\documentclass[12pt,titlepage]{article} \usepackage{amsmath} \usepackage{mathrsfs} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsthm} \usepackage{mathtools} \usepackage{graphicx} \usepackage{color} \usepackage{ucs} \usepackage[utf8x]{inputenc} \usepackage{xparse} \usepackage{hyperref} %----Macros---------- % % Unresolved issues: % % \righttoleftarrow % \lefttorightarrow % % \color{} with HTML colorspec % \bgcolor % \array with options (without options, it's equivalent to the matrix environment) % Of the standard HTML named colors, white, black, red, green, blue and yellow % are predefined in the color package. Here are the rest. \definecolor{aqua}{rgb}{0, 1.0, 1.0} \definecolor{fuschia}{rgb}{1.0, 0, 1.0} \definecolor{gray}{rgb}{0.502, 0.502, 0.502} \definecolor{lime}{rgb}{0, 1.0, 0} \definecolor{maroon}{rgb}{0.502, 0, 0} \definecolor{navy}{rgb}{0, 0, 0.502} \definecolor{olive}{rgb}{0.502, 0.502, 0} \definecolor{purple}{rgb}{0.502, 0, 0.502} \definecolor{silver}{rgb}{0.753, 0.753, 0.753} \definecolor{teal}{rgb}{0, 0.502, 0.502} % Because of conflicts, \space and \mathop are converted to % \itexspace and \operatorname during preprocessing. % itex: \space{ht}{dp}{wd} % % Height and baseline depth measurements are in units of tenths of an ex while % the width is measured in tenths of an em. \makeatletter \newdimen\itex@wd% \newdimen\itex@dp% \newdimen\itex@thd% \def\itexspace#1#2#3{\itex@wd=#3em% \itex@wd=0.1\itex@wd% \itex@dp=#2ex% \itex@dp=0.1\itex@dp% \itex@thd=#1ex% \itex@thd=0.1\itex@thd% \advance\itex@thd\the\itex@dp% \makebox[\the\itex@wd]{\rule[-\the\itex@dp]{0cm}{\the\itex@thd}}} \makeatother % \tensor and \multiscript \makeatletter \newif\if@sup \newtoks\@sups \def\append@sup#1{\edef\act{\noexpand\@sups={\the\@sups #1}}\act}% \def\reset@sup{\@supfalse\@sups={}}% \def\mk@scripts#1#2{\if #2/ \if@sup ^{\the\@sups}\fi \else% \ifx #1_ \if@sup ^{\the\@sups}\reset@sup \fi {}_{#2}% \else \append@sup#2 \@suptrue \fi% \expandafter\mk@scripts\fi} \def\tensor#1#2{\reset@sup#1\mk@scripts#2_/} \def\multiscripts#1#2#3{\reset@sup{}\mk@scripts#1_/#2% \reset@sup\mk@scripts#3_/} \makeatother % \slash \makeatletter \newbox\slashbox \setbox\slashbox=\hbox{$/$} \def\itex@pslash#1{\setbox\@tempboxa=\hbox{$#1$} \@tempdima=0.5\wd\slashbox \advance\@tempdima 0.5\wd\@tempboxa \copy\slashbox \kern-\@tempdima \box\@tempboxa} \def\slash{\protect\itex@pslash} \makeatother % math-mode versions of \rlap, etc % from Alexander Perlis, "A complement to \smash, \llap, and lap" % http://math.arizona.edu/~aprl/publications/mathclap/ \def\clap#1{\hbox to 0pt{\hss#1\hss}} \def\mathllap{\mathpalette\mathllapinternal} \def\mathrlap{\mathpalette\mathrlapinternal} \def\mathclap{\mathpalette\mathclapinternal} \def\mathllapinternal#1#2{\llap{$\mathsurround=0pt#1{#2}$}} \def\mathrlapinternal#1#2{\rlap{$\mathsurround=0pt#1{#2}$}} \def\mathclapinternal#1#2{\clap{$\mathsurround=0pt#1{#2}$}} % Renames \sqrt as \oldsqrt and redefine root to result in \sqrt[#1]{#2} \let\oldroot\root \def\root#1#2{\oldroot #1 \of{#2}} \renewcommand{\sqrt}[2][]{\oldroot #1 \of{#2}} % Manually declare the txfonts symbolsC font \DeclareSymbolFont{symbolsC}{U}{txsyc}{m}{n} \SetSymbolFont{symbolsC}{bold}{U}{txsyc}{bx}{n} \DeclareFontSubstitution{U}{txsyc}{m}{n} % Manually declare the stmaryrd font \DeclareSymbolFont{stmry}{U}{stmry}{m}{n} \SetSymbolFont{stmry}{bold}{U}{stmry}{b}{n} % Manually declare the MnSymbolE font \DeclareFontFamily{OMX}{MnSymbolE}{} \DeclareSymbolFont{mnomx}{OMX}{MnSymbolE}{m}{n} \SetSymbolFont{mnomx}{bold}{OMX}{MnSymbolE}{b}{n} \DeclareFontShape{OMX}{MnSymbolE}{m}{n}{ <-6> MnSymbolE5 <6-7> MnSymbolE6 <7-8> MnSymbolE7 <8-9> MnSymbolE8 <9-10> MnSymbolE9 <10-12> MnSymbolE10 <12-> MnSymbolE12}{} % Declare specific arrows from txfonts without loading the full package \makeatletter \def\re@DeclareMathSymbol#1#2#3#4{% \let#1=\undefined \DeclareMathSymbol{#1}{#2}{#3}{#4}} \re@DeclareMathSymbol{\neArrow}{\mathrel}{symbolsC}{116} \re@DeclareMathSymbol{\neArr}{\mathrel}{symbolsC}{116} \re@DeclareMathSymbol{\seArrow}{\mathrel}{symbolsC}{117} \re@DeclareMathSymbol{\seArr}{\mathrel}{symbolsC}{117} \re@DeclareMathSymbol{\nwArrow}{\mathrel}{symbolsC}{118} \re@DeclareMathSymbol{\nwArr}{\mathrel}{symbolsC}{118} \re@DeclareMathSymbol{\swArrow}{\mathrel}{symbolsC}{119} \re@DeclareMathSymbol{\swArr}{\mathrel}{symbolsC}{119} \re@DeclareMathSymbol{\nequiv}{\mathrel}{symbolsC}{46} \re@DeclareMathSymbol{\Perp}{\mathrel}{symbolsC}{121} \re@DeclareMathSymbol{\Vbar}{\mathrel}{symbolsC}{121} \re@DeclareMathSymbol{\sslash}{\mathrel}{stmry}{12} \re@DeclareMathSymbol{\bigsqcap}{\mathop}{stmry}{"64} \re@DeclareMathSymbol{\biginterleave}{\mathop}{stmry}{"6} \re@DeclareMathSymbol{\invamp}{\mathrel}{symbolsC}{77} \re@DeclareMathSymbol{\parr}{\mathrel}{symbolsC}{77} \makeatother % \llangle, \rrangle, \lmoustache and \rmoustache from MnSymbolE \makeatletter \def\Decl@Mn@Delim#1#2#3#4{% \if\relax\noexpand#1% \let#1\undefined \fi \DeclareMathDelimiter{#1}{#2}{#3}{#4}{#3}{#4}} \def\Decl@Mn@Open#1#2#3{\Decl@Mn@Delim{#1}{\mathopen}{#2}{#3}} \def\Decl@Mn@Close#1#2#3{\Decl@Mn@Delim{#1}{\mathclose}{#2}{#3}} \Decl@Mn@Open{\llangle}{mnomx}{'164} \Decl@Mn@Close{\rrangle}{mnomx}{'171} \Decl@Mn@Open{\lmoustache}{mnomx}{'245} \Decl@Mn@Close{\rmoustache}{mnomx}{'244} \makeatother % Widecheck \makeatletter \DeclareRobustCommand\widecheck[1]{{\mathpalette\@widecheck{#1}}} \def\@widecheck#1#2{% \setbox\z@\hbox{\m@th$#1#2$}% \setbox\tw@\hbox{\m@th$#1% \widehat{% \vrule\@width\z@\@height\ht\z@ \vrule\@height\z@\@width\wd\z@}$}% \dp\tw@-\ht\z@ \@tempdima\ht\z@ \advance\@tempdima2\ht\tw@ \divide\@tempdima\thr@@ \setbox\tw@\hbox{% \raise\@tempdima\hbox{\scalebox{1}[-1]{\lower\@tempdima\box \tw@}}}% {\ooalign{\box\tw@ \cr \box\z@}}} \makeatother % \mathraisebox{voffset}[height][depth]{something} \makeatletter \NewDocumentCommand\mathraisebox{moom}{% \IfNoValueTF{#2}{\def\@temp##1##2{\raisebox{#1}{$\m@th##1##2$}}}{% \IfNoValueTF{#3}{\def\@temp##1##2{\raisebox{#1}[#2]{$\m@th##1##2$}}% }{\def\@temp##1##2{\raisebox{#1}[#2][#3]{$\m@th##1##2$}}}}% \mathpalette\@temp{#4}} \makeatletter % udots (taken from yhmath) \makeatletter \def\udots{\mathinner{\mkern2mu\raise\p@\hbox{.} \mkern2mu\raise4\p@\hbox{.}\mkern1mu \raise7\p@\vbox{\kern7\p@\hbox{.}}\mkern1mu}} \makeatother %% Fix array \newcommand{\itexarray}[1]{\begin{matrix}#1\end{matrix}} %% \itexnum is a noop \newcommand{\itexnum}[1]{#1} %% Renaming existing commands \newcommand{\underoverset}[3]{\underset{#1}{\overset{#2}{#3}}} \newcommand{\widevec}{\overrightarrow} \newcommand{\darr}{\downarrow} \newcommand{\nearr}{\nearrow} \newcommand{\nwarr}{\nwarrow} \newcommand{\searr}{\searrow} \newcommand{\swarr}{\swarrow} \newcommand{\curvearrowbotright}{\curvearrowright} \newcommand{\uparr}{\uparrow} \newcommand{\downuparrow}{\updownarrow} \newcommand{\duparr}{\updownarrow} \newcommand{\updarr}{\updownarrow} \newcommand{\gt}{>} \newcommand{\lt}{<} \newcommand{\map}{\mapsto} \newcommand{\embedsin}{\hookrightarrow} \newcommand{\Alpha}{A} \newcommand{\Beta}{B} \newcommand{\Zeta}{Z} \newcommand{\Eta}{H} \newcommand{\Iota}{I} \newcommand{\Kappa}{K} \newcommand{\Mu}{M} \newcommand{\Nu}{N} \newcommand{\Rho}{P} \newcommand{\Tau}{T} \newcommand{\Upsi}{\Upsilon} \newcommand{\omicron}{o} \newcommand{\lang}{\langle} \newcommand{\rang}{\rangle} \newcommand{\Union}{\bigcup} \newcommand{\Intersection}{\bigcap} \newcommand{\Oplus}{\bigoplus} \newcommand{\Otimes}{\bigotimes} \newcommand{\Wedge}{\bigwedge} \newcommand{\Vee}{\bigvee} \newcommand{\coproduct}{\coprod} \newcommand{\product}{\prod} \newcommand{\closure}{\overline} \newcommand{\integral}{\int} \newcommand{\doubleintegral}{\iint} \newcommand{\tripleintegral}{\iiint} \newcommand{\quadrupleintegral}{\iiiint} \newcommand{\conint}{\oint} \newcommand{\contourintegral}{\oint} \newcommand{\infinity}{\infty} \newcommand{\bottom}{\bot} \newcommand{\minusb}{\boxminus} \newcommand{\plusb}{\boxplus} \newcommand{\timesb}{\boxtimes} \newcommand{\intersection}{\cap} \newcommand{\union}{\cup} \newcommand{\Del}{\nabla} \newcommand{\odash}{\circleddash} \newcommand{\negspace}{\!} \newcommand{\widebar}{\overline} \newcommand{\textsize}{\normalsize} \renewcommand{\scriptsize}{\scriptstyle} \newcommand{\scriptscriptsize}{\scriptscriptstyle} \newcommand{\mathfr}{\mathfrak} \newcommand{\statusline}[2]{#2} \newcommand{\tooltip}[2]{#2} \newcommand{\toggle}[2]{#2} % Theorem Environments \theoremstyle{plain} \newtheorem{theorem}{Theorem} \newtheorem{lemma}{Lemma} \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*{internal sheaf} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{topos_theory}{}\paragraph*{{Topos Theory}}\label{topos_theory} [[!include topos theory - contents]] \hypertarget{internal_category_theory}{}\paragraph*{{Internal category theory}}\label{internal_category_theory} [[!include internal infinity-categories contents]] \hypertarget{topos_theory_2}{}\paragraph*{{$(\infty,2)$-Topos theory}}\label{topos_theory_2} [[!include (infinity,2)-topos theory - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{InTermsOf2Sheaves}{In terms of external 2-sheaves}\dotfill \pageref*{InTermsOf2Sheaves} \linebreak \noindent\hyperlink{ExplicitDefinition}{Explicit definition}\dotfill \pageref*{ExplicitDefinition} \linebreak \noindent\hyperlink{properties}{Properties}\dotfill \pageref*{properties} \linebreak \noindent\hyperlink{related_concepts}{Related concepts}\dotfill \pageref*{related_concepts} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} The notions of \emph{[[presheaf]]}, \emph{[[site]]} and \emph{[[sheaf]]} can be formulated [[internalization|internal]] to any [[topos]]. The ordinary such notions are recovered by internalization into [[Set]]. More precisely, the direct internalization of these notions is into the [[universe enlargement|very large]] [[2-topos]] $\hat Sh_2(\mathcal{S},can)$ of the given ambient topos $\mathcal{S}$, since an internal presheaf is to be an $\mathcal{S}$-valued internal functor, but $\mathcal{S}$ does not quite sit inside itself. It does, however, sit inside $\hat Sh_2(\mathcal{S},can)$, incarnated as the [[2-sheaf]] $\bar \mathcal{S}$ corresponding to its [[codomain fibration]]. Therefore, regarding $\hat Sh_2(\mathcal{S}, can)$ as the [[2-category]] of [[internal categories]] in $\mathcal{S}$, an [[internal site]] in $\mathcal{S}$ is an object $\bar \mathbb{C}$ of $\hat Sh_2(\mathcal{S}, can)$ and an internal presheaf is a morphism $F : \bar \mathbb{C}^{op} \to \bar \mathcal{S}$. \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} While it is straightforward to define an [[internal site]], hence the [[domain]] of an internal (pre)sheaf, the definition of the codomain is slightly more subtle, for that needs to be a copy of the ambient [[universe]] internalized into itself. One way to naturally say this is by passing to the \emph{external} [[2-sheaves]] [[2-topos]]. This version of the definition we state in \begin{itemize}% \item \emph{\hyperlink{InTermsOf2Sheaves}{In terms of external 2-sheaves}}. \end{itemize} But the resulting notion can of course be expressed entirely in terms of data in the ambient topos. This we spell out in \begin{itemize}% \item \emph{\hyperlink{ExplicitDefinition}{Explicit definition}}. \end{itemize} \hypertarget{InTermsOf2Sheaves}{}\subsubsection*{{In terms of external 2-sheaves}}\label{InTermsOf2Sheaves} Let $\mathcal{S}$ be a [[topos]] and let $\mathbb{C}$ be an [[internal category]] in $\mathcal{S}$: \begin{displaymath} \mathbb{C} = \left( C_1 \stackrel{\overset{s}{\to}}{\underset{t}{\to}} C_0 \right) \in \mathcal{S} \,. \end{displaymath} \begin{defn} \label{}\hypertarget{}{} Write $\bar \mathbb{C}$ for the [[2-sheaf]] on $\mathcal{S}$ \begin{displaymath} \bar \mathbb{C} : \mathcal{S}^{op} \to Cat \end{displaymath} that is [[representable functor|represented]] by $\mathbb{C}$. More explicitly, this is the [[pseudofunctor]] which to an [[object]] $X \in \mathcal{S}$ assigns \begin{displaymath} \bar \mathbb{C} : X \mapsto \left( \mathcal{S}(X,C_1) \stackrel{\overset{\mathcal{S}(X,s)}{\to}}{\underset{\mathcal{S}(X,t)}{\to}} \mathcal{S}(X,C_0) \right) \,. \end{displaymath} \end{defn} \begin{defn} \label{}\hypertarget{}{} Write \begin{displaymath} \bar \mathcal{S} : \mathcal{S}^{op} \to Cat \end{displaymath} for the [[2-sheaf]] that [[Grothendieck construction|classifies]] the [[codomain fibration]] of $\mathcal{S}$, the [[pseudofunctor]] which sends an object to the corresponding [[slice topos]] and morphisms to [[base change]] \begin{displaymath} \bar \mathcal{S} : X \mapsto \mathcal{S}_{/X} \end{displaymath} (also called the ``[[indexed category|self-indexing]] of $\mathcal{S}$''). \end{defn} \begin{remark} \label{}\hypertarget{}{} This is the incarnation of $\mathcal{S}$ itself, regard internal to its [[2-sheaf]] [[2-topos]] $\hat Sh_{2}(\mathcal{T}, can)$. \end{remark} \begin{defn} \label{}\hypertarget{}{} An \textbf{internal presheaf} on $\mathbb{C}$ (internal to $\mathcal{T}$) is a morphism \begin{displaymath} F : \bar \mathbb{C}^{op} \to \bar \mathcal{S} \end{displaymath} of [[2-sheaves]] on $\mathcal{S}$. (Also called an ``[[indexed functor]]'' between [[indexed categories]]``.) Suppose moreover that $\mathbb{C}$ is equipped with the structure of an [[internal site]]. Then $F$ above is an \textbf{internal sheaf} on $\mathbb{C}$ if it satisfies the evident [[descent]] condition. A [[morphism]] of internal presheaves is simply a [[2-morphism]] in $\hat Sh_2(\mathcal{S}, can)$ (also called an ``[[indexed functor|indexed natural transformation]]''). This yields a [[category]] \begin{displaymath} PSh(\mathbb{C}) \end{displaymath} of internal presheaves. Accordingly we have the [[full subcategory]] \begin{displaymath} Sh(\mathbb{C}, \mathcal{S}) \hookrightarrow PSh(\mathbb{C}, \mathcal{S}) \end{displaymath} of internal sheaves. \end{defn} \hypertarget{ExplicitDefinition}{}\subsubsection*{{Explicit definition}}\label{ExplicitDefinition} We unwind what the above amounts to