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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*{points-to-pieces transform} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{cohesive_toposes}{}\paragraph*{{Cohesive $\infty$-Toposes}}\label{cohesive_toposes} [[!include cohesive infinity-toposes - contents]] \hypertarget{discrete_and_concrete_objects}{}\paragraph*{{Discrete and concrete objects}}\label{discrete_and_concrete_objects} [[!include discrete and concrete objects - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{definition_and_basic_properties}{Definition and basic properties}\dotfill \pageref*{definition_and_basic_properties} \linebreak \noindent\hyperlink{relation_to_points_to_copieces}{Relation to points to co-pieces}\dotfill \pageref*{relation_to_points_to_copieces} \linebreak \noindent\hyperlink{RelationToAufhebung}{Relation to Aufhebung of the initial opposition}\dotfill \pageref*{RelationToAufhebung} \linebreak \noindent\hyperlink{examples}{Examples}\dotfill \pageref*{examples} \linebreak \noindent\hyperlink{BundleEquivalenceAndConcordance}{Bundle equivalence and concordance}\dotfill \pageref*{BundleEquivalenceAndConcordance} \linebreak \noindent\hyperlink{InGlobalEquivariantHomotopyTheory}{In global equivariant homotopy theory}\dotfill \pageref*{InGlobalEquivariantHomotopyTheory} \linebreak \noindent\hyperlink{in_tangent_cohesion_the_differential_cohomology_diagram}{In tangent cohesion: the differential cohomology diagram}\dotfill \pageref*{in_tangent_cohesion_the_differential_cohomology_diagram} \linebreak \noindent\hyperlink{comparison_map_between_algebraic_and_topological_ktheory}{Comparison map between algebraic and topological K-theory}\dotfill \pageref*{comparison_map_between_algebraic_and_topological_ktheory} \linebreak \noindent\hyperlink{in_infinitesimal_cohesion}{In infinitesimal cohesion}\dotfill \pageref*{in_infinitesimal_cohesion} \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} In a [[cohesive (∞,1)-topos]] $\mathbf{H}$, the canonical [[natural transformation]] \begin{displaymath} \flat \to \Pi \end{displaymath} from the [[flat modality]] to the [[shape modality]] may be thought of as sending ``points to the pieces in which they sit''. \hypertarget{definition_and_basic_properties}{}\subsection*{{Definition and basic properties}}\label{definition_and_basic_properties} Notice the existence of the following canonical [[natural transformations]] induced from the structure of a cohesive topos (a special case of the construction at \emph{[[unity of opposites]]}). \begin{defn} \label{TransformationFromPointsToPieces}\hypertarget{TransformationFromPointsToPieces}{} Given a [[cohesive topos]] $\mathcal{E}$ with ($ʃ \dashv \flat$) its ([[shape modality]] $\dashv$ [[flat modality]])-[[adjunction]], then the [[natural transformation]] \begin{displaymath} \flat X \longrightarrow X \longrightarrow ʃ X \end{displaymath} (given by the [[composition]] of the $\flat$-[[counit of a comonad|counit]] followed by the $ʃ$-[[unit of a monad|unit]]) may be called the transformation \textbf{from points to their pieces} or the \textbf{points-to-pieces-transformation}, for short. If this is an [[epimorphism]] for all $X$, we say that \emph{pieces have points} or that the \emph{[[Nullstellensatz]]} is verified. \end{defn} \begin{remark} \label{}\hypertarget{}{} The $(f^\ast \dashv f_\ast)$-[[adjunct]] of the transformation from pieces to points, def. \ref{TransformationFromPointsToPieces}, \begin{displaymath} \flat X \longrightarrow X \longrightarrow ʃ X \end{displaymath} is (by the rule of forming right [[adjuncts]] by first applying the [[right adjoint]] functor and then precomposing with the [[unit of an adjunction|unit]] and