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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*{coherent object} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{topos_theory}{}\paragraph*{{Topos Theory}}\label{topos_theory} [[!include topos theory - contents]] \hypertarget{topos_theory_2}{}\paragraph*{{$(\infty,1)$-Topos Theory}}\label{topos_theory_2} [[!include (infinity,1)-topos - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{in_abelian_categories}{In Abelian categories}\dotfill \pageref*{in_abelian_categories} \linebreak \noindent\hyperlink{in_1topos_theory}{In 1-topos theory}\dotfill \pageref*{in_1topos_theory} \linebreak \noindent\hyperlink{InInfinityToposTheory}{In $(\infty,1)$-topos theory}\dotfill \pageref*{InInfinityToposTheory} \linebreak \noindent\hyperlink{examples}{Examples}\dotfill \pageref*{examples} \linebreak \noindent\hyperlink{see_also}{See also}\dotfill \pageref*{see_also} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} \hypertarget{in_abelian_categories}{}\subsubsection*{{In Abelian categories}}\label{in_abelian_categories} An object $X$ in an [[AB5-category]] $C$ is \textbf{of [[finite type]]} if one of the following equivalent conditions hold: (i) any complete directed set $\{X_i\}_{i\in I}$ of [[subobject]]s of $X$ is stationary (ii) for any complete directed set $\{Y_i\}_{i\in I}$ of subobjects of an object $Y$ the natural morphism $\operatorname{colim}_i C(X,Y_i) \to C(X,Y)$ is an isomorphism. An object $X$ is \textbf{finitely presented} if it is of finite type and if for any epimorphism $p : Y \to X$ where $Y$ is of finite type, it follows that $\operatorname{ker} p$ is also of finite type. An object $X$ in an AB5 category is \textbf{coherent} if it is of finite type and for any morphism $f : Y \to X$ where $Y$ is of finite type $\operatorname{ker} f$ is of finite type. For an exact sequence $0 \to X' \to X \to X'' \to 0$ in an AB5 category the following hold: \begin{enumerate}% \item if $X'$ and $X''$ are finitely presented, then $X$ is finitely presented; \item if $X$ is finitely presented and $X'$ of finite type, then $X''$ is finitely presented; \item if $X$ is coherent and $X'$ of finite type then $X''$ is also coherent. \end{enumerate} For a module $M$ over a ring $R$ this is equivalent to $M$ being finitely generated $R$-module. It is finitely presented if it is finitely presented in the usual sense of existence of short exact sequence of the form $R^I \to R^J \to M \to 0$ where $I$ and $J$ are finite. An AB5-category is \textbf{locally coherent} if it has a [[generator|generating set]] of coherent objects. If it is such, than every finitely presented object is coherent, and the full subcategory of finitely presented objects is therefore abelian. \hypertarget{in_1topos_theory}{}\subsubsection*{{In 1-topos theory}}\label{in_1topos_theory} Let $C$ be a [[topos]]. \begin{defn} \label{}\hypertarget{}{} An object $X$ of $C$ is called \textbf{compact} if the [[top]] element of the [[poset of subobjects]] $Sub(X)$ is a [[compact element]]. \end{defn} \begin{defn} \label{}\hypertarget{}{} An [[object]] $X$ of $C$ is called \textbf{stable} if for all morphisms $Y \to X$ from a compact object $Y$, the [[domain]] of the [[kernel pair]] $R \rightrightarrows Y$ of $f$ is also a compact object. \end{defn} \begin{defn} \label{}\hypertarget{}{} An object $X$ of $C$ is called \textbf{coherent} if it is compact and stable. \end{defn} \begin{theorem} \label{}\hypertarget{}{} Let $(C, \tau)$ be a [[small site|small]] [[cartesian site|cartesian]] [[site]], and suppose that $\tau$ is generated by finite [[covering families]]. For an object $X$ of $C$, let $l(X)$ denote the [[sheaf]] [[sheafification|associated]] to the [[presheaf]] [[representable functor|represented]] by $X$. Then \begin{itemize}% \item $l(X)$ is a coherent object of the [[topos]] $Sh(C, \tau)$, for all objects $X$ in $C$, \item if $(C, \tau)$ is further a [[pretopos]] with its [[coherent coverage]], then every [[coherent object]] of $Sh(C, \tau)$ is isomorphic to $l(X)$ for some $X$. \end{itemize} \end{theorem} This is (\hyperlink{JohnstoneSketches}{Johnstone, Theorem D3.3.7}). \hypertarget{InInfinityToposTheory}{}\subsubsection*{{In $(\infty,1)$-topos theory}}\label{InInfinityToposTheory} \begin{defn} \label{}\hypertarget{}{} An [[object]] $X$ in an [[(∞,1)-topos]] $\mathbf{H}$ is an \emph{$n$-coherent object} if the [[slice (∞,1)-topos]] $\mathbf{H}_{/X}$ is an [[n-coherent (∞,1)-topos]] \end{defn} (\hyperlink{Lurie}{Lurie, def. 3.1}). \begin{remark} \label{}\hypertarget{}{} A coherent object which is also [[n-truncated object of an (∞,1)-category|n-truncated]] for some $n$ is called a \emph{[[finitely constructible object]]}. \end{remark} ([[Rational and p-adic Homotopy Theory|Lurie pAdic, def. 2.3.1]]) \hypertarget{examples}{}\subsection*{{Examples}}\label{examples} \begin{example} \label{}\hypertarget{}{} [[∞Grpd]] is a [[coherent (∞,1)-topos]] and a [[locally coherent (∞,1)-topos]]. An [[object]] $X$, hence an [[∞-groupoid]], is an $n$-coherent object if all its [[homotopy groups]] in degree $k \leq n$ are [[finite set|finite]]. Hence the fully coherent objects here are the [[homotopy types with finite homotopy groups]]. \end{example} ([[Spectral Schemes|Lurie SpecSchm, example 3.13]]) \hypertarget{see_also}{}\subsection*{{See also}}\label{see_also} \begin{itemize}% \item [[compact object]] \item [[coherent topos]] \item [[coherent cohomology]] \item [[(∞,1)-pretopos]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} \begin{itemize}% \item N. Popescu, \emph{Abelian categories with applications to rings and modules}, London Math. Soc. Monographs 3, Academic Press 1973. xii+467 pp. \href{http://www.ams.org/mathscinet-getitem?mr=0340375}{MR0340375} \item [[Peter Johnstone]], [[Sketches of an elephant]], D3.3. \item [[Jacob Lurie]], section 3 of \emph{[[Spectral Schemes]]} \item Ivo Herzog, \emph{Contravariant functors on the category of finitely presented modules}, Israel J. Math. \href{http://lima.osu.edu/people/iherzog/contra.pdf}{pdf}: \emph{The Ziegler spectrum of a locally coherent Grothendieck category}, Proc. London Math. Soc. 74(3) (1997), 503-558 \href{http://lima.osu.edu/people/iherzog/Ziegler%20spec.pdf}{pdf} \end{itemize} [[!redirects coherent objects]] [[!redirects locally coherent category]] [[!redirects n-coherent object]] [[!redirects n-coherent objects]] \end{document}