\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*{locally connected geometric morphism} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{topos_theory}{}\paragraph*{{Topos Theory}}\label{topos_theory} [[!include 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{properties}{Properties}\dotfill \pageref*{properties} \linebreak \noindent\hyperlink{relation_to_connectedness}{Relation to connectedness}\dotfill \pageref*{relation_to_connectedness} \linebreak \noindent\hyperlink{over_}{Over $Set$}\dotfill \pageref*{over_} \linebreak \noindent\hyperlink{StrongAdjunctions}{Strong adjunctions}\dotfill \pageref*{StrongAdjunctions} \linebreak \noindent\hyperlink{coreflectivity}{Coreflectivity}\dotfill \pageref*{coreflectivity} \linebreak \noindent\hyperlink{other_characterizations}{Other characterizations}\dotfill \pageref*{other_characterizations} \linebreak \noindent\hyperlink{variations_in_the_context_of_the_nullstellensatz}{Variations in the context of the Nullstellensatz}\dotfill \pageref*{variations_in_the_context_of_the_nullstellensatz} \linebreak \noindent\hyperlink{examples}{Examples}\dotfill \pageref*{examples} \linebreak \noindent\hyperlink{related_entries}{Related entries}\dotfill \pageref*{related_entries} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} A [[geometric morphism]] is \emph{locally connected} if it behaves as though its [[fiber]]s are [[locally connected space]]s. In particular, a [[Grothendieck topos]] $E$ is [[locally connected topos|locally connected]] iff the unique [[geometric morphism]] to [[Set]] (the terminal Grothendieck topos, i.e. the [[point]] in the [[category]] [[Topos]] of toposes) is locally connected. \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} A [[geometric morphism]] $(f^* \dashv f_*) : F \underoverset{f_*}{f^*}{\leftrightarrows} E$ is \textbf{locally connected} if it satisfies the following equivalent conditions: \begin{enumerate}% \item It is [[essential geometric morphism|essential]], i.e. $f^*$ has a [[left adjoint]] $f_!$, and moreover $f_!$ can be made into an $E$-[[indexed functor]]. \item For every $A\in E$, the functor $f^* \colon E/A \to F/f^*A$ is [[cartesian closed functor|cartesian closed]]. \item $f^*$ commutes with [[dependent products]] -- For any morphism $h\colon A\to B$ in $E$, the canonically defined [[natural transformation]] $f^* \circ \Pi_h \to \Pi_{f^*h} \circ f^*$ is an [[isomorphism]]. \end{enumerate} \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \hypertarget{relation_to_connectedness}{}\subsubsection*{{Relation to connectedness}}\label{relation_to_connectedness} If $f$ is locally connected, then it makes sense to think of the left adjoint $f_!$ as assigning to an object of $F$ its ``set of connected components'' in $E$. In particular, if $f$ is locally connected, then it is moreover [[connected geometric morphism|connected]] if and only if $f_!$ preserves the [[terminal object]]. However, not every connected geometric morphism is locally connected. \hypertarget{over_}{}\subsubsection*{{Over $Set$}}\label{over_} Over the [[base topos]] $E =$ [[Set]] every [[connected topos]] which is [[essential geometric morphism|essential]] is automatically locally connected. This is because the required [[Frobenius reciprocity]] condition \begin{displaymath} f_!(A \times f^* (B)) \simeq f_!(A) \times B \end{displaymath} is automatically satisfied, using that [[cartesian product]] with a [[set]] is equivalently a [[coproduct]] \begin{displaymath} A \times B = \coprod_{a \in A} B \,, \end{displaymath} that the [[left adjoint]] $f_!$ preserves coproducts, and that for $f^*$ [[full and faithful]] we have $f_! f^* \simeq Id$. \hypertarget{StrongAdjunctions}{}\subsubsection*{{Strong adjunctions}}\label{StrongAdjunctions} The pair of [[adjoint functors]] $(f_! \dashv f^*)$ in a locally connected geometric morphisms forms a ``strong adjunction'' in that it holds also for the [[internal homs]] in the sense that there is a [[natural isomorphism]] \begin{displaymath} [f_!(X), A] \simeq f_* [X, f^* A] \end{displaymath} for all $X, A$. This follows by duality from the [[Frobenius reciprocity]] that characterizes $f_*$ as being a [[cartesian closed functor]]: by the [[Yoneda lemma]], the morphism in question is an [[isomorphism]] if for all objects $A,B, X$ the morphism \begin{displaymath} Hom(X, [f_!(A), B]) \stackrel{}{\to} Hom(X,f_*[A,f^*(B)]) \end{displaymath} is a bijection. By [[adjunction]] this is the same as \begin{displaymath} Hom(X \times f_!(A), B) \stackrel{\simeq}{\to} Hom(f_!(f^*(X) \times A), B) \,. \end{displaymath} Again by Yoneda, this is a bijection precisely if \begin{displaymath} f_!(f^*(X) \times A) \to X \times f_!