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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*{composition law for factorizations} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{factorization_systems}{}\paragraph*{{Factorization systems}}\label{factorization_systems} [[!include factorization systems - contents]] \hypertarget{composition_laws_for_factorizations}{}\section*{{Composition laws for factorizations}}\label{composition_laws_for_factorizations} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{relation_to_awfs}{Relation to awfs}\dotfill \pageref*{relation_to_awfs} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} A \emph{composition law} for a [[functorial factorization]] is a functorial way to compose lifting structures for its algebraic right maps. A functorial factorization whose pointed endofunctor extends to a monad over $cod$ is an [[algebraic weak factorization system]] if and only if it has a composition law. Moreover, subject to smallness conditions, any functorial factorization with a composition law freely generates an algebraic weak factorization system. \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} Suppose $\mathcal{C}$ is a category equipped with a [[functorial factorization]], sending every arrow $f:A\to B$ to a factorization $A \xrightarrow{f_L} E f \xrightarrow{f_R} B$. As noted at [[functorial factorization]], a functorial factorization is equivalent to a pointed endofunctor $R$ on $\mathcal{C}^{\mathbf{2}}$ over $cod$, which maps each morphism $f$ (regarded as an object of the [[arrow category]] $\mathcal{C}^{\mathbf{2}}$) to its right factor $f_R$, the point being given by the left factor $f_L$ and the identity: \begin{displaymath} \itexarray{ & \xrightarrow{f_L} & \\ ^f \downarrow & & \downarrow^{f_R} \\ & \xrightarrow{id}&. } \end{displaymath} As with any pointed endofunctor, we can consider the category of [[algebra over a pointed endofunctor|algebras]] for $R$. Such an $R$-algebra is an arrow $f:A\to B$ equipped with a map $s : E f \to A$ such that $s \circ f_L = id_A$ and $f \circ s = f_R$. Equivalently, it is a diagonal lifting in the square \begin{displaymath} \itexarray{ A & \xrightarrow{id} & A \\ ^{f_L}\downarrow & & \downarrow^{f} \\ E f & \xrightarrow{f_R} & B.} \end{displaymath} In particular, this means that if $(L,R)$ is a factorization for a [[weak factorization system]], then the arrows of $\mathcal{C}$ that admit some structure of $R$-algebra are precisely those in the right class of the weak factorization system. The morphisms of $R$-algebras are commuting squares $g \circ h = k \circ f$ that additionally commute with the actions, i.e. $h \circ s_f = s_g \circ E(h,k)$. This defines a category $R Alg$ with a forgetful functor $U : R Alg \to \mathcal{C}^{\mathbf{2}}$. If it should happen that the pointed endofunctor $R$ is actually a [[monad]] over $cod$, i.e. it also has a multiplication $R R \to R$ that is also the identity on codomains, then we can also consider the smaller category $\mathbb{R} Alg$ of [[algebra over a monad|monad algebras]], the $R$-algebras as above such that $s$ also satisfies an associativity condition. \begin{udefn} A \textbf{right weak composition law} for a functorial factorization is a functor $R Alg \times_{\mathcal{C}} R Alg \to R Alg$, where the pullback is over $dom \circ U : R Alg \to \mathcal{C}$ and $cod \circ U : R Alg \to \mathcal{C}$, lying over the composition functor $\mathcal{C}^{\mathbf{2}} \times \mathcal{C}^{\mathbf{2}}\to \mathcal{C}$. If $R$ is a monad over $cod$, then a \textbf{right strong composition law} is defined analogously using $\mathbb{R} Alg$ instead. \end{udefn} More explicitly, this means that \begin{enumerate}% \item whenever $(f, s)$ and $(g, t)$ are $R$-algebras (resp. $\mathbb{R}$-algebras) such that $cod(f) = dom(g)$, we have a specified $R$-algebra structure (resp. $\mathbb{R}$-algebra structure) $t \bullet s$ for $g f$, such that \item for any morphisms of $R$-algebras (resp. $\mathbb{R}$-algebras) $(u, v) : (f, s) \to (f' , s')$ and $(v, w) : (g, t) \to (g' , t')$ between composable pairs $(f, s), (g, t)$ and $(f' , s' ),(g' , t' )$, the pasted square $(u, w) : (g f, t \bullet s) \to (g ' f' , t' \bullet s ' )$ is also a map of $R$-algebras (resp. $\mathbb{R}$-algebras). \end{enumerate} In other words, a composition law is an operation with the requisite shape to be the vertical composition in a [[double category]] whose vertical arrows are algebras, whose horizontal arrows are arbitrary arrows, and whose 2-cells are commutative squares. Associativity is not assumed, but as noted below it often comes for free. \hypertarget{relation_to_awfs}{}\subsection*{{Relation to awfs}}\label{relation_to_awfs} \begin{utheorem} Suppose $(L,R)$ is a functorial factorization whose underlying pointed endofunctor $R$ over $cod$ has the structure of a [[monad]] on $\mathcal{C}^{\mathbf{2}}$ over $cod$. Then $(L,R)$ is an [[algebraic weak factorization system]] if and only if it admits a right strong composition law. \end{utheorem} \begin{proof} See \hyperlink{Garner09}{Garner 09}, \hyperlink{Garner10}{Garner 10}, \hyperlink{Riehl11}{Riehl 11}, and \hyperlink{BR13}{Barthel-Riehl 13} for proofs in varying degrees of explicitness. \end{proof} \begin{utheorem} Suppose $(L,R)$ is a functorial factorization with a right weak composition law, and that the [[algebraically free monad]] on the pointed endofunctor $R$ exists and is over $cod$ (for instance if it can be constructed by a [[transfinite construction of free algebras]]). Then the latter monad has a right strong composition law, hence underlies an algebraic weak factorization system whose right-monad-algebras coincide with the $R$-pointed-endofunctor-algebras, including their natural composition law. \end{utheorem} \begin{proof} By definition, the algebraically-free monad $\mathbb{F}(R)$ satisfies $\mathbb{F}(R) Alg = R Alg$. Thus, the weak composition law for $R$ extends to a strong one for $\mathbb{F}(R)$; now we can apply the previous theorem. \end{proof} \hypertarget{references}{}\subsection*{{References}}\label{references} \begin{itemize}% \item [[Richard Garner]]. \emph{Understanding the small object argument}. Appl. Categ. Structures. 17(3) (2009) 247--285. \item [[Richard Garner]]. \emph{Homomorphisms of higher categories}. Adv. Math. 224(6) (2010), 2269--2311. \item [[Emily Riehl]]. \emph{Algebraic model structures}. New York J. Math. 17 (2011) 173-231. \item [[Tobias Barthel]] and [[Emily Riehl]]. \emph{On the construction of functorial factorizations for model categories}. Algebr. Geom. Topol. Volume 13, Number 2 (2013), 1089-1124. \href{https://projecteuclid.org/euclid.agt/1513715550}{projecteuclid} \end{itemize} [[!redirects composition law for factorization]] [[!redirects composition law for factorizations]] [[!redirects composition laws for factorization]] [[!redirects composition laws for factorizations]] [[!redirects composition law for a functorial factorization]] [[!redirects composition law for functorial factorizations]] [[!redirects composition laws for a functorial factorization]] [[!redirects composition laws for functorial factorizations]] [[!redirects composition law for a factorization system]] [[!redirects composition law for factorization systems]] [[!redirects composition laws for a factorization system]] [[!redirects composition laws for factorization systems]] [[!redirects composition law for a weak factorization system]] [[!redirects composition law for weak factorization systems]] [[!redirects composition laws for a weak factorization system]] 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