by the fact that the [[adjunct]] of a [[unit of an adjunction|unit]] is the [[identity]]) the map \begin{displaymath} (f_\ast X \longrightarrow f_! X) \coloneqq \left( f_\ast X \longrightarrow f_\ast f^\ast f_! X \stackrel{\simeq}{\longrightarrow} f_!X \right) \,. \end{displaymath} Observe that going backwards by applying $f^\ast$ to this and postcomposing with the $(f^\ast \dashv f_\ast)$-[[counit of an adjunction|counit]] is equivalent to just applying $f^\ast$, since by [[idempotent monad|idempotency]] of $\flat$ the counit is an [[isomorphism]] on the [[discrete object]] $f^\ast f_! X$. Therefore the points-to-pieces transformation and its adjunct are related by \begin{displaymath} \left( \flat X \longrightarrow X \longrightarrow ʃ X \right) = f^\ast \left( f_\ast X \longrightarrow f_! X \right). \end{displaymath} Observe then finally that since $f^\ast$ is a [[full and faithful functor|full and faithful]] [[left adjoint|left]] and [[right adjoint]], the points-to-pieces transform is an [[epimorphism]]/[[isomorphism]]/[[monomorphism]] precisely if its [[adjunct]] $f_\ast X \longrightarrow f_! X$ is, respectively. If this adjunct \begin{displaymath} \flat X \overset{\epsilon^{\flat}}{\longrightarrow} X \overset{\eta^\sharp}{\longrightarrow} \sharp X \end{displaymath} is a monomorphism, we say that \emph{[[discrete objects]] are [[concrete objects|concrete]]}. \end{remark} \hypertarget{relation_to_points_to_copieces}{}\subsection*{{Relation to points to co-pieces}}\label{relation_to_points_to_copieces} \begin{prop} \label{PiecesHavePointsIfDiscreteObjectsAreConcrete}\hypertarget{PiecesHavePointsIfDiscreteObjectsAreConcrete}{} \textbf{([[pieces have points]] iff [[discrete objects are concrete]])} For a [[cohesive topos]] $\mathbf{H}$, the the following two conditions are equivalent: \begin{enumerate}% \item \emph{pieces have points}, i.e. $\flat X \to X \to ʃ X$ is an [[epimorphism]] for all $X \in \mathbf{H}$; \item \emph{[[discrete objects]] are [[concrete objects|concrete]]}, i.e. $\flat X \overset{ \eta^{\sharp}_{\flat X} }{\longrightarrow} \sharp \flat X$ is a [[monomorphism]]. \end{enumerate} \end{prop} See at \emph{[[cohesive topos]]} \href{cohesive+topos#PiecesHavePointsEquivalentToDiscreteObjectsAreConcrete}{this prop.}. \hypertarget{RelationToAufhebung}{}\subsection*{{Relation to Aufhebung of the initial opposition}}\label{RelationToAufhebung} For a cohesive 1-topos, if the pieces-to-points transform is an [[epimorphism]] then there is [[Aufhebung]] of the initial opposition $(\emptyset \dashv \ast)$ in that $\sharp \emptyset \simeq \emptyset$ (\hyperlink{LawvereMenni15}{Lawvere-Menni 15, lemma 4.1}, see also \hyperlink{Shulman15}{Shulman 15, section 3}). Conversely, if the [[base topos]] is a [[Boolean topos]], then this Aufhebung implies that the pieces-to-points transform is an [[epimorphism]] (\hyperlink{LawvereMenni15}{Lawvere-Menni 15, lemma 4.2}). \hypertarget{examples}{}\subsection*{{Examples}}\label{examples} \hypertarget{BundleEquivalenceAndConcordance}{}\subsubsection*{{Bundle equivalence and concordance}}\label{BundleEquivalenceAndConcordance} Given an [[∞-group]] $G$ in a [[cohesive (∞,1)-topos]] $\mathbf{H}$, with [[delooping]] $\mathbf{B}G$, then for any other object $X$ the [[∞-groupoid]] $\mathbf{H}(X,\mathbf{B}G)$ is that of $G$-[[principal ∞-bundles]] with [[equivalences]] between them. Alternatively one may form the [[internal hom]] $[X,\mathbf{B}G]$. Applying the [[shape modality]] to this yields the $\infty$-groupoid $\mathbf{H}^\infty(X,\mathbf{B}G) \coloneqq ʃ [X,\mathbf{B}G]$ of $G$-principal $\infty$-bundles and [[concordances]] between them. Alternatively, the [[flat modality]] applied to the internal hom