\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*{weighted limit} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{category_theory}{}\paragraph*{{Category theory}}\label{category_theory} [[!include category theory - contents]] \hypertarget{enriched_category_theory}{}\paragraph*{{Enriched category theory}}\label{enriched_category_theory} [[!include enriched category theory contents]] \hypertarget{limits_and_colimits}{}\paragraph*{{Limits and colimits}}\label{limits_and_colimits} [[!include infinity-limits - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{contents}{Contents}\dotfill \pageref*{contents} \linebreak \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{examples}{Examples}\dotfill \pageref*{examples} \linebreak \noindent\hyperlink{homotopy_limits}{Homotopy limits}\dotfill \pageref*{homotopy_limits} \linebreak \noindent\hyperlink{homotopy_pullback}{Homotopy pullback}\dotfill \pageref*{homotopy_pullback} \linebreak \noindent\hyperlink{references_for_homotopy_limits_in_terms_of_weighted_limits}{References for homotopy limits in terms of weighted limits}\dotfill \pageref*{references_for_homotopy_limits_in_terms_of_weighted_limits} \linebreak \noindent\hyperlink{related_pages}{Related pages}\dotfill \pageref*{related_pages} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} The notion of \emph{weighted limit} is naturally understood from the point of view on [[limits]] as described at [[representable functor]]. Weighted limits make sense and are considered in the general context of $V$-[[enriched category theory]], but restrict attention to $V=$ [[Set]] for the moment, in order to motivate the concept. Let $K$ denote the small category which indexes [[diagrams]] over which we want to consider limits and eventually weighted limits. Notice that for \begin{displaymath} F : K \to Set \end{displaymath} a [[Set]]-valued functor on $K$, the limit of $F$ is canonically identified simply with the set of [[cones]] with tip the singleton set $pt = \{\bullet\}$: \begin{displaymath} lim F = [K,Set](\Delta pt, F) \,. \end{displaymath} This means, more generally, that for \begin{displaymath} F : K \to C \end{displaymath} a functor with values in an arbitrary category $C$, the object-wise limit of the functor $F$ under the [[Yoneda embedding]] \begin{displaymath} C(-,F(-)) : K \stackrel{F}{\to} C \stackrel{Y}{\to} Set^{C^{op}} \end{displaymath} which appears in the discussion in example 1 at [[representable functor]] can be expressed by the right side of \begin{displaymath} lim C(-,F(-)) = [K,Set](\Delta pt, C(-,F(-))) \,. \end{displaymath} (Recall that this is the limit over the diagram $C(-,F(-)) : K \to Set^{C^{op}}$ which, if [[representable functor|representable]] defines the desired limit of $F$.) The \textbf{idea} of weighted limits is to \begin{enumerate}% \item allow in the formula above the particular functor $\Delta pt$ to be replaced by any other functor $W : K \to Set$; \item to generalize everything straightforwardly from the [[Set]]-[[enriched category|enriched]] context to arbitrary $V$-enriched contexts. \end{enumerate} The idea is that the weight $W : K \to V$ encodes the way in which one generalizes the concept of a [[cone]] over a diagram $F$ (that is, something with just a tip from which morphisms are emanating down to $F$) to a more intricate structure over the diagram $F$. For instance in the application to [[homotopy limits]] discussed below with $V$ set to [[SimpSet]] the weight is such that it ensures that not only 1-morphisms are emanating from the tip, but that any triangle formed by these is filled by a 2-cell, every tetrahedron by a 3-cell, etc. \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} Let $V$ be a [[closed category|closed]] [[symmetric monoidal category]]. All categories in the following are $V$-[[enriched category|enriched categories]], all functors are $V$-functors. A \textbf{weighted limit} over a functor \begin{displaymath} F : K \to C \end{displaymath} with respect to a \emph{weight} or \emph{indexing type} functor \begin{displaymath} W : K \to V \end{displaymath} is, if it exists, the object $lim^W F \in C$ which [[representable functor|represents]] the functor (in $c \in C$) \begin{displaymath} [K,V](W, C(c,F(-))) : C^{op} \to V \,, \end{displaymath} i.e. such that for all objects $c \in C$ there is an isomorphism \begin{displaymath} C(c, lim^W F) \simeq [K,V](W(-), C(c,F(-))) \end{displaymath} natural in $c$. (Here $[K,V]$ is the $V$-[[enriched functor category]], as usual.) In particular, if $C = V$ itself, then we get the direct formula \begin{displaymath} lim^W F \simeq [K,V](W,F) \,. \end{displaymath} This follows from the above by the [[coend]] manipulation \begin{displaymath} \begin{aligned} [K,V](W(-),C(c,F(-))) &:= \int_{k \in K} V(W(k),V(c,F(k))) \\ & \simeq \int_{k \in K} V(c,V(W(k),F(k)) \\ & \simeq V(c, \int_{k \in K} V(W(k),F(k)) \\ & =: V(c, [K,V](W,F)) \,. \end{aligned} \end{displaymath} \hypertarget{examples}{}\subsection*{{Examples}}\label{examples} \hypertarget{homotopy_limits}{}\subsubsection*{{Homotopy