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filtered object in nLab
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content="application/xhtml+xml;charset=utf-8" /><title>Contents</title></head> <body> <div class="rightHandSide"> <div class="toc clickDown" tabindex="0"> <h3 id="context">Context</h3> <h4 id="category_theory">Category theory</h4> <div class="hide"><div> <p><strong><a class="existingWikiWord" href="/nlab/show/category+theory">category theory</a></strong></p> <h2 id="sidebar_concepts">Concepts</h2> <ul> <li> <p><a class="existingWikiWord" href="/nlab/show/category">category</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/functor">functor</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/natural+transformation">natural transformation</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/Cat">Cat</a></p> </li> </ul> <h2 id="sidebar_universal_constructions">Universal constructions</h2> <ul> <li> <p><a class="existingWikiWord" href="/nlab/show/universal+construction">universal construction</a></p> <ul> <li> <p><a class="existingWikiWord" href="/nlab/show/representable+functor">representable functor</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/adjoint+functor">adjoint functor</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/limit">limit</a>/<a class="existingWikiWord" href="/nlab/show/colimit">colimit</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/weighted+limit">weighted limit</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/end">end</a>/<a class="existingWikiWord" href="/nlab/show/coend">coend</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/Kan+extension">Kan extension</a></p> </li> </ul> </li> </ul> <h2 id="sidebar_theorems">Theorems</h2> <ul> <li> <p><a class="existingWikiWord" href="/nlab/show/Yoneda+lemma">Yoneda lemma</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/Isbell+duality">Isbell duality</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/Grothendieck+construction">Grothendieck construction</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/adjoint+functor+theorem">adjoint functor theorem</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/monadicity+theorem">monadicity theorem</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/adjoint+lifting+theorem">adjoint lifting theorem</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/Tannaka+duality">Tannaka duality</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/Gabriel-Ulmer+duality">Gabriel-Ulmer duality</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/small+object+argument">small object argument</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/Freyd-Mitchell+embedding+theorem">Freyd-Mitchell embedding theorem</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/relation+between+type+theory+and+category+theory">relation between type theory and category theory</a></p> </li> </ul> <h2 id="sidebar_extensions">Extensions</h2> <ul> <li> <p><a class="existingWikiWord" href="/nlab/show/sheaf+and+topos+theory">sheaf and topos theory</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/enriched+category+theory">enriched category theory</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/higher+category+theory">higher category theory</a></p> </li> </ul> <h2 id="sidebar_applications">Applications</h2> <ul> <li><a class="existingWikiWord" href="/nlab/show/applications+of+%28higher%29+category+theory">applications of (higher) category theory</a></li> </ul> <div> <p> <a href="/nlab/edit/category+theory+-+contents">Edit this sidebar</a> </p> </div></div></div> </div> </div> <h1 id="contents">Contents</h1> <div class='maruku_toc'> <ul> <li><a href='#definition'>Definition</a></li> <li><a href='#properties'>Properties</a></li> <li><a href='#related_concepts'>Related concepts</a></li> <li><a href='#references'>References</a></li> </ul> </div> <h2 id="definition">Definition</h2> <div class="num_defn" id="Filtration"> <h6 