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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*{locally bounded category} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{category_theory}{}\paragraph*{{Category theory}}\label{category_theory} [[!include category theory - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{context_2}{Context}\dotfill \pageref*{context_2} \linebreak \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{features}{Features}\dotfill \pageref*{features} \linebreak \noindent\hyperlink{examples}{Examples}\dotfill \pageref*{examples} \linebreak \noindent\hyperlink{related_notions}{Related notions}\dotfill \pageref*{related_notions} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} Local boundedness of a [[category]] is a generalization of the notion of [[locally presentable category|local presentability]] that includes the category of [[topological spaces]]. \hypertarget{context_2}{}\subsection*{{Context}}\label{context_2} Let $C$ be a [[small category|small]] [[cocomplete category]] with a proper [[factorization system]], i.e., an [[orthogonal factorization system]] $(E,M)$ where every [[morphism|map]] in $E$ is an [[epimorphism]] and every map in $M$ is a [[monomorphism]]. The $M$-\textbf{union} of a small family of $M$-[[subobjects]] $(A_j \to B)_{j \in J}$ is the unique $M$-subobject $A \to B$ containing the $A_j$ and so that the induced map $\sum_j A_j \to A$ is in $E$. The union is calculated by applying the $(E,M)$ factorization to the canonical map $\sum_j A_j \to B$. If the map $\sum_j A_j \to B$ is in $E$, we say $(A_j \to B)_{j \in J}$ is an $M$-\textbf{union}. The set $J$ is a preorder under the relation $j \leq k$ if $A_j \leq A_k$ as $M$-subobjects of $B$. Regarding the $A_j$ as a diagram of shape $J$, the family $(A_j \to B)_{j \in J}$ is an $M$-union if and only if the map colim$A_j \to B$ is in $E$. We say $(A_j \to B)_{j \in J}$ is a \textbf{filtered union of} $M$-\textbf{subobjects} if it is a union of $M$-subobjects and if the category $J$ is [[filtered category|filtered]]. A [[representable functor]] $C(X,-)$ \textbf{preserves} the $M$-union of $(A_j \to B)_{j \in J}$ if the functions $C(X,A_j) \to C(X,A)$ are jointly surjective, i.e., if each $X \to A$ factors through some $X \to A_j$. \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} Let $\lambda$ be a [[regular cardinal]], and let $C$ be a cocomplete category with a proper factorization system $(E,M)$. \begin{defn} \label{}\hypertarget{}{} An object $X$ in $C$ is $\lambda$-\textbf{bounded} if $C(X,-)$ preserves $\lambda$-[[filtered colimit|filtered]] [[unions]] of $M$-subobjects. \end{defn} \begin{defn} \label{}\hypertarget{}{} A small set $G$ of objects of $C$ is an $(E,M)$-\textbf{generator} if $f \colon A \to B$ in $M$ is invertible whenever $f_* \colon C(X,A) \to C(X,B)$ is [[bijection|bijective]] for all $X \in G$. Equivalently, $G$ is an $(E,M)$-\textbf{generator} if for each $A \in C$ the family of maps $X \to A$ is jointly in $E$, i.e., if the map $\sum_{X \in G} \sum_{C(X,A)} X \to A$ is in $E$. \end{defn} \begin{defn} \label{}\hypertarget{}{} A [[category]] $C$ is called \textbf{locally} $\lambda$-\textbf{bounded} with respect to a proper factorization system $(E,M)$ if \begin{itemize}% \item it has an $(E,M)$-generator $G$ each of whose objects is $\lambda$-bounded \item it has arbitrary cointersections (even large ones) of maps in $E$ --- that is, it is [[E-cocomplete category|E-cocomplete]]. \end{itemize} \end{defn} \hypertarget{features}{}\subsection*{{Features}}\label{features} \begin{prop} \label{}\hypertarget{}{} In a locally $\lambda$-presentable category, every $\lambda$-presentable object is $\lambda$-bounded. Hence a $\lambda$-presentable category is $\lambda$-bounded. \end{prop} \begin{proof} This appears as Lemma 2.3.1 of \hyperlink{FreydKelly}{Freyd-Kelly} \end{proof} \begin{prop} \label{}\hypertarget{}{} Locally bounded categories are necessarily complete \end{prop} \begin{proof} This appears as Corollary 2.2 of \hyperlink{KellyLack}{Kelly-Lack}. The essential point is an $(E,M)$-variant of the special adjoint functor theorem: if $C$ is cocomplete, has a proper factorization system $(E,M)$, admits arbitrary $E$-cointersections, and has an $(E,M)$-generator, then every cocontinuous functor $C \to D$ has a right adjoint. \end{proof} \hypertarget{examples}{}\subsection*{{Examples}}\label{examples} The following examples are discussed in Section 6.1 of Kelly's \emph{Basic concepts of enriched category theory}. \begin{itemize}% \item [[locally presentable category|Locally presentable categories]] such as [[simplicial sets]], [[categories]], [[abelian groups]], [[Set|sets]]. \item [[compactly generated topological space|Compactly generated spaces]], and likewise based compactly generated spaces, with $E$ the surjections and $M$ the subspace inclusions. The point is an $(E,M)$-generator. \item [[quasi-topological space|Quasi-topological spaces]]. Note that this category is not $E$-well-copowered. \item [[Banach spaces]] with $E$ the epimorphisms, equivalently the dense maps, and $M$ the [[extremal monomorphisms]], equivalently the inclusions of closed subspaces with the induced norm. The base field is an $(E,M)$-generator. \end{itemize} \hypertarget{related_notions}{}\subsection*{{Related notions}}\label{related_notions} The term \textbf{locally ranked} is sometimes used to refer to a locally bounded category which in addition is co-wellpowered. For example, this terminology is used in \hyperlink{AHRT}{Ad\'a{}mek et. al.}. \hypertarget{references}{}\subsection*{{References}}\label{references} The contents of this page are taken from: \begin{itemize}% \item [[Max Kelly]], [[Steve Lack]], \emph{$V$-cat is locally presentable or locally bounded if $V$ is so} TAC (2001) \end{itemize} See also: \begin{itemize}% \item [[Max Kelly]], \emph{Basic concepts of enriched category theory}. \item [[Peter Freyd]], [[Max Kelly]], \emph{Categories of continuous functors} J. Pure. Appl. Algebra 2 (1972) 169-191. \end{itemize} \begin{itemize}% \item [[Jírí Adámek]], [[Horst Herrlich]], [[Jírí Rosickỳ]], [[Walter Tholen]], \emph{On a generalized small-object argument for the injective subcategory problem}. Cah. Topol. G\'e{}om. Diff\'e{}r. Cat\'e{}g 43 (2002) 83--106. \end{itemize} [[!redirects locally bounded categories]] \end{document}