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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} \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{special_cases_and_models}{Special cases and models}\dotfill \pageref*{special_cases_and_models} \linebreak \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{related_entries}{Related entries}\dotfill \pageref*{related_entries} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} Given a [[stable (∞,1)-category]] $C$, its naive [[decategorification]] (its set of objects modulo the relation of [[equivalence]]) naturally inherits the structure of an abelian [[monoid]] from the [[biproducts]] in $C$. That is, we define $[a]+[b] = [a\oplus b]$. We can then further pass to the [[Grothendieck group]], adding formal additive inverses to get an abelian [[group]]. Note that such a biproduct sits in a [[split exact sequence|split]] (co)[[fibration sequence]] $a\to a\oplus b \to b$. Often we would like to obtain an analogous additivity result for \emph{arbitrary} (co)fibration sequences (a.k.a. [[distinguished triangles]]). This requires imposing the [[generators and relations|relation]] $[c] = [a] + [b]$ for any fibration sequence $a\to c\to b$, thereby passing to a quotient abelian group called $K_0(C)$, the \textbf{K-group} or [[Grothendieck group]] of $C$; see the latter entry for more details. The ``K'' is chosen by Grothendieck for the German word \emph{Klasse} for ``class''. The K-group of $C$ is the group of equivalence classes of $C$: it is a group due to the existence of a notion of exact sequences in $C$. K-theory starts with the study of these K-groups and their higher analogues $K_n(C)$, collectively denoted $K(C)$. Sometimes the K-groups themselves are called ``K-theory''. One would say for instance: ``$K(C)$ is the K-theory of $C$.'' More generally, there is a [[symmetric monoidal (infinity,1)-category|symmetric groupal ∞-groupoid]] $\mathbf{K}(C)$ -- i.e. a connective [[spectrum]] -- in between the [[decategorification]] from $C$ to $K(C)$, for which $K_0(C)$ is the set of [[simplicial homotopy group|connected components]] \begin{displaymath} C \mapsto \mathbf{K}(C) \to \pi_0 \mathbf{K}(C) = K_0(C) \,. \end{displaymath} and more generally $K_n(C) = \pi_n \mathbf{K}(C)$. In nice cases this is the degree 0 part of a non-connective [[spectrum]] which is then the \textbf{K-theory spectrum} of $C$. This is also called the \textbf{Waldhausen K-theory} of $C$. \hypertarget{special_cases_and_models}{}\subsection*{{Special cases and models}}\label{special_cases_and_models} Much of the literature on K-theory discusses constructions that \emph{model} the above abstract setup in terms of [[model category|model categories]], or just their [[homotopy category|homotopy categories]], often of the [[derived category|derived catgeories]] type and then often expressed in terms of the [[abelian category]] or more generally [[Quillen exact category]] from which the derived category is derived. Only a subset of the structure on a [[model category]] is necessary in order to conveniently extract the K-groups of the [[presentable (infinity,1)-category|presented]] [[stable (∞,1)-category]]. For that reason the axioms of a [[Waldhausen category]] have been devised to provide just the necessary convenient prerequisites to compute the K-groups of the [[(∞,1)-category]] [[presentable (infinity,1)-category|presented]] by the underlying [[homotopical category]]. \begin{itemize}% \item In particular, the K-group associated to the [[stable (∞,1)-category]] $Ch^b(A)$ of \emph{bounded} [[chain complex]]es in an [[abelian category]] or [[exact category]] $A$ is often called the K-group of $A$ itself and just denoted \begin{displaymath} K(A) := K(Ch^b(A)) \,. \end{displaymath} Most explicit constructions of K-theory spectra start with the data of an [[exact category]], such as notably Quillen's [[Q-construction]] and the [[Waldhausen S-construction]]. \item In particular if the exact category $A$ is that of [[vector bundle]]s on a [[topological space]] $X$ \begin{displaymath} A = VectBund(X) \end{displaymath} the corresponding K-group is degree 0 [[topological K-theory]]. This was the original of the notion and the term K-theory. \end{itemize} \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} Recall that given a [[(∞,1)-category]] $C$, we may regard it as a [[complete Segal space]] $C_{\bullet,\bullet}$, a [[bisimplicial object|bisimplicial set]]. For instance if $C$ is originally given as a [[quasicategory]] then \begin{displaymath} C_{\bullet,\bullet} : [n],[m] \mapsto Core(Func(\Delta^n,C))_{m} \,, \end{displaymath} where $Core(Func(\Delta^n,C))$ denotes the maximal [[Kan complex]] inside the [[(∞,1)-category of (∞,1)-functors]] from $\Delta^n$ to $C$. \begin{defn} \label{Gap}\hypertarget{Gap}{} For $\mathcal{C}$ an [[(∞,1)-category]] and $n \in \mathbb{N}$, write $Gap(\Delta^n, \mathcal{C})$ for the full sub-$\infty$-category on $Func(Arr(\Delta^n),\mathcal{C} )$ on those objects $F$ for which \begin{itemize}% \item the [[diagonal]] $F(n,n)$ is inhabited by [[zero objects]], for