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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*{module over a monoid} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{higher_algebra}{}\paragraph*{{Higher algebra}}\label{higher_algebra} [[!include higher algebra - contents]] \hypertarget{higher_linear_algebra}{}\paragraph*{{Higher linear algebra}}\label{higher_linear_algebra} [[!include homotopy - 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{examples}{Examples}\dotfill \pageref*{examples} \linebreak \noindent\hyperlink{modules_over_monoids_in_abelian_groups}{Modules over monoids in abelian groups}\dotfill \pageref*{modules_over_monoids_in_abelian_groups} \linebreak \noindent\hyperlink{GSets}{$G$-sets}\dotfill \pageref*{GSets} \linebreak \noindent\hyperlink{AbelianGroupsWithGAction}{Abelian groups with $G$-action as modules over the group ring}\dotfill \pageref*{AbelianGroupsWithGAction} \linebreak \noindent\hyperlink{more_examples}{more examples}\dotfill \pageref*{more_examples} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} In a [[monoidal category]], there is a notion of [[modules]] over [[monoid objects]] which generalizes the classical notion of [[modules]] over [[rings]]. This is a special case of [[module over a monad]] where the monad is taken to be $A \otimes -$, with $A$ some [[monoid object]]. \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} Let $(\mathcal{V}, \otimes, I)$ be a [[monoidal category]] and $A$ a [[monoid object]] in $\mathcal{V}$, hence an object $A \in \mathcal{V}$ equipped with a multiplication morphism \begin{displaymath} \cdot : A \otimes A \to A \end{displaymath} and a unit element \begin{displaymath} e : I \to A \end{displaymath} satisfying the [[associativity law]] and the [[unit law]]. \begin{defn} \label{ModuleInMonoidalCategory}\hypertarget{ModuleInMonoidalCategory}{} A (left) \textbf{module} over $A$ in $(\mathcal{V}, \otimes, I)$ is \begin{itemize}% \item an [[object]] $N \in \mathcal{V}$ \item equipped with a morphism \begin{displaymath} \lambda : A \otimes N \to N \end{displaymath} in $\mathcal{V}$ \end{itemize} such that this satisfies the axioms of an [[action]], in that the following are [[commuting diagrams]] in $\mathcal{V}$: \begin{displaymath} \itexarray{ A \otimes A \otimes N &\stackrel{id_A \otimes \lambda}{\to}& A \otimes N \\ \downarrow^{\mathrlap{\cdot \otimes id_n}} && \downarrow^{\mathrlap{\lambda}} \\ A \otimes N &\stackrel{\lambda}{\to}& N } \end{displaymath} and \begin{displaymath} \itexarray{ I \otimes N &&\stackrel{e \otimes id_N}{\to}&& A \otimes N \\ & \searrow && \swarrow_{\mathrlap{\lambda}} \\ && N } \,. \end{displaymath} \end{defn} \hypertarget{examples}{}\subsection*{{Examples}}\label{examples} \hypertarget{modules_over_monoids_in_abelian_groups}{}\subsubsection*{{Modules over monoids in abelian groups}}\label{modules_over_monoids_in_abelian_groups} Recall that a [[ring]], in the classical sense, is a [[monoid object]] in the category [[Ab]] of [[abelian groups]] with [[monoidal structure]] given by the [[tensor product of abelian groups]] $\otimes$. Accordingly a \emph{module over $R$} is a module in $(Ab,\otimes)$ according to def. \ref{ModuleInMonoidalCategory}. We unwind what this means in terms of [[abelian groups]] regarded as [[sets]] with extra [[structure]]: \begin{defn} \label{ModuleOverARing}\hypertarget{ModuleOverARing}{} A \textbf{module} $N$ over a ring $R$ is \begin{enumerate}% \item