the evident sense. For every object $c \in \mathcal{C}$, there is a topologically enriched [[representable functor]], denoted $y(c) or \mathcal{C}(c,-)$ which sends objects to \begin{displaymath} y(c)(d) = \mathcal{C}(c,d) \in Top_{cg} \end{displaymath} and whose action on morphisms is, under the above identification, just the [[composition]] operation in $\mathcal{C}$. \end{example} There is a full blown $Top_{cg}$-[[enriched Yoneda lemma]]. The following records a slightly simplified version. \begin{prop} \label{TopologicallyEnrichedYonedaLemma}\hypertarget{TopologicallyEnrichedYonedaLemma}{} \textbf{(topologically enriched Yoneda-lemma)} Let $\mathcal{C}$ be a [[topologically enriched category]], def. \ref{TopEnrichedCategory}, write $[\mathcal{C}, Top_{cg}]$ for its category of topologically enriched (co-)presheaves, and for $c\in Obj(\mathcal{C})$ write $y(c) = \mathcal{C}(c,-) \in [\mathcal{C}, Top_k]$ for the topologically enriched functor that it represents, all according to example \ref{TopologicallyEnrichedFunctorsToTopk}. Recall also the $Top_{cg}$-tensored functors $F \cdot X$ from that example. For $c\in Obj(\mathcal{C})$, $X \in Top$ and $F \in [\mathcal{C}, Top_{cg}]$, there is a [[natural bijection]] between \begin{enumerate}% \item morphisms $y(c) \cdot X \longrightarrow F$ in $[\mathcal{C}, Top_{cg}]$; \item morphisms $X \longrightarrow F(c)$ in $Top_{cg}$. \end{enumerate} \end{prop} \begin{proof} Given a morphism $\eta \colon y(c) \cdot X \longrightarrow F$ consider its component \begin{displaymath} \eta_c \;\colon\; \mathcal{C}(c,c)\times X \longrightarrow F(c) \end{displaymath} and restrict that to the identity morphism $id_c \in \mathcal{C}(c,c)$ in the first argument \begin{displaymath} \eta_c(id_c,-) \;\colon\; X \longrightarrow F(c) \,. \end{displaymath} We claim that just this $\eta_c(id_c,-)$ already uniquely determines all components \begin{displaymath} \eta_d \;\colon\; \mathcal{C}(c,d)\times X \longrightarrow F(d) \end{displaymath} of $\eta$, for all $d \in Obj(\mathcal{C})$: By definition of the transformation $\eta$ (def. \ref{TopologicallyEnrichedFunctor}), the two functions \begin{displaymath} F(-) \circ \eta_c \;\colon\; \mathcal{C}(c,d) \longrightarrow F(d)^{\mathcal{C}(c,c) \times X} \end{displaymath} and \begin{displaymath} \eta_d \circ \mathcal{C}(c,-) \times X \;\colon\; \mathcal{C}(c,d) \longrightarrow F(d)^{\mathcal{C}(c,c) \times X} \end{displaymath} agree. This means that they may be thought of jointly as a function with values in commuting squares in $Top$ of this form: \begin{displaymath} f \;\;\;\; \mapsto \;\;\;\; \itexarray{ \mathcal{C}(c,c) \times X &\overset{\eta_c}{\longrightarrow}& F(c) \\ {}^{\mathllap{\mathcal{C}(c,f)}}\downarrow && \downarrow^{\mathrlap{F(f)}} \\ \mathcal{C}(c,d) \times X &\underset{\eta_d}{\longrightarrow}& F(d) } \end{displaymath} For any $f \in \mathcal{C}(c,d)$, consider the restriction of \begin{displaymath} \eta_d \circ \mathcal{C}(c,f) \in F(d)^{\mathcal{C}(c,c) \times X} \end{displaymath} to $id_c \in \mathcal{C}(c,c)$, hence restricting the above commuting squares to \begin{displaymath} f \;\;\;\; \mapsto \;\;\;\; \itexarray{ \{id_c\} \times X &\overset{\eta_c}{\longrightarrow}& F(c) \\ {}^{\mathllap{\mathcal{C}(c,f)}}\downarrow && \downarrow^{\mathrlap{F}(f)} \\ \{f\} \times X &\underset{\eta_d}{\longrightarrow}& F(d) } \end{displaymath} This shows that $\eta_d$ is fixed to be the function \begin{displaymath} \eta_d(f,x) = F(f)\circ \eta_c(id_c,x) \end{displaymath} and this is a continuous function since all the operations it is built from are continuous. Conversely, given a continuous function $\alpha \colon X \longrightarrow F(c)$, define for each $d$ the function \begin{displaymath} \eta_d \colon (f,x) \mapsto F(f) \circ \alpha \,. \end{displaymath} Running the above analysis backwards shows that this determines a transformation $\eta \colon y(c)\times X \to F$. \end{proof} \begin{remark} \label{}\hypertarget{}{} With $Top_{cg}$ equipped with the [[classical model structure on topological spaces]], which is a [[locally presentable (∞,1)-category|presentation]] for the archetypical [[(∞,1)-category]] [[∞Grpd]] of [[∞-groupoids]], then the topological functor category \begin{displaymath} [\mathcal{C},Top_{cg}] \end{displaymath} (def. \ref{TopologicallyEnrichedFunctor}, def. \ref{TopkAsATopologicallyEnrichedCategory}) is a model for the [[(∞,1)-category of (∞,1)-presheaves]] on $\mathcal{C}^{op}$. This is made precise by the \emph{[[model structure on enriched functors]]}, $[\mathcal{C},Top_{Quillen}]_{proj}$. See at \emph{\href{classical+model+structure+on+topological+spaces#ModelStructureOnTopEnrichedFunctors}{classical model structure on topological spaces -- Model structure on functors}} for details. \end{remark} \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} \begin{itemize}% \item [[simplicially enriched category]] \item [[model structure on enriched functors]] \item [[(infinity,1)-category]] \end{itemize} [[!redirects topologically enriched categories]] [[!redirects Top-enriched category]] [[!redirects Top-enriched categories]] [[!redirects topologically enriched functor]] [[!redirects topologically enriched functors]] [[!redirects Top-enriched functor]] [[!redirects Top-enriched functors]] [[!redirects pointed topologically enriched category]] [[!redirects pointed topologically enriched categories]] [[!redirects pointed topologically enriched functor]] [[!redirects pointed topologically enriched functors]] \end{document}