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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*{model structure for quasi-categories} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{model_category_theory}{}\paragraph*{{Model category theory}}\label{model_category_theory} [[!include model category theory - contents]] \hypertarget{category_theory}{}\paragraph*{{$(\infty,1)$-Category theory}}\label{category_theory} [[!include quasi-category 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{as_a_cisinski_model_structure}{As a Cisinski model structure}\dotfill \pageref*{as_a_cisinski_model_structure} \linebreak \noindent\hyperlink{GeneralProperties}{General properties}\dotfill \pageref*{GeneralProperties} \linebreak \noindent\hyperlink{RelToInfGrpds}{Relation to the model structure for $\infty$-groupoids}\dotfill \pageref*{RelToInfGrpds} \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} A [[quasi-category]] is a [[simplicial set]] satisfying [[weak Kan complex|weak Kan filler conditions]] that make it behave like the [[nerve]] of an [[(∞,1)-category]]. There is a [[model category]] structure on the category [[SSet]] -- the \textbf{[[Andre Joyal|Joyal]] model structure} or \textbf{model structure for quasi-categories} -- such that the fibrant objects are precisely the quasi-categories and the weak equivalences precisely the correct \emph{categorical equivalences} that generalize the notion of [[equivalence of categories]]. \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} \begin{defn} \label{}\hypertarget{}{} The \textbf{model structure for quasi-categories} or \textbf{Joyal model structure} $sSet_{Joyal}$ on [[sSet]] has \begin{itemize}% \item [[cofibrations]] are the [[monomorphisms]] \item [[weak equivalences]] are those maps that are taken by the [[left adjoint]] of the [[homotopy coherent nerve]] to a weak equivalence in the [[model structure on simplicial categories]]. \end{itemize} \end{defn} \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \hypertarget{as_a_cisinski_model_structure}{}\subsubsection*{{As a Cisinski model structure}}\label{as_a_cisinski_model_structure} The model structure for quasi-categories is the [[Cisinski model structure]] on [[sSet]] whose class of weak equivalences is the localizer generated by the [[spine]] inclusions $\{Sp^n \hookrightarrow \Delta^n\}$. See (\hyperlink{Ara}{Ara}). \hypertarget{GeneralProperties}{}\subsubsection*{{General properties}}\label{GeneralProperties} \begin{prop} \label{}\hypertarget{}{} The model structure for quasi-categories is \begin{itemize}% \item a [[left proper model category]], \item a [[combinatorial model category]]. \end{itemize} \end{prop} \begin{remark} \label{}\hypertarget{}{} It is also a [[monoidal model category]] with respect to cartesian product and thus is naturally an [[enriched model category]] over itself, hence is $sSet_{Joyal}$-enriched (reflecting the fact that it tends to present an [[(infinity,2)-category]]). It is however \emph{not} $sSet_{Quillen}$-enriched and thus not a ``[[simplicial model category]]'' with respect to this enrichment. \end{remark} \begin{prop} \label{}\hypertarget{}{} For $p \colon \mathcal{C} \to \mathcal{D}$ a morphism of [[simplicial sets]] such that $\mathcal{D}$ is a [[quasi-category]]. Then $p$ is a fibration in $sSet_{Joyal}$ precisely if \begin{enumerate}% \item it is an [[inner fibration]]; \item it is an ``[[isofibration]]'': for every [[equivalence in an (∞,1)-category|equivalence]] in $\mathcal{D}$ and a lift of its [[domain]] through $p$, there is also a lift of the whole equivalence through $p$ to an equivalence in $\mathcal{C}$. \end{enumerate} \end{prop} This is due to [[Joyal]]. (\hyperlink{Lurie}{Lurie, cor. 2.4.6.5}). So every [[fibration]] in $sSet_{Joyal}$ is an [[inner fibration]], but the converse is in general false. A notably exception are the fibrations to the point: \begin{prop} \label{}\hypertarget{}{} The [[fibrant objects]] in $sSet_{Joyal}$ are precisely those that are [[inner fibration|inner fibrant]] over the point, hence those simplicial sets which are [[quasi-categories]]. \end{prop} (\hyperlink{Lurie}{Lurie, theorem 2.4.6.1}) \hypertarget{RelToInfGrpds}{}\subsubsection*{{Relation to the model structure for $\infty$-groupoids}}\label{RelToInfGrpds} The inclusion of [[(∞,1)-category|(∞,1)-catgeories]] [[∞Grpd]] $\stackrel{i}{\hookrightarrow}$ [[(∞,1)Cat]] has a left and a right [[adjoint (∞,1)-functor]] \begin{displaymath} (grpdfy \dashv i \dashv Core) \;\; : \;\; (\infty,1)Cat \stackrel{\overset{grpdfy}{\to}}{\stackrel{\overset{i}{\leftarrow}}{\overset{Core}{\to}}} \infty Grpd \,, \end{displaymath} where \begin{itemize}% \item $Core$ is the