all $n$; \item all [[diagrams]] of the form \begin{displaymath} \itexarray{ F(i,j) &\to& F(i,k) \\ \downarrow && \downarrow \\ F(j,j) &\to& F(j,k) } \end{displaymath} is an [[(∞,1)-pushout]]. \end{itemize} \end{defn} \begin{defn} \label{}\hypertarget{}{} Let $C$ be a [[stable (∞,1)-category]]. Then its \textbf{Waldhausen K-theory} \begin{displaymath} \mathbf{K}(C) := \underset{\rightarrow}{\lim}_n Core(Gap(C^{\Delta^n})) \end{displaymath} is the [[geometric realization of simplicial topological spaces|geometric realization]] of/[[homotopy colimit]] of the degreewise [[core]] of the $Gap$, def. \ref{Gap}, of the corresponding [[complete Segal space]] (as a simplicial diagram of $\infty$-groupoids). \end{defn} This is \href{http://arxiv.org/PS_cache/math/pdf/0608/0608228v5.pdf#page=43}{remark 11.4} in [[stable (infinity,1)-category|StCat]]. See also \hyperlink{BGT}{Blumberg-Gepner-Tabuada, section 7}. This construction is also conjectured in the last section of Toen-Vezzosi's \emph{A remark on K-theory} . \begin{remark} \label{}\hypertarget{}{} In the case that $C$ is the [[simplicial localization]] of a [[Waldhausen category]] $\bar C$ the explicit way to obtain this is the [[Waldhausen S-construction]]. \end{remark} \begin{remark} \label{}\hypertarget{}{} It should be true that with this definition we have an isomorphism of groups \begin{displaymath} K(C) \simeq \pi_0 \mathbf{K}(C) \,. \end{displaymath} \end{remark} \begin{remark} \label{}\hypertarget{}{} This Waldhausen/hocolim-construction gives the \emph{connective} K-theory, taking values in [[connective spectra]]. The [[universal construction|universal]] completion to functor that sends [[homotopy cofibers]] of [[stable (infinity,1)-categories]] to homotopy cofibers of spectra is the corresponding unconnective $\mathbb{K}$-functor. \end{remark} There is a universal characterization of the construction of the $\mathbb{K}$-theory spectrum of a stable $(\infty,1)$-category $A$: there is an $(\infty,1)$-functor \begin{displaymath} U : (\infty,1)StabCat \to N \end{displaymath} to a stable $(\infty,1)$-category which is universal with the property that it respects colimits and exact sequences in a suitable way. Given any stable $(\infty,1)$-category $A$, its (connective or non-connective, depending on details) algebraic K-theory spectrum is the hom-object \begin{displaymath} K(A) \simeq Hom(U(Sp), U(A)) \,, \end{displaymath} where $Sp$ denotes the stable $(\infty,1)$-category of compact [[spectra]]. (\hyperlink{BGT}{BGT}) \hypertarget{related_entries}{}\subsection*{{Related entries}}\label{related_entries} \begin{itemize}% \item [[Grothendieck group]] \item [[K-theory spectrum]] \item [[topological K-theory]] \begin{itemize}% \item [[fiber integration in K-theory]] \end{itemize} \item [[Tate K-theory]] \item [[K-orientation]] \item [[algebraic K-theory]] \begin{itemize}% \item [[K-theory of a permutative category]] \item [[K-theory of a bipermutative category]] \item [[differential algebraic K-theory]] \item [[noncommutative motive]] \end{itemize} \item [[Waldhausen K-theory of a dg-category]] \item [[groupoid K-theory]] \item [[operator K-theory]] \begin{itemize}% \item [[equivariant operator K-theory]] \item [[KK-theory]] \item [[E-theory]] \end{itemize} \item [[equivariant K-theory]] \item [[Karoubi K-theory]] \item [[Morava K-theory]] \item [[L-theory]] \item [[twisted K-theory]] \item [[differential K-theory]] \item [[K-theory and physics]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} It was in \begin{itemize}% \item [[Bertrand Toen]], Gabriele Vezzosi, \emph{A remark on K-theory and $S$-categories} (\href{http://arxiv.org/PS_cache/math/pdf/0210/0210125v2.pdf}{arXiv}). \end{itemize} that it was proven that the the [[Waldhausen S-construction]] of the [[K-theory spectrum]] depends precisely on the [[simplicial localization]] of the [[Waldhausen category]], i.e. of the [[(∞,1)-category]] that it presents. In view of this remark 11.4 in \begin{itemize}% \item [[Jacob Lurie]], [[stable (infinity,1)-category|Stable ∞-Categories]] . \end{itemize} interprets the construction of the K-theory spectrum as a natural operation of [[stable (infinity,1)-category|stable (∞,1)-categories]], as described above. The universal property of the $(\infty,1)$-categorical definition is studied in \begin{itemize}% \item [[Andrew Blumberg]], [[David Gepner]], [[Goncalo Tabuada]], \emph{A universal characterization of higher algebraic K-theory} (\href{http://arxiv.org/abs/1001.2282}{arXiv:1001.2282}). \end{itemize} The standard constructions of K-theory spectra from [[Quillen exact categories]] are discussed in detail in chapter 1 of \begin{itemize}% \item [[Eric Friedlander]], [[Daniel Grayson]], \emph{Handbook of K-theory}, Springer Verlag . \end{itemize} A useful introduction to the definition and computation of K-groups (with a little on K-spectra) is \begin{itemize}% \item Charles Weibel, \emph{The K-book: An introduction to algebraic K-theory} (\href{http://www.math.rutgers.edu/~weibel/Kbook.html}{web}) \end{itemize} [[!redirects Waldhausen K-theory]] \end{document}