limits}}\label{homotopy_limits} For $V$ some category of higher structures, the \emph{local} definition of [[homotopy limit]] over a diagram $F : K \to C$ replaces the ordinary notion of [[cone]] over $F$ by a higher cone in which all triangles of 1-morphisms are filled by 2-cells, all tetrahedra by 3-cells, etc. One can convince oneself that for the choice of [[SimpSet]] for $V$ this is realized in terms of the weighted limit $lim^W F$ with the weight $W$ taken to be \begin{displaymath} W : K \to \Simp\Set \end{displaymath} \begin{displaymath} W : k \mapsto N(K/k) \,, \end{displaymath} where $K/k$ denotes the [[over category]] of $K$ over $k$ and $N(K/k)$ denotes its [[nerve]]. This leads to the classical definition of homotopy limits in $\Simp\Set$-enriched categories due to \begin{itemize}% \item A.K. Bousfield and D.M. Kan, \emph{Homotopy limits, completions, and localizations} \end{itemize} See for instance also \begin{itemize}% \item Nicola Gambino, \emph{Weighted limits in simplicial homotopy theory} (\href{http://www.crm.cat/Publications/08/Pr790.pdf}{pdf} or \href{http://www.math.unipa.it/%7Engambino/Research/Papers/weighted.pdf}{pdf}) \end{itemize} In some nice cases the weight $N(K/-)$ can be replaced by a simpler weight; an example is discussed at [[Bousfield-Kan map]]. \hypertarget{homotopy_pullback}{}\subsubsection*{{Homotopy pullback}}\label{homotopy_pullback} For instance in the case that $K = \{r \to t \leftarrow s\}$ is the [[pullback]] diagram we have \begin{displaymath} W(r) = \{r\} \end{displaymath} \begin{displaymath} W(s) = \{s\} \end{displaymath} \begin{displaymath} W(t) = N( \{r \to t \leftarrow s\} ) \end{displaymath} and $W(r \to t) : \{r\} \to \{r \to t \leftarrow s\}$ injects the vertex $r$ into $\{r \to t \leftarrow s\}$ and similarly for $W(s \to t)$. This implies that for $F : K \to C$ a pullback diagram in the [[SimpSet]]-eriched category $C$, a $W$-weighted [[cone]] over $F$ with tip some object $c \in C$, i.e. a natural transformation \begin{displaymath} W \Rightarrow C(c, F(-)) \end{displaymath} is \begin{itemize}% \item over $r$ a ``morphism'' from the tip $c$ to $F(r)$ (i.e. a vertex in the Hom-simplicial set $C(c,F(r))$); \item similarly over $s$; \item over $t$ three ``morphisms'' from $c$ to $F(t)$ together with 2-cells between them (i.e. a 2-[[horn]] in the Hom-simplicial set $C(c,F(t))$) \item such that the two outer morphisms over $t$ are identified with the morphisms over $r$ and $s$, respectively, postcompoised with the morphisms $F(r \to t)$ and $F(s \to t)$, respectively. \end{itemize} So in total such a $W$-weighted cone looks like \begin{displaymath} \itexarray{ &&& c \\ & \swarrow &\Rightarrow& \downarrow &\Leftarrow& \searrow \\ F(r) && \stackrel{F(r \to t)}{\to} & F(t) & \stackrel{F(s \to t)}{\leftarrow} && F(s) } \end{displaymath} as one would expect for a ``homotopy cone''. \hypertarget{references_for_homotopy_limits_in_terms_of_weighted_limits}{}\subsection*{{References for homotopy limits in terms of weighted limits}}\label{references_for_homotopy_limits_in_terms_of_weighted_limits} Details of this are discussed for instance in the book \begin{itemize}% \item Hirschhorn, \emph{Model categories and their localization} \end{itemize} To compare with the above discussion notice that \begin{itemize}% \item The functor \begin{displaymath} W := N(K/-) \end{displaymath} is discussed there in definition 14.7.8 on p. 269. \item the $V$-enriched hom-category $[K,V]$ which on $V$-functors $S,T$ is the [[end]] $[K,V](S,T) = \int_{k \in K} V(S(k), T(k))$ appears as $hom^K(S,T)$ in definition 18.3.1 (see bottom of the page). \item for $V$ set to [[SimpSet]] the above definition of homotopy limit appears in example 18.3.6 (2). \end{itemize} \hypertarget{related_pages}{}\section*{{Related pages}}\label{related_pages} \begin{itemize}% \item [[strict 2-limit]] \item [[saturated class of limits]] \item [[weighted colimit]] \end{itemize} \hypertarget{references}{}\section*{{References}}\label{references} A standard reference is \begin{itemize}% \item [[Max Kelly]], \emph{Basic concepts of enriched category theory}, \href{http://www.emis.de/journals/TAC/reprints/articles/10/tr10.pdf#page=37}{section 3.1, p. 37} \end{itemize} In \begin{itemize}% \item [[Emily Riehl]], \emph{Weighted limits and colimits} (2008) (\href{http://www.math.jhu.edu/~eriehl/weighted.pdf}{pdf}) is given an account of lectures by [[Mike Shulman]] on the subject. The definition appears there as \href{http://www.math.jhu.edu/~eriehl/weighted.pdf#page=4}{definition 3.1, p. 4} (in a form a bit more general than the one above). \end{itemize} The analogous notion of weighted [[(infinity,1)-limit]] is discussed in \begin{itemize}% \item Martina Rovelli, \emph{Weighted limits in an (∞,1)-category}, 2019, \href{https://arxiv.org/abs/1902.00805}{arxiv:1902.00805} \end{itemize} [[!redirects weighted limits]] \end{document}