an [[object]] $N \in$ [[Ab]], hence an [[abelian group]]; \item equipped with a [[morphism]] \begin{displaymath} \alpha : R \otimes N \to N \end{displaymath} in [[Ab]]; hence a [[function]] of the underlying [[sets]] that sends elements \begin{displaymath} (r,n) \mapsto r n \coloneqq \alpha(r,n) \end{displaymath} and which is a [[bilinear function]] in that it satisfies \begin{displaymath} (r, n_1 + n_2) \mapsto r n_1 + r n_2 \end{displaymath} and \begin{displaymath} (r_1 + r_2, n) \mapsto r_1 n + r_2 n \end{displaymath} for all $r, r_1, r_2 \in R$ and $n,n_1, n_2 \in N$; \item such that the [[diagram]] \begin{displaymath} \itexarray{ R \otimes R \otimes N &\stackrel{\cdot_R \otimes Id_N}{\to}& R \otimes N \\ {}^{\mathllap{Id_R \otimes \alpha}}\downarrow && \downarrow^{\mathrlap{\alpha}} \\ R \otimes N &\to& N } \end{displaymath} [[commuting diagram|commutes]] in [[Ab]], which means that for all elements as before we have \begin{displaymath} (r_1 \cdot r_2) n = r_1 (r_2 n) \,. \end{displaymath} \item such that the diagram \begin{displaymath} \itexarray{ 1 \otimes N &&\stackrel{1 \otimes id_N}{\to}&& R \otimes N \\ & \searrow && \swarrow_{\mathrlap{\alpha}} \\ && N } \end{displaymath} commutes, which means that on elements as above \begin{displaymath} 1 \cdot n = n \,. \end{displaymath} \end{enumerate} \end{defn} \begin{remark} \label{}\hypertarget{}{} The category of all modules over all commutative rings is [[Mod]]. It is a [[bifibration]] \begin{displaymath} Mod \to CRing \end{displaymath} over [[CRing]]. This fibration may be characterized intrinsically, which gives yet another way of defining $R$-modules. This we turn to \hyperlink{ModulesOverARingInTermsOfStabilizedSlices}{below}. \end{remark} \hypertarget{GSets}{}\subsubsection*{{$G$-sets}}\label{GSets} Simpler than the traditionally default notion of a module in $(Ab,\otimes)$, as \hyperlink{Rings}{above} is that of a module in [[Set]], equipped with its [[cartesian monoidal category|cartesian monoidal structure]]. (These days one may want to think of this as a notion of modules over [[F1]].) A [[monoid object]] in $(Set,\times)$ is just a [[monoid]], for instance a [[discrete group]] $G$. A $G$-module in $(Set,\times)$ is simpy an \emph{[[action]]}, say a [[group action]]. \begin{defn} \label{ActionOfDiscreteGroupOnSet}\hypertarget{ActionOfDiscreteGroupOnSet}{} For $S \in$ [[Set]] and $G$ a [[discrete group]], a \textbf{$G$-[[action]]} of $G$ on $S$ is a [[function]] \begin{displaymath} \lambda \colon G \times S \to S \end{displaymath} such that \begin{enumerate}% \item the neutral element acts trivially \begin{displaymath} \itexarray{ * \times S &&\stackrel{\simeq}{\to}&& S \\ & {}_{(e,id_S)}\searrow && \nearrow_{\mathrlap{\lambda}} \\ && G \times S } \end{displaymath} \item the action property holds: for all $g_1, g_2 \in G$ and $s \in S$ we have $\lambda(g_1,\lambda(g_2, s)) = \lambda(g_1 \cdot g_2, s)$. \end{enumerate} \end{defn} \hypertarget{AbelianGroupsWithGAction}{}\subsubsection*{{Abelian groups with $G$-action as modules over the group ring}}\label{AbelianGroupsWithGAction} If a [[discrete group]] acts, as in def. \ref{ActionOfDiscreteGroupOnSet}, on the set underlying an [[abelian group]] and acts by [[linear maps]] (abelian group [[homomorphisms]]), then this action is equivalently a module over the [[group ring]] $\mathbb{Z}[G]$ as in def. \ref{ModuleOverARing}. \begin{defn} \label{}\hypertarget{}{} For $G$ a [[discrete