operation of taking the [[core]], the maximal $\infty$-groupoid inside an $(\infty,1)$-category; \item $grpdfy$ is the operation of \emph{groupoidification} that freely generates an $\infty$-groupoid on a given $(\infty,1)$-category \end{itemize} (see [[Higher Topos Theory|HTT, around remark 1.2.5.4]]) The adjunction $(grpdfy \dashv i)$ is modeled by the [[Bousfield localization of model categories|left Bousfield localization]] \begin{displaymath} (Id \dashv Id) \; :\; sSet_{Joyal} \stackrel{\leftarrow}{\to} sSet_{Quillen} \,. \end{displaymath} Notice that the left [[derived functor]] $\mathbb{L} Id : (sSet_{Joyal})^\circ \to (sSet_{Quillen})^\circ$ takes a fibrant object on the left -- a [[quasi-category]] -- then does nothing to it but regarding it now as an object in $sSet_{Quillen}$ and then producing its fibrant replacement there, which is [[Kan fibrant replacement]]. This is indeed the operation of \emph{groupoidification} . The other adjunction is given by the following \begin{prop} \label{}\hypertarget{}{} There is a [[Quillen adjunction]] \begin{displaymath} (k_! \dashv k^!) \;\; : sSet_{Quillen} \stackrel{\overset{k^!}{\leftarrow}}{\overset{k_!}{\to}} sSet_{Joyal} \end{displaymath} which arises as [[nerve and realization]] for the [[cosimplicial object]] \begin{displaymath} k : \Delta \to sSet : [n] \mapsto \Delta'[n] \,, \end{displaymath} where $\Delta^'[n] = N(\{0 \stackrel{\simeq}{\to} 1 \stackrel{\simeq}{\to} \cdots \stackrel{\simeq}{\to} n\})$ is the [[nerve]] of the [[groupoid]] freely generated from the linear [[quiver]] $[n]$. This means that for $X \in SSet$ we have \begin{itemize}% \item $k^!(X)_n = Hom_{sSet}(\Delta'[n],X)$. \item and $k_!(X)_n = \int^{[k]} X_k \cdot \Delta'[k]$. \end{itemize} \end{prop} This is (\hyperlink{JoyalTierney}{JoTi, prop 1.19}) The following proposition shows that $(k_! \dashv k^!)$ is indeed a model for $(i \dashv Core)$: \begin{prop} \label{}\hypertarget{}{} \begin{itemize}% \item For any $X \in sSet$ the canonical morphism $X \to k_!(X)$ is an acyclic cofibration in $sSet_{Quillen}$; \item for $X \in sSet$ a [[quasi-category]], the canonical morphism $k^!(X) \to Core(X)$ is an acyclic fibration in $sSet_{Quillen}$. \end{itemize} \end{prop} This is (\hyperlink{JoyalTierney}{JoTi, prop 1.20}) \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} A similar model for [[(∞,n)-categories]] is discussed at \begin{itemize}% \item [[model structure on cellular sets]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} The original construction of the Joyal model structure is in \begin{itemize}% \item [[Andre Joyal]], \emph{Theory of quasi-categories I} \end{itemize} Unfortunately, this is still not publicly available, but see the lecture notes \begin{itemize}% \item [[Andre Joyal]], \emph{The theory of quasi-categories and its applications}, (\href{http://mat.uab.cat/~kock/crm/hocat/advanced-course/Quadern45-2.pdf}{pdf}) \end{itemize} or the construction of the model structure in Cisinski's book \begin{itemize}% \item [[Denis-Charles Cisinski]], \emph{Higher categories and homotopical algebra}, (\href{http://www.mathematik.uni-regensburg.de/cisinski/CatLR.pdf}{pdf}) \end{itemize} which closely follows Joyal's original construction. A proof that proceeds via [[homotopy coherent nerve]] and [[simplicially enriched categories]] is given in detail following theorem 2.2.5.1 in \begin{itemize}% \item [[Jacob Lurie]], \emph{[[Higher Topos Theory]]} \end{itemize} The relation to the model structure for [[complete Segal space]]s is in \begin{itemize}% \item [[Andre Joyal]], [[Myles Tierney]], \emph{Quasi-categories vs. Segal spaces} (\href{http://arxiv.org/abs/math/0607820}{arXiv}) \end{itemize} Discussion with an eye towards [[Cisinski model structures]] and the [[model structure on cellular sets]] is in \begin{itemize}% \item [[Dimitri Ara]], \emph{Higher quasi-categories vs higher Rezk spaces} (\href{http://arxiv.org/abs/1206.4354}{arXiv}) \end{itemize} See also \begin{itemize}% \item [[Denis-Charles Cisinski]], \emph{Alg\`e{}bre homotopique et cat\'e{}gories sup\'e{}rieures}, course notes, 2009, \href{http://www.math.univ-toulouse.fr/~dcisinsk/coursinftycat.pdf}{pdf}. \end{itemize} A model structure for [[(infinity,2)-sheaves]] with values in quasicategories is discussed in \begin{itemize}% \item [[Nicholas Meadows]], \emph{The Local Joyal Model Structure} (\href{http://arxiv.org/abs/1507.08723}{arXiv:1507.08723}) \end{itemize} [[!redirects Joyal model structure]] [[!redirects Joyal model structure on simplicial sets]] [[!redirects model structure on quasicategories]] [[!redirects model structure on quasi-categories]] [[!redirects model structure for quasicategories]] \end{document}