id="definition_2">Definition</h6> <p>Given a <a class="existingWikiWord" href="/nlab/show/category">category</a> <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><mi>𝒞</mi></mrow><annotation encoding="application/x-tex">\mathcal{C}</annotation></semantics></math>, then a <strong>filtered object</strong> is an <a class="existingWikiWord" href="/nlab/show/object">object</a> <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><mi>X</mi></mrow><annotation encoding="application/x-tex">X</annotation></semantics></math> of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><mi>𝒞</mi></mrow><annotation encoding="application/x-tex">\mathcal{C}</annotation></semantics></math> equipped with a <em>filtration</em>:</p> <p>A <strong>descending filtration</strong> or <strong>decreasing filtrations</strong> of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><mi>X</mi></mrow><annotation encoding="application/x-tex">X</annotation></semantics></math> is a sequence of <a class="existingWikiWord" href="/nlab/show/morphisms">morphisms</a> (often required to be <a class="existingWikiWord" href="/nlab/show/monomorphisms">monomorphisms</a>) of the form</p> <div class="maruku-equation"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block" class="maruku-mathml"><semantics><mrow><mi>⋯</mi><mo>⟶</mo><msub><mi>X</mi> <mrow><mi>n</mi><mo>+</mo><mn>1</mn></mrow></msub><mo>⟶</mo><msub><mi>X</mi> <mi>n</mi></msub><mo>⟶</mo><msub><mi>X</mi> <mrow><mi>n</mi><mo>−</mo><mn>1</mn></mrow></msub><mo>⟶</mo><mi>⋯</mi><mo>⟶</mo><mi>X</mi></mrow><annotation encoding="application/x-tex"> \cdots \longrightarrow X_{n+1} \longrightarrow X_n \longrightarrow X_{n-1} \longrightarrow \cdots \longrightarrow X </annotation></semantics></math></div> <p>An <strong>ascending filtration</strong> or <strong>increasing filtration</strong> of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><mi>X</mi></mrow><annotation encoding="application/x-tex">X</annotation></semantics></math> is of the form</p> <div class="maruku-equation"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block" class="maruku-mathml"><semantics><mrow><mi>X</mi><mo>⟶</mo><mi>⋯</mi><mo>⟶</mo><msub><mi>X</mi> <mrow><mi>n</mi><mo>−</mo><mn>1</mn></mrow></msub><mo>⟶</mo><msub><mi>X</mi> <mi>n</mi></msub><mo>⟶</mo><msub><mi>X</mi> <mrow><mi>n</mi><mo>+</mo><mn>1</mn></mrow></msub><mo>⟶</mo><mi>⋯</mi></mrow><annotation encoding="application/x-tex"> X \longrightarrow \cdots \longrightarrow X_{n-1} \longrightarrow X_n \longrightarrow X_{n+1} \longrightarrow \cdots </annotation></semantics></math></div></div> <p>(In more generality, it is also possible to index using any <a class="existingWikiWord" href="/nlab/show/ordered+group">ordered abelian group</a>.)</p> <div class="num_defn" id="ExhaustiveHausdorffAndCompleteFiltrations"> <h6 id="definition_3">Definition</h6> <p>A decreasing filtration <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><mo stretchy="false">{</mo><msub><mi>X</mi> <mi>s</mi></msub><msub><mo stretchy="false">}</mo> <mi>s</mi></msub></mrow><annotation encoding="application/x-tex">\{X_s\}_s</annotation></semantics></math> of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><mi>X</mi></mrow><annotation encoding="application/x-tex">X</annotation></semantics></math> (def. <a class="maruku-ref" href="#Filtration"></a>) is called</p> <ul> <li> <p><strong>exhaustive</strong> if <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><msub><munder><mi>lim</mi><mo>⟶</mo></munder> <mi>s</mi></msub><msub><mi>X</mi> <mi>s</mi></msub><mo>≃</mo><mi>X</mi></mrow><annotation encoding="application/x-tex">\underset{\longrightarrow}{\lim}_s X_s \simeq X</annotation></semantics></math> (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><mi>X</mi></mrow><annotation encoding="application/x-tex">X</annotation></semantics></math> is the <a class="existingWikiWord" href="/nlab/show/colimit">colimit</a> of the filter stages)</p> </li> <li> <p><strong>Hausdorff</strong> if <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><msub><munder><mi>lim</mi><mo>⟵</mo></munder> <mi>s</mi></msub><msub><mi>X</mi> <mi>s</mi></msub><mo>≃</mo><mn>0</mn></mrow><annotation encoding="application/x-tex">\underset{\longleftarrow}{\lim}_s X_s \simeq 0</annotation></semantics></math> (the <a class="existingWikiWord" href="/nlab/show/limit">limit</a> of the the filter stages is <a class="existingWikiWord" href="/nlab/show/initial+object">initial</a> (<a class="existingWikiWord" href="/nlab/show/zero+object">zero</a> in the case of an <a class="existingWikiWord" href="/nlab/show/abelian+category">abelian category</a>))</p> </li> </ul> <p>and for a filtration of <a class="existingWikiWord" href="/nlab/show/abelian+groups">abelian groups</a>:</p> <ul> <li><strong>complete</strong> if <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><msubsup><munder><mi>lim</mi><mo>⟵</mo></munder> <mi>s</mi> <mn>1</mn></msubsup><msub><mi>X</mi> <mi>n</mi></msub><mo>=</mo><mn>0</mn></mrow><annotation encoding="application/x-tex">\underset{\longleftarrow}{\lim}^1_s X_n = 0</annotation></semantics></math> (also the first <a class="existingWikiWord" href="/nlab/show/derived+functor">derived</a> <a class="existingWikiWord" href="/nlab/show/limit">limit</a> (<a class="existingWikiWord" href="/nlab/show/lim%5E1">lim^1</a>) vanishes)</li> </ul> </div> <p>(<a href="#Boardman99">Boardman 99, def. 2.1</a>, see also <a href="#Rognes12">Rognes 12, section 2.1</a>)</p> <div class="num_remark"> <h6 id="remark">Remark</h6> <p>Beware that for decreasing filtrations (def. <a class="maruku-ref" href="#Filtration"></a>) often exhaustiveness (def. <a class="maruku-ref" href="#ExhaustiveHausdorffAndCompleteFiltrations"></a>) is understood by default, and often it is even assumed by default that <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><msub><mi>X</mi> <mi>s</mi></msub><mo>=</mo><mi>X</mi></mrow><annotation encoding="application/x-tex">X_s = X</annotation></semantics></math> for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><mi>s</mi><mo>≤</mo><mn>0</mn></mrow><annotation encoding="application/x-tex">s \leq 0</annotation></semantics></math>.</p> </div> <h2 id="properties">Properties</h2> <div class="num_prop" id="ReconstructingAbelianGroupFromExhaustiveCompleteHausdorffFiltering"> <h6 id="proposition">Proposition</h6> <p>If a decreasing filtration (def. <a class="maruku-ref" href="#Filtration"></a>) of <a class="existingWikiWord" href="/nlab/show/abelian+groups">abelian</a> <a class="existingWikiWord" href="/nlab/show/subgroups">subgroups</a></p> <div class="maruku-equation"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block" class="maruku-mathml"><semantics><mrow><mi>⋯</mi><mo>↪</mo><msub><mi>A</mi> <mrow><mi>n</mi><mo>+</mo><mn>1</mn></mrow></msub><mo>↪</mo><msub><mi>A</mi> <mi>n</mi></msub><mo>↪</mo><msub><mi>A</mi> <mrow><mi>n</mi><mo>−</mo><mn>1</mn></mrow></msub><mo>↪</mo><mi>⋯</mi><mo>↪</mo><mi>A</mi></mrow><annotation encoding="application/x-tex"> \cdots \hookrightarrow A_{n+1} \hookrightarrow A_n \hookrightarrow A_{n-1} \hookrightarrow \cdots \hookrightarrow A </annotation></semantics></math></div> <p>is exhaustive and complete Hausdorff (def. <a class="maruku-ref" href="#ExhaustiveHausdorffAndCompleteFiltrations"></a>) then <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" class="maruku-mathml"><semantics><mrow><mi>A</mi></mrow><annotation encoding="application/x-tex">A</annotation></semantics></math> may be reobtained from the subquotients of the filtering as the <a class="existingWikiWord" href="/nlab/show/limit">limit</a>/<a class="existingWikiWord" href="/nlab/show/colimit">colimit</a></p> <div class="maruku-equation"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block" class="maruku-mathml"><semantics><mrow><mrow><mtable displaystyle="true" columnalign="right left right left right left right left right