(A) \end{displaymath} is an [[isomorphism]]. But this is the [[Frobenius reciprocity]] condition on $f^*$. \hypertarget{coreflectivity}{}\subsubsection*{{Coreflectivity}}\label{coreflectivity} Locally connected toposes are [[coreflective subcategory|coreflective]] in [[Topos]]. See (\hyperlink{Funk}{Funk (1999)}). \hypertarget{other_characterizations}{}\subsubsection*{{Other characterizations}}\label{other_characterizations} \begin{itemize}% \item Let $(\mathcal{C}, J)$ be a [[site]] and $S$ be a [[sieve]] on the object $U$. $S$ is called \emph{connected} when $S$ viewed as a full subcategory of $\mathcal{C}/U$ is connected. The \textbf{site} is called \emph{locally connected} if every sieve is connected. For a [[bounded geometric morphism]] $p:\mathcal{E}\to\mathcal{S}$ the following holds: \emph{$p$ is locally connected iff there exists a locally connected internal site in $\mathcal{S}$ such that $\mathcal{E}\simeq Sh(\mathcal{C},J)$.} (cf. \hyperlink{elephant}{Johnstone (2002)}, pp.656-658) \item \hyperlink{Cara12}{Caramello (2012)} gives syntactic characterizations of [[geometric theories]] whose [[classifying topos]] is locally connected. \end{itemize} The same paper also contains the following characterization: \begin{itemize}% \item A Grothendieck topos is locally connected iff it has a [[separator|separating set]] of (coproduct) indecomposable objects. \end{itemize} \hypertarget{variations_in_the_context_of_the_nullstellensatz}{}\subsubsection*{{Variations in the context of the Nullstellensatz}}\label{variations_in_the_context_of_the_nullstellensatz} \hyperlink{JS11}{Johnstone (2011)} studies several subclasses of locally connected geometric morphisms in the context of [[William Lawvere|Lawvere]]`s theory of [[cohesion]] and the [[Nullstellensatz]]. He calls a locally connected morphism $p$ \emph{stably locally connected} if $p_!$ preserves finite products. According to the \hyperlink{connect}{above remark} this implies that $p$ is connected. Slightly stronger is the preservation of all finite limits by $p_!$: these $p$ are called [[totally connected geometric morphisms]]. \hypertarget{examples}{}\subsection*{{Examples}}\label{examples} \begin{itemize}% \item If the terminal [[global section]] geometric morphism $E \to Set$ is locally connected, one calls $E$ a [[locally connected topos]]. More generally, if $E\to S$ is locally connected, we may call $E$ a \emph{locally connected $S$-topos}. \item Let $X$ be a [[topological space]] (or a [[locale]]) and $U\subseteq X$ an [[open subset]], with corresponding [[geometric embedding]] $j\colon Sh(U)\to Sh(X)$. Then any $A\in Sh(X)$ can be identified with a space (or locale) $A$ equipped with a [[local homeomorphism]] $A\to X$, in such a way that $Sh(X)/A \simeq Sh(A)$. Moreover, $j^*A \in Sh(U)$ can be identified with the pullback of $A\to X$ along $U$, and so $Sh(U)/j^*A \simeq Sh(j^*A)$ similarly. Noting that $j^*A \to A$ is again the inclusion of an open subset, and using the fact that the inverse image part of any open [[geometric embedding]] is cartesian closed, we see that $(j/A)^*\colon Sh(X)/A \to Sh(U)/j^*A$ is cartesian closed for any $A$. Hence $j$ is locally connected. \end{itemize} \hypertarget{related_entries}{}\subsection*{{Related entries}}\label{related_entries} \begin{itemize}% \item [[connected geometric morphism]] \item [[local geometric morphism]] \item [[totally connected geometric morphism]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} The case of $Sh(X)$ for a topological space $X$ was an exercise (p.417) in [[SGA4]]: \begin{itemize}% \item [[M. Artin]], [[A. Grothendieck]], [[J. L. Verdier]], \emph{Th\'e{}orie des Topos et Cohomologie Etale des Sch\'e{}mas ([[SGA4]])}, Springer LNM vol.269 (1972). \end{itemize} The concept relative to other bases was introduced in the following paper: \begin{itemize}% \item [[Michael Barr]], [[Robert Paré]], \emph{Molecular Toposes} , JPAA \textbf{17} (1980) pp.127-152. \end{itemize} The standard reference is section C3.3 of \begin{itemize}% \item [[Peter Johnstone]], \emph{[[Sketches of an Elephant]]} , Oxford UP 2002. \end{itemize} Further references include \begin{itemize}% \item [[Olivia Caramello]], \emph{Syntactic Characterizations of Properties of Classifying Toposes} , TAC \textbf{26} no.6 (2012) pp.176-193. (\href{http://www.tac.mta.ca/tac/volumes/26/6/26-06.pdf}{pdf}) \item [[Jonathon Funk]], \emph{The locally connected coclosure of a Grothendieck topos}, JPAA \textbf{137} (1999) pp.17-27. \item [[Peter Johnstone]], \emph{Remarks on Punctual Local Connectedness} , TAC \textbf{25} no.3 (2011) pp.51-63. (\href{http://www.tac.mta.ca/tac/volumes/25/3/25-03abs.html}{pdf}) \item [[Ieke Moerdijk]], \emph{Continuous fibrations and inverse limits of toposes} , Comp. Math. \textbf{58} (1986) pp.45-72. (\href{http://archive.numdam.org/article/CM_1986__58_1_45_0.pdf}{pdf}) \item [[Ieke Moerdijk]], [[Gavin Wraith]], \emph{Connected and locally connected toposes are path connected} , Trans. AMS \textbf{295} (1986) pp.849-859. (\href{http://www.ams.org/journals/tran/1986-295-02/S0002-9947-1986-0833712-3/S0002-9947-1986-0833712-3.pdf}{pdf}) \end{itemize} [[!redirects locally connected geometric morphisms]] \end{document}