more explicitly. Let $(\mathbb{C},J)$ be an [[internal site]] in $\mathcal{S}$, i.e. an [[internal category]] $\mathbb{C}$ equipped with an internal [[coverage]] $J$. Let $\mathcal{S}^{\mathbb{C}^{op}}$ be the topos of [[internal diagram|internal diagrams]] on $\mathbb{C}^{op}$. \begin{defn} \label{}\hypertarget{}{} \begin{enumerate}% \item An \textbf{internal presheaf} on $\mathbb{C}$ is an internal diagram $F \in \mathcal{S}^{\mathbb{C}^{op}}$. \item An \textbf{internal sheaf} on $\mathbb{C}$ (with respect to $J$) is an internal presheaf on $\mathbb{C}$ satisfying one of the following equivalent conditions: \begin{enumerate}% \item $F$ satisfies the \href{/nlab/show/sheaf#GeneralComponentwiseDefinition}{usual sheaf condition} interpreted in the [[internal language]] of $\mathcal{S}$. \item $F$ is a $j$-sheaf for the [[Lawvere-Tierney topology]] on $\mathcal{S}^{\mathbb{C}^{op}}$ induced by $J$. (The equivalence is because the usual proof for $\mathcal{S} =$[[Set]] is constructive and can thus be internalized in an arbitrary topos.) \end{enumerate} \end{enumerate} \end{defn} \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} Let $\mathcal{S}$ and $\mathbb{C}$ be as above. \begin{prop} \label{}\hypertarget{}{} The category of internal presheaves $PSh(\mathbb{C}, \mathcal{S})$ is a [[topos]]. \end{prop} This appears as (\hyperlink{Johnstone}{Johnstone, cor. B.2.3.17}). \begin{prop} \label{}\hypertarget{}{} Let $f : \mathbb{C} \to \mathbb{D}$ be an [[internal functor]]. Write $\bar f : \bar \mathbb{C} \to \bar \mathbb{D}$ for the corresponding morphism in $\hat Sh_2(\mathcal{S}, can)$. Precomposition with this morphism induces a [[functor]] of internal presheaf catgeories \begin{displaymath} f^* \colon PSh(\mathbb{D}, \mathcal{S}) \to PSh(\mathbb{C}, \mathcal{S}) \,. \end{displaymath} This is the [[inverse image]] of a [[geometric morphism]] of [[toposes]] \begin{displaymath} f \colon PSh(\mathbb{C}, \mathcal{S}) \to PSh(\mathbb{D}, \mathcal{S}) \,. \end{displaymath} \end{prop} This appears as (\hyperlink{Johnstone}{Johnstone, cor. B.2.3.22}). The global section functors of internal sheaf toposes in $\mathcal{S}$ are [[bounded geometric morphisms]] over $\mathcal{S}$. \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} \begin{itemize}% \item [[internal diagram]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} Internal presheaves in a [[Grothendieck topos]] are discussed in Section V.7 of \begin{itemize}% \item [[Saunders MacLane]], [[Ieke Moerdijk]], \emph{[[Sheaves in Geometry and Logic]]} \end{itemize} and in section B2.3 of \begin{itemize}% \item [[Peter Johnstone]], \emph{[[Sketches of an Elephant]]} \end{itemize} In these references internal presheaves are introduced in components as in the \hyperlink{ExplicitDefinition}{explicit definition} above. The equivalence to the \hyperlink{InTermsOf2Sheaves}{abstract formulation} above, in terms of morphisms between 2-sheaves, follows for instance with (\hyperlink{Johnstone}{Johnstone, lemma B.2.3.13}). category: sheaf theory [[!redirects internal sheaves]] [[!redirects internal presheaf]] [[!redirects internal presheaves]] [[!redirects category of internal sheaves]] [[!redirects category of internal presheaves]] [[!redirects categories of internal sheaves]] [[!redirects categories of internal presheaves]] [[!redirects internal presheaf topos]] [[!redirects internal sheaf topos]] [[!redirects internal presheaf toposes]] [[!redirects internal sheaf toposes]] \end{document}