Not signed in

Want to take part in these discussions? Sign in if you have an account, or apply for one below

• Username
• Password
• Remember me

• Sign in using OpenID
• Remember me

• Forgot your password?
• Apply for membership

2-category 2-category-theory abelian-categories adjoint algebra algebraic algebraic-geometry algebraic-topology analysis analytic-geometry arithmetic arithmetic-geometry book bundles calculus categorical categories category category-theory chern-weil-theory cohesion cohesive-homotopy-type-theory cohomology colimits combinatorics complex complex-geometry computable-mathematics computer-science constructive cosmology deformation-theory descent diagrams differential differential-cohomology differential-equations differential-geometry digraphs duality elliptic-cohomology enriched fibration foundation foundations functional-analysis functor gauge-theory gebra geometric-quantization geometry graph graphs gravity grothendieck group group-theory harmonic-analysis higher higher-algebra higher-category-theory higher-differential-geometry higher-geometry higher-lie-theory higher-topos-theory homological homological-algebra homotopy homotopy-theory homotopy-type-theory index-theory integration integration-theory internal-categories k-theory lie-theory limits linear linear-algebra locale localization logic mathematics measure measure-theory modal modal-logic model model-category-theory monad monads monoidal monoidal-category-theory morphism motives motivic-cohomology nlab noncommutative noncommutative-geometry number-theory of operads operator operator-algebra order-theory pages pasting philosophy physics pro-object probability probability-theory quantization quantum quantum-field quantum-field-theory quantum-mechanics quantum-physics quantum-theory question representation representation-theory riemannian-geometry scheme schemes set set-theory sheaf simplicial space spin-geometry stable-homotopy-theory stack string string-theory superalgebra supergeometry svg symplectic-geometry synthetic-differential-geometry terminology theory topology topos topos-theory tqft type type-theory universal variational-calculus

1. • CommentRowNumber1.
   • CommentAuthorHarry Gindi
   • CommentTimeNov 17th 2010
   • (edited Nov 17th 2010)
   • PermaLink
   Author: Harry Gindi
   Format: MarkdownItexRecall that a marked simplicial set is simplicial set $S$ together with a fixed subset of $S_1$ containing all degeneracies. Recall that the category of "marked simplices" is the category of pairs $([n],W)$ where $W$ is a class of morphisms of $[n]$ containing all identities with no further restrictions. We require simply that morphisms between relative simplices preserve weak equivalences. Call this category $\Delta_m$. We can define a functor $\Delta_m\to sSet^+$ in a somewhat obvious way by sending a marked simplex to the corresponding marked simplicial set. This yields a realization-nerve pair $Psh(\Delta_m)\leftrightarrows sSet^+$. Why is this adjunction not an equivalence of categories? If we take the subcategory of $\Delta_m$ where we require the morphisms to be "strict on weak equivalences", that is, the lluf subcategory where the morphisms are strict on markings, i.e. a morphism $f:([n],W)\to ([m],W')$ of marked simplices is called strict on weak equivalences if $f(W)=f([n])\cap W'$, is there still a chance of realizing $sSet^+$ as a presheaf category on this category? If we further restrict $\Delta_m$ by requiring that morphisms are saturated, ($f^{-1}(W')=W$) are we still screwed?

   Recall that a marked simplicial set is simplicial set $S$ together with a fixed subset of $S_1$ containing all degeneracies.

   Recall that the category of “marked simplices” is the category of pairs $([n],W)$ where $W$ is a class of morphisms of $[n]$ containing all identities with no further restrictions. We require simply that morphisms between relative simplices preserve weak equivalences. Call this category $\Delta_m$.

   We can define a functor $\Delta_m\to sSet^+$ in a somewhat obvious way by sending a marked simplex to the corresponding marked simplicial set. This yields a realization-nerve pair $Psh(\Delta_m)\leftrightarrows sSet^+$. Why is this adjunction not an equivalence of categories?

   If we take the subcategory of $\Delta_m$ where we require the morphisms to be “strict on weak equivalences”, that is, the lluf subcategory where the morphisms are strict on markings, i.e. a morphism $f:([n],W)\to ([m],W')$ of marked simplices is called strict on weak equivalences if $f(W)=f([n])\cap W'$, is there still a chance of realizing $sSet^+$ as a presheaf category on this category?

   If we further restrict $\Delta_m$ by requiring that morphisms are saturated, ($f^{-1}(W')=W$) are we still screwed?

