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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*{K-theory of a permutative category} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{cohomology}{}\paragraph*{{Cohomology}}\label{cohomology} [[!include cohomology - 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{ViaTopologicalGroupCompletion}{Via topological group completion}\dotfill \pageref*{ViaTopologicalGroupCompletion} \linebreak \noindent\hyperlink{via_gamma_spaces}{Via Gamma spaces}\dotfill \pageref*{via_gamma_spaces} \linebreak \noindent\hyperlink{properties}{Properties}\dotfill \pageref*{properties} \linebreak \noindent\hyperlink{monoidal_functoriality}{Monoidal functoriality}\dotfill \pageref*{monoidal_functoriality} \linebreak \noindent\hyperlink{Examples}{Examples}\dotfill \pageref*{Examples} \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} To a [[permutative category]] $C$ is naturally associated a [[Gamma-space]], hence a [[symmetric spectrum]]. The [[generalized (Eilenberg-Steenrod) cohomology]] theory [[Brown representability theorem|represented]] by this is called the \emph{([[algebraic K-theory|algebraic]]) [[K-theory]]} of (or represented by) $C$. If the category is even a [[bipermutative category]] then the corresponding [[K-theory of a bipermutative category]] in addition has [[E-infinity ring]] structure, hence is a [[multiplicative cohomology theory]]. \hypertarget{Definition}{}\subsection*{{Definition}}\label{Definition} \hypertarget{ViaTopologicalGroupCompletion}{}\subsubsection*{{Via topological group completion}}\label{ViaTopologicalGroupCompletion} For $\mathcal{C}$ a [[category]], write ${\vert \mathcal{C}\vert} \in$ [[classical model structure on simplicial sets|sSet]] $\simeq_{Qu}$ [[classical model structure on topological spaces|Top]] for its [[nerve]]/[[geometric realization]]. (Beware that this is often denoted $B C$, instead, but that notation clashes with that for [[delooping]], which we also need in the following.) For $\mathcal{C}$ a [[permutative category]] its [[nerve]]/[[geometric realization]] $\vert \mathcal{C} \vert$ is naturally a [[topological monoid]] (\hyperlink{Quillen70}{Quillen 70} see e.g. \hyperlink{May13}{May 13, theorem 4.10}), hence admits a [[bar construction]]/[[classifying space]] $B {\vert \mathcal{C}\vert}$. The [[loop space]] of that \begin{displaymath} (\mathbb{K}\mathcal{C})_0 \;\coloneqq\; \Omega B {\vert \mathcal{C}\vert} \,, \end{displaymath} being an [[∞-group]], may be regarded as the [[homotopy theory|homotopy theoretic]] [[group completion]] of the [[topological monoid]] ${\vert \mathcal{C}\vert}$. This is the degree-0 space in the \emph{algebraic K-theory [[spectrum]] $\mathbb{K}\mathcal{C}$ of the permutative category} $\mathcal{C}$ (see e.g. \hyperlink{May13}{May 13, def 4.11}). By (\hyperlink{DwyerKan80}{Dwyer-Kan 80, prop. 3.7, prop. 9.2, remark 9.7}) the operation $\Omega B (-)$ is the [[derived functor]] of [[group completion]], so that this construction ought to be a model for the [[K-theory of a symmetric monoidal (∞,1)-category]]. In particular, under [[decategorification]], its [[group]] of [[connected components]] is the actual [[Grothendieck group]] $K(-)$ of the [[isomorphism classes]] of [[objects]] in $\mathcal{C}$: \begin{displaymath} \pi_0 \left( \mathbb{K} \mathcal{C} \right) \;\simeq\; K\left( \mathcal{C}/_\sim, \oplus \right) \end{displaymath} (recalled e.g. in \hyperlink{BohmannOsorno14}{Bohmann-Osorno 14, p. 14}). In particular for $R$ a [[topological ring]] one considers $C$ a [[skeletal category|skeleton]] of the [[groupoid]] of ([[finitely generated module|finitely generated]]) [[projective modules]] over $R$. Then the K-theory of $C$ is the [[algebraic K-theory]] of $R$ (e.g. \hyperlink{May13}{May 13, p. 25}) \begin{displaymath} \mathbb{K}\left( R Mod^{fin}_{proj} \right) \;\simeq\; K R \,. \end{displaymath} \hypertarget{via_gamma_spaces}{}\subsubsection*{{Via Gamma spaces}}\label{via_gamma_spaces} Write $FinSet^{*/}$ for the [[category]] of [[pointed objects|pointed]] [[finite sets]]. For $C$ a [[permutative category]], there is naturally a [[functor]] \begin{displaymath} \widebar {C}_{(-)} \;\colon\; FinSet^{*/} \to Cat \end{displaymath} \begin{displaymath} A \mapsto \widebar C_A \end{displaymath} such that (\ldots{}). (\hyperlink{ElmendorfMandell06}{Elmendorf-Mandell, theorem 4.2}) Accordingly, postcomposition with the [[nerve]] $N : Cat \to sSet$ produces from $C$ a [[Gamma-space]] $N \widebar C$. To this corresponds a [[spectrum]] \begin{displaymath} K^{Seg} C \coloneqq \{N \widebar C_{S_\bullet^n}\} \,. \end{displaymath} This is the \emph{K-theory spectrum of $C$}. (\hyperlink{ElmendorfMandell06}{Elmendorf-Mandell, def. 4.3}) \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \hypertarget{monoidal_functoriality}{}\subsubsection*{{Monoidal functoriality}}\label{monoidal_functoriality} \begin{prop} \label{MonoidalFunctoriality}\hypertarget{MonoidalFunctoriality}{} The construction of K-theory spectra of permutative categories constitutes a [[multifunctor]] \begin{displaymath} \mathbb{K} \;\colon\; PermCat \longrightarrow Spectra \end{displaymath} between [[multicategories]] of [[permutative categories]] (under [[Deligne tensor product]]) and [[spectra]] (under [[smash product of spectra]]). Hence for a multilinear functor of permutative categories \begin{displaymath} F \;\colon\; \mathcal{A}_1 \times \cdots \times \mathcal{A}_n \longrightarrow \mathcal{B} \end{displaymath} there is a compatibly induced morphism of K-[[spectra]] out of the [[smash product of spectra|smash product]] \begin{displaymath} \mathbb{K}(\mathcal{A}_1) \wedge \cdots \wedge \mathbb{K}(\mathcal{A}_n) \longrightarrow \mathbb{K}(\mathcal{B}) \,. \end{displaymath} This implies that the construction further extends to a [[2-functor]] from the [[2-category]] $PermCat Cat$ of [[enriched categories]] over the [[multicategory]] of [[permutative categories]] to that of [[enriched categories]] of [[spectra]]: \begin{displaymath} \mathbb{K}_\bullet \;\colon\; PermCat Cat \longrightarrow Spectra Cat \end{displaymath} which applies $\mathbb{K}$ to each [[hom-object]]. \end{prop} (\hyperlink{May80}{May 80, theorem 1.6, Theorem 2.1}, \hyperlink{ElmendorfMandell04}{Elmendorf-Mandell 04, theorem 1.1}, \hyperlink{Guillou10}{Guillou 10, Theorem 1.1} \hypertarget{Examples}{}\subsection*{{Examples}}\label{Examples} \begin{example} \label{OrdinaryAlgebraicKTheoryFromPermutativeCategoryOfProjectiveModules}\hypertarget{OrdinaryAlgebraicKTheoryFromPermutativeCategoryOfProjectiveModules}{} \textbf{(ordinary [[algebraic K-theory]])} For $R$ a [[commutative ring]], let $\mathcal{C} = R Mod_{pr}$ its [[category of modules|category of]] [[finitely generated module|finitely generated]] [[projective modules]] regarded as a [[permutative category]]. Then \begin{displaymath} K (R Mod^{pr}_{fin}) \;\simeq\; K R \end{displaymath} is the classical [[algebraic K-theory]] spectrum of the ring $R$. \end{example} (e.g. \hyperlink{ElmendorfMandell04}{Elmendorf-Mandell 04, p. 10}). \begin{example} \label{StableCohomotopyIsKTheoryOfFinSet}\hypertarget{StableCohomotopyIsKTheoryOfFinSet}{} \textbf{([[stable cohomotopy]] is K-theory of [[FinSet]])} Let $\mathcal{C} =$ [[FinSet]] be a [[skeleton]] of the category of [[finite sets]], regarded as a [[permutative category]]. Then \begin{displaymath} K(FinSet) \;\simeq\; \mathbb{S} \end{displaymath} is the [[sphere spectrum]], hence represents the [[cohomology theory]] called \emph{[[stable cohomotopy]]}. \end{example} (due to \hyperlink{Segal74}{Segal 74, Prop. 3.5}, see also \hyperlink{Priddy73}{Priddy 73}) \begin{remark} \label{StableCohomotopyIsAlgebraicKTheoryOverFieldWithOneElement}\hypertarget{StableCohomotopyIsAlgebraicKTheoryOverFieldWithOneElement}{} \textbf{([[stable cohomotopy]] as [[algebraic K-theory]] over the [[field with one element]])} Since ([[pointed sets|pointed]]) [[finite sets]] may be regarded as the modules over the ``[[field with one element]]'' $\mathbb{F}_1$ (see \href{field+with+one+element#Modules}{there}), \begin{displaymath} \mathbb{F}_1 Mod \;=\; FinSet^{\ast/} \end{displaymath} one may read example \ref{StableCohomotopyIsKTheoryOfFinSet} in view of example \ref{OrdinaryAlgebraicKTheoryFromPermutativeCategoryOfProjectiveModules} as saying that [[stable cohomotopy]] is the algebraic