group]], write $\mathbb{Z}[G] \in$ [[Ring]] for the [[ring]] \begin{enumerate}% \item whose underlying [[abelian group]] is the [[free abelian group]] on the set underlying $G$; \item whose multiplication is given on [[basis]] elements by the group operation in $G$. \end{enumerate} \end{defn} \begin{remark} \label{}\hypertarget{}{} For $G$ a [[finite group]] an element $r$r of $\mathbb{Z}[G]$ is for the form \begin{displaymath} r = \sum_{g \in G} r_g g \end{displaymath} with $r_g \in \mathbb{Z}$. Addition is given by addition of the [[coefficients]] $r_g$ and multiplication is given by the formula \begin{displaymath} \begin{aligned} r \cdot \tilde r & = \sum_{g \in G} \sum_{\tilde g \in G} (r_g \tilde r_{\tilde g}) g \cdot \tilde g \\ & = \sum_{q \in G} \left( \sum_{g \tilde g = q} r_g \tilde r_{\tilde g} \right) q \end{aligned} \,. \end{displaymath} \end{remark} \begin{prop} \label{}\hypertarget{}{} For $A \in$ [[Ab]] an [[abelian group]] with underlying set $U(A)$, $G$-[[actions]] $\lambda \colon G \times U(A) \to U(A)$ such that for each element $g \in G$ the function $\lambda(g,-) \colon U(A) \to U(A)$ is an abelian group homomorphism are equivalently $\mathbb{Z}[G]$-[[module]] structures on $A$. \end{prop} \begin{proof} Since the underlying abelian group of $\mathbb{Z}[G]$ is a [[free abelian group|free]] by definition, a [[bilinear map]] $\mathbb{Z}[G] \times A \to A$ is equivalently for each [[basis]] element $g \in G$ a [[linear map]] $A \to A$. Similarly the module property is determined on basis elements, where it reduces manifestly to the action property of $G$ on $U(A)$. \end{proof} \begin{remark} \label{}\hypertarget{}{} This reformulation of linear $G$-[[actions]] in terms of [[modules]] allows to treat $G$-actions in terms of [[homological algebra]]. See at \emph{\href{Ext#RelationToGroupCohomology}{Ext -- Relation to group cohomology}}. \end{remark} \hypertarget{more_examples}{}\subsubsection*{{more examples}}\label{more_examples} \begin{itemize}% \item In a [[symmetric monoidal category|symmetric monoidal]] [[category of chain complexes]] equipped with the [[tensor product of chain complexes]], then a [[monoid]] is a [[dg-algebra]], and a module is a \emph{[[dg-module]]}. See there for more. \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} The basic properties of categories of modules over [[monoid objects]] in [[symmetric monoidal categories]] are spelled out in sections 1.2 and 1.3 of \begin{itemize}% \item Florian Marty, \emph{Des Ouverts Zariski et des Morphismes Lisses en G\'e{}om\'e{}trie Relative}, Ph.D. Thesis, 2009, \href{http://thesesups.ups-tlse.fr/540/}{web} \end{itemize} A summary is in section 4.1 of \begin{itemize}% \item [[Martin Brandenburg]], \emph{Tensor categorical foundations of algebraic geometry}, \href{http://arxiv.org/abs/1410.1716}{arXiv:1410.1716}. \end{itemize} See also \href{http://mathoverflow.net/questions/180673/category-of-modules-over-commutative-monoid-in-symmetric-monoidal-category}{MO/180673}, and the references at [[modules over a monad]]. For the classical case of the [[symmetric monoidal category]] [[Ab]], a standard textbook is \begin{itemize}% \item F.W. Anderson, K.R. Fuller, \emph{Rings and Categories of Modules}, Graduate Texts in Mathematics, Vol. 13, Springer-Verlag, New York, (1992). \end{itemize} [[!redirects modules over a monoid]] [[!redirects modules over monoids]] [[!redirects modules over a monoid]] \end{document}