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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*{cartesian closed functor} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{category_theory}{}\paragraph*{{Category theory}}\label{category_theory} [[!include category theory - contents]] \hypertarget{monoidal_categories}{}\paragraph*{{Monoidal categories}}\label{monoidal_categories} [[!include monoidal categories - 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{Examples}{Examples}\dotfill \pageref*{Examples} \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{}{} A \textbf{cartesian closed functor} is a [[functor]] $F\colon \mathcal{C}\to \mathcal{D}$ between [[cartesian closed categories]] which preserves both [[products]] and [[exponential objects]]/[[internal homs]] (all the structure of cartesian closed categories). More precisely, if $F\colon C\to D$ preserves products, then the canonical [[morphisms]] $F(A\times B) \to F A \times F B$ (for all [[objects]] $A,B \mathcal{C}$) are [[isomorphisms]], and we therefore have canonical induced morphism $F[A,B] \to [F A, F B]$ --- the [[adjuncts]] of the composite $F[A,B] \times F A \xrightarrow{\cong} F([A,B] \times A) \to F B$. $F$ is \textbf{cartesian closed} if these maps $F[A,B] \to [F A, F B]$ are also isomorphisms. \end{defn} \begin{remark} \label{}\hypertarget{}{} When cartesian closed categories are identified with [[cartesian monoidal categories]] that are also [[closed monoidal category|closed monoidal]], a cartesian closed functor can be identified with a [[strong monoidal functor]] which is also [[strong closed functor|strong closed]]. \end{remark} \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \begin{prop} \label{FrobeniusReciprocity}\hypertarget{FrobeniusReciprocity}{} \textbf{(Frobenius reciprocity)} Let $R : \mathcal{C} \to \mathcal{D}$ be a [[functor]] between [[cartesian closed categories]] with a [[left adjoint]] $L$. Then $R$ is cartesian closed precisely if the [[natural transformation]] \begin{displaymath} (L \pi_1, \epsilon_A L \pi_2) : L(B \times R(A)) \to L(B) \times A \end{displaymath} is an [[isomorphism]]. \end{prop} \begin{proof} The above natural transformation is the [[mate]] of the exponential comparison natural transformation $R[A,B] \to [R A, R B]$ under the composite adjunctions \begin{displaymath} \mathcal{C} \underoverset{[R A, -]}{- \times R A}{\rightleftarrows} \mathcal{C} \underoverset{R}{L}{\rightleftarrows} \mathcal{D} \end{displaymath} and \begin{displaymath} \mathcal{C} \underoverset{R}{L}{\rightleftarrows} \mathcal{D} \underoverset{[A,-]}{A\times -}{\rightleftarrows} \mathcal{D} \end{displaymath} \end{proof} This is called the \textbf{[[Frobenius reciprocity]]} law. It is discussed, for instance, as (\hyperlink{Johnstone}{Johnstone, lemma 1.5.8}). More generally for closed monoidal categories (not necessarily cartesian monoidal) it is discussed in ``[[Wirthmüller contexts]]'' in Let still $R$ and $L$ be as above. \begin{cor} \label{}\hypertarget{}{} If $R$ is [[full and faithful]] and $L$ preserves binary [[products]], then $R$ is cartesian closed. \end{cor} For instance (\hyperlink{Johnstone}{Johnstone, corollary A1.5.9}). \hypertarget{Examples}{}\subsection*{{Examples}}\label{Examples} \begin{prop} \label{}\hypertarget{}{} For $\mathcal{C}$ a [[locally cartesian closed category]] and $f : X_1 \to X_2$ a [[morphism]], the [[base change]]/[[pullback]] functor between the [[slice categories]] \begin{displaymath} f^* : \mathcal{C}_{/X_2} \to \mathcal{C}_{/X_1} \end{displaymath} is cartesian closed. In particular the [[inverse image]] functor of an [[étale geometric morphism]] between [[toposes]] is cartesian closed and hence constitutes a cartesian [[Wirthmüller context]]. \end{prop} \begin{proof} The functor $f^*$ has a [[left adjoint]] \begin{displaymath} \sum_f : \mathcal{C}_{/X_1} \to \mathcal{C}_{/X_2} \end{displaymath} given by postcomposition with $f$ (the [[dependent sum]] along $f$). Therefore by prop. \ref{FrobeniusReciprocity} it is sufficient to show that for all $(A \to X_2)$ in $\mathcal{C}_{/X_2}$ and $(B \stackrel{b}{\to} X_1) \in \mathcal{C}_{/X_1}$ that \begin{displaymath} B \times_{X_1} f^* A \simeq B \times_{X_2} A \end{displaymath} in $\mathcal{C}$. But this is the [[pasting law]] for pullbacks in $\mathcal{C}$, which says that the two consecutive pullbacks on the left of \begin{displaymath} \itexarray{ B \times_{X_1} f^* A &\to& f^* A &\to& A \\ \downarrow && \downarrow && \downarrow \\ B &\stackrel{b}{\to}& X_1 &\stackrel{f}{\to}& X_2 } \;\;\; \simeq \;\;\; \itexarray{ (b \circ f)^* A &\to& &\to& A \\ \downarrow && && \downarrow \\ B &\stackrel{b}{\to}& X_1 &\stackrel{f}{\to}& X_2 } \end{displaymath} are isomorphic to the direct pullback along the composite on the right. \end{proof} \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} \begin{itemize}% \item [[strong functor]] \item [[cartesian closed category]], [[locally cartesian closed category]] \item \textbf{cartesian closed functor}, [[locally cartesian closed functor]] \item [[cartesian closed model category]], [[locally cartesian closed model category]] \item [[cartesian closed (∞,1)-category]] [[locally cartesian closed (∞,1)-category]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} For instance section A1.5 of \begin{itemize}% \item [[Peter Johnstone]], \emph{[[Sketches of an Elephant]]} \end{itemize} Also \begin{itemize}% \item H. Fausk, P. Hu, [[Peter May]], \emph{Isomorphisms between left and right adjoints}, Theory and Applications of Categories , Vol. 11, 2003, No. 4, pp 107-131. (\href{http://www.tac.mta.ca/tac/volumes/11/4/11-04abs.html}{TAC}, \href{http://www.math.uiuc.edu/K-theory/0573/FormalFeb16.pdf}{pdf}) \end{itemize} [[!redirects cartesian closed functors]] \end{document}