K-theory of the [[field with one element]]: \begin{displaymath} K \mathbb{F}_1 \;=\; \mathbb{S} \,. \end{displaymath} This perspective is highlighted for instance in (\hyperlink{Deitmar06}{Deitmar 06, p. 2}, \hyperlink{Guillot06}{Guillot 06}). \end{remark} \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} \begin{itemize}% \item [[K-theory of a bipermutative category]] \item [[algebraic K-theory of a symmetric monoidal (∞,1)-category]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} \begin{itemize}% \item [[Daniel Quillen]], \emph{Cohomology of groups}, Proceedings of the international congress of mathematics 1970 \item [[Daniel Quillen]], \emph{On the group completion of a simplicial monoid} \item [[Stewart Priddy]], \emph{Transfer, symmetric groups, and stable homotopy theory}, in \emph{Higher K-Theories}, Springer, Berlin, Heidelberg, 1973. 244-255 (\href{https://link.springer.com/content/pdf/10.1007/BFb0067060.pdf}{pdf}) \item [[Graeme Segal]], \emph{Catgeories and cohomology theories}, Topology vol 13 (1974) (\href{http://ncatlab.org/nlab/files/SegalCategoriesAndCohomologyTheories.pdf}{pdf}) \item [[Peter May]], \emph{The spectra associated to permutative categories}, Topology 17 (1978) (\href{http://www.math.uchicago.edu/~may/PAPERS/23.pdf}{pdf}) \item [[Peter May]], \emph{$E_\infty$-Spaces, group completions, and permutative categories}, in [[Graeme Segal]] (ed.) \emph{New Developments in Topology}, Cambirdge University Press 2013 (\href{http://www.math.uchicago.edu/~may/PAPERS/13.pdf}{pdf}, \href{https://doi.org/10.1017/CBO9780511662607.008}{doi:10.1017/CBO9780511662607.008}) \item [[Peter May]], \emph{Pairings of categories and spectra}, J. Pure Appl. Algebra, 19:299–346, 1980 \item [[William Dwyer]], [[Daniel Kan]], \emph{Simplicial localization of categories}, Journal of pure and applied algebra 17 (1980) 267-284 \item [[Anthony Elmendorf]], [[Michael Mandell]], \emph{Permutative categories as a model of connective stable homotopy}, in: [[Birgit Richter]] (ed.) \emph{Structured Ring spectra}, Cambridge University Press (2004) \item [[Anthony Elmendorf]], [[Michael Mandell]], \emph{Rings, modules and algebras in infinite loop space theory}, Adv. in Math. 205 (2006), no. 1, 163-228 (\href{https://arxiv.org/abs/math/0403403}{arXiv:math/0403403}) \item [[Anthony Elmendorf]], [[Michael Mandell]], \emph{Permutative categories, multicategories, and algebraic K-theory}, Algebraic \& Geometric Topology 9 (2009) 2391-2441 (\href{http://arxiv.org/abs/0710.0082}{arXiv:0710.0082}, \href{https://projecteuclid.org/euclid.agt/1513797088}{euclid:1513797088}) \item [[Bertrand Guillou]], \emph{Strictification of categories weakly enriched in symmetric monoidal categories}, Theory Appl. Categ., 24:No. 20, 564–579, 2010. \end{itemize} The interpretation of [[stable cohomotopy]] as the algebraic K-theory over the [[field with one element]] is adopted in \begin{itemize}% \item [[Anton Deitmar]], \emph{Remarks on zeta functions and K-theory over $\mathbb{F}_1$} (\href{https://arxiv.org/abs/math/0605429}{arXiv:math/0605429}) \item [[Pierre Guillot]], \emph{Adams operations in cohomotopy} (\href{https://arxiv.org/abs/math/0612327}{arXiv:0612327}) \end{itemize} The generalization of K-theory of permutative categories to [[Mackey functors]] is discussed in \begin{itemize}% \item [[Anna Marie Bohmann]], [[Angélica Osorno]], \emph{Constructing equivariant spectra via categorical Mackey functors}, Algebraic \& Geometric Topology 15.1 (2015): 537-563 (\href{http://arxiv.org/abs/1405.6126}{arXiv:1405.6126}) \end{itemize} Generalization to [[equivariant stable homotopy theory]] and [[G-spectra]] is discussed in \begin{itemize}% \item [[Bert Guillou]], [[Peter May]], around section 4.4 of \emph{Permutative $G$-categories in equivariant infinite loop space theory} (\href{http://arxiv.org/abs/1207.3459}{arXiv:1207.3459}) \end{itemize} [[!redirects K-theory of permutative categories]] [[!redirects algebraic K-theory of a permutative category]] [[!redirects algebraic K-theory of permutative categories]] [[!redirects K-theory spectrum of a permutative category]] [[!redirects K-theory spectra of permutative categories]] \end{document}