is again just the external hom $\flat [X,\mathbf{B}G] \simeq \mathbf{H}(X,\mathbf{B}G)$. In conclusion, in this situation the points-to-pieces transform is the canonical map \begin{displaymath} \mathbf{H}(X,\mathbf{B}G) \longrightarrow \mathbf{H}^\infty(X,\mathbf{B}G) \end{displaymath} from $G$-principal $\infty$-bundles with bundle equivalences between them, to $G$-principal $\infty$-bundles with concordances between them. \hypertarget{InGlobalEquivariantHomotopyTheory}{}\subsubsection*{{In global equivariant homotopy theory}}\label{InGlobalEquivariantHomotopyTheory} In [[global equivariant homotopy theory]] an incarnation of the points to pieces transform is the comparison map from [[homotopy quotients]] to ordinary [[quotients]] \begin{displaymath} X//G \longrightarrow X/G \end{displaymath} which in terms of the [[Borel construction]] is induced by the map $E G \to \ast$ \begin{displaymath} E G \times_G X \longrightarrow \ast \times_G X = X/G \,. \end{displaymath} (see at [[global equivariant homotopy theory]] \href{global+equivariant+homotopy+theory#OrdinaryQuotientAndHomotopyQuotientViaCohesion}{this prop.}) \hypertarget{in_tangent_cohesion_the_differential_cohomology_diagram}{}\subsubsection*{{In tangent cohesion: the differential cohomology diagram}}\label{in_tangent_cohesion_the_differential_cohomology_diagram} In a [[tangent cohesive (∞,1)-topos]] on [[stable homotopy types]] the points-to-pieces transform is one stage in a natural hexagonal [[long exact sequence]], the \emph{[[differential cohomology diagram]]}. See there for more. \hypertarget{comparison_map_between_algebraic_and_topological_ktheory}{}\subsubsection*{{Comparison map between algebraic and topological K-theory}}\label{comparison_map_between_algebraic_and_topological_ktheory} Applied to [[stable homotopy types]] in $Stab(\mathbf{H}) \hookrightarrow T\mathbf{H}$ the [[tangent cohesive (∞,1)-topos]] which arise from a [[symmetric monoidal (∞,1)-category]] $V \in CMon_\infty(Cat_\infty(\mathbf{H}))$ [[internal (∞,1)-category|internal]] to $\mathbf{H}$ under internal [[algebraic K-theory of a symmetric monoidal (∞,1)-category]], the points-to-pieces transform interprets as the [[comparison map between algebraic and topological K-theory]]. See there for more \hypertarget{in_infinitesimal_cohesion}{}\subsubsection*{{In infinitesimal cohesion}}\label{in_infinitesimal_cohesion} In [[infinitesimal cohesion]] the points-to-pieces transform is an [[equivalence]]. \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} [[!include cohesion - table]] \hypertarget{references}{}\subsection*{{References}}\label{references} \begin{itemize}% \item [[William Lawvere]], [[Matías Menni]], \emph{Internal choice holds in the discrete part of any cohesive topos satisfying stable connected codiscreteness}, Theory and Applications of Categories, Vol. 30, 2015, No. 26, pp 909-932. (\href{http://www.tac.mta.ca/tac/volumes/30/26/30-26abs.html}{TAC}) \item [[Mike Shulman]], \emph{Brouwer's fixed-point theorem in real-cohesive homotopy type theory}, Mathematical Structures in Computer Science Vol 28 (6) (2018): 856-941 (\href{https://arxiv.org/abs/1509.07584}{arXiv:1509.07584}, \href{https://doi.org/10.1017/S0960129517000147}{doi:10.1017/S0960129517000147}) [[!redirects points-to-pieces transforms]] \end{itemize} [[!redirects points-to-pieces transformation]] [[!redirects points-to-pieces transformations]] [[!redirects points-to-pieces-transformation]] [[!redirects points-to-pieces-transformations]] [[!redirects points-to-pieces-transform]] [[!redirects points-to-pieces-transforms]] [[!redirects pieces have points]] [[!redirects pieces-have-points]] [[!redirects discrete objects are concrete]] \end{document}