left" columnspacing="0em"><mtr><mtd><mi>A</mi></mtd> <mtd><mo>≃</mo><msub><munder><mi>lim</mi><mo>⟵</mo></munder> <mi>s</mi></msub><mo stretchy="false">(</mo><mi>A</mi><mo stretchy="false">/</mo><msub><mi>A</mi> <mi>s</mi></msub><mo stretchy="false">)</mo></mtd></mtr> <mtr><mtd></mtd> <mtd><mo>≃</mo><msub><munder><mi>lim</mi><mo>⟵</mo></munder> <mi>s</mi></msub><mo stretchy="false">(</mo><msub><munder><mi>lim</mi><mo>⟶</mo></munder> <mi>t</mi></msub><mo stretchy="false">(</mo><msub><mi>A</mi> <mi>t</mi></msub><mo stretchy="false">/</mo><msub><mi>A</mi> <mi>s</mi></msub><mo stretchy="false">)</mo><mo stretchy="false">)</mo></mtd></mtr></mtable></mrow><mspace width="thinmathspace"></mspace><mo>.</mo></mrow><annotation encoding="application/x-tex"> \begin{aligned} A & \simeq \underset{\longleftarrow}{\lim}_s (A/A_s) \\ & \simeq \underset{\longleftarrow}{\lim}_s (\underset{\longrightarrow}{\lim}_t ( A_t / A_s )) \end{aligned} \,. </annotation></semantics></math></div></div> <p>(<a href="#Boardman99">Boardman 99, prop. 2.5</a>)</p> <h2 id="related_concepts">Related concepts</h2> <div> <p><strong><a class="existingWikiWord" href="/nlab/show/filtered+objects">filtered objects</a></strong></p> <ul> <li> <p><a class="existingWikiWord" href="/nlab/show/filtered+topological+space">filtered topological space</a></p> <ul> <li><a class="existingWikiWord" href="/nlab/show/spectral+sequence+of+a+tower+of+fibrations">spectral sequence of a tower of fibrations</a></li> </ul> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/filtered+vector+space">filtered vector space</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/filtered+chain+complex">filtered chain complex</a></p> <ul> <li> <p><a class="existingWikiWord" href="/nlab/show/spectral+sequence+of+a+filtered+complex">spectral sequence of a filtered complex</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/Hodge+filtration">Hodge filtration</a></p> </li> </ul> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/filtered+ring">filtered ring</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/filtered+object+in+an+%28%E2%88%9E%2C1%29-category">filtered object in an (∞,1)-category</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/filtered+stable+homotopy+type">filtered stable homotopy type</a></p> <ul> <li><a class="existingWikiWord" href="/nlab/show/spectral+sequence+of+a+filtered+stable+homotopy+type">spectral sequence of a filtered stable homotopy type</a></li> </ul> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/filtered+probability+space">filtered probability space</a></p> </li> </ul> <p><strong><a class="existingWikiWord" href="/nlab/show/associated+graded+objects">associated graded objects</a></strong></p> <ul> <li> <p><a class="existingWikiWord" href="/nlab/show/associated+graded+vector+space">associated graded vector space</a></p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/associated+graded+ring">associated graded ring</a></p> </li> </ul> </div> <h2 id="references">References</h2> <ul> <li id="Boardman99"> <p><a class="existingWikiWord" href="/nlab/show/Michael+Boardman">Michael Boardman</a>, section I.2 of <em>Conditionally convergent spectral sequences</em>, 1999 (<a href="http://www.uio.no/studier/emner/matnat/math/MAT9580/v12/undervisningsmateriale/boardman-conditionally-1999.pdf">pdf</a>)</p> </li> <li id="Rognes12"> <p><a class="existingWikiWord" href="/nlab/show/John+Rognes">John Rognes</a>, <em>The Adams spectral sequence</em> (following <a href="Adams+spectral+sequence#Bruner09">Bruner 09</a>), 2012 (<a href="http://folk.uio.no/rognes/papers/notes.050612.pdf">pdf</a>)</p> </li> <li> <p><a class="existingWikiWord" href="/nlab/show/Paolo+Perrone">Paolo Perrone</a>, <em>Starting Category Theory</em>, World Scientific, 2024, Section 3.2.7. (<a href="https://www.worldscientific.com/worldscibooks/10.1142/13670">website</a>)</p> </li> </ul> </body></html> </div> <div class="revisedby"> <p> Last revised on July 21, 2024 at 18:05:19. 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