2. • CommentRowNumber2.
   • CommentAuthorUrs
   • CommentTimeNov 17th 2010
   • (edited Nov 17th 2010)
   • PermaLink
   Author: Urs
   Format: MarkdownItex> Why is this adjunction not an equivalence of categories? Because $sSet^+$ is not a presheaf topos, but a quasi-topos: the category of separated presheaves on a category $\Delta^+$ obtained from $\Delta$ by adding a single object $[1/2]$ and factoring $[1] \to [0]$ through this object. Currently this is described at [[model structure for Cartesian fibrations]] in the section <a href="http://ncatlab.org/nlab/show/model+structure+for+Cartesian+fibrations#CartesianClosure">Cartesian closure</a>. But I'll move the discussion to a dedicated section now.

   Why is this adjunction not an equivalence of categories?

   Because $sSet^+$ is not a presheaf topos, but a quasi-topos: the category of separated presheaves on a category $\Delta^+$ obtained from $\Delta$ by adding a single object $[1/2]$ and factoring $[1] \to [0]$ through this object.

   Currently this is described at model structure for Cartesian fibrations in the section Cartesian closure. But I’ll move the discussion to a dedicated section now.

3. • CommentRowNumber3.
   • CommentAuthorHarry Gindi
   • CommentTimeNov 17th 2010
   • PermaLink
   Author: Harry Gindi
   Format: MarkdownItexDear Urs, are you familiar with a proof that $sSet^+$ is not a presheaf topos?

   Dear Urs, are you familiar with a proof that $sSet^+$ is not a presheaf topos?

4. • CommentRowNumber4.
   • CommentAuthorUrs
   • CommentTimeNov 17th 2010
   • PermaLink
   Author: Urs
   Format: MarkdownItex> Dear Urs, are you familiar with a proof that $sSet^+$ is not a presheaf topos? It is not a [[balanced category]]: the canonical morphisms $X^\flat \to X^{#}$ are mono and epi, but not iso.

   Dear Urs, are you familiar with a proof that $sSet^+$ is not a presheaf topos?

   It is not a balanced category: the canonical morphisms $X^\flat \to X^{#}$ are mono and epi, but not iso.

5. • CommentRowNumber5.
   • CommentAuthorHarry Gindi
   • CommentTimeNov 17th 2010
   • (edited Nov 17th 2010)
   • PermaLink
   Author: Harry Gindi
   Format: MarkdownItexmhm. Are you familiar with any attempts to find two quillen-biequivalent (to the cartesian and cocartesian model structures) model structures on a presheaf topos that's ever-so-slightly bigger than $sSet^+$?

   mhm. Are you familiar with any attempts to find two quillen-biequivalent (to the cartesian and cocartesian model structures) model structures on a presheaf topos that’s ever-so-slightly bigger than $sSet^+$?

6. • CommentRowNumber6.
   • CommentAuthorUrs
   • CommentTimeNov 17th 2010
   • (edited Nov 17th 2010)
   • PermaLink
   Author: Urs
   Format: MarkdownItex> Are you familiar with any attempts Haven't heard and haven't thought about it, no. What do you mean by "biequivalent"? Do you want to consider $sSet_{Joyal}$-enriched model categories ("$\infty$-bicategories")? By the way, what did you mean in your very first message by > We require simply that morphisms between relative simplices preserve weak equivalences. Didn't you say in the sentence before that this is supposed to be a category whose objects are all simplices with markings? Where is the relative simplices and what notion of weak equivalence do you have in mind?

   Are you familiar with any attempts

   Haven’t heard and haven’t thought about it, no.

   What do you mean by “biequivalent”? Do you want to consider $sSet_{Joyal}$-enriched model categories (“$\infty$-bicategories”)?

   By the way, what did you mean in your very first message by

   We require simply that morphisms between relative simplices preserve weak equivalences.

   Didn’t you say in the sentence before that this is supposed to be a category whose objects are all simplices with markings? Where is the relative simplices and what notion of weak equivalence do you have in mind?

7. • CommentRowNumber7.
   • CommentAuthorHarry Gindi
   • CommentTimeNov 17th 2010
   • PermaLink
   Author: Harry Gindi
   Format: MarkdownItexBy weak equivalences, I mean the same thing as markings, sorry for the confusion. By biequivalent, I mean that we have left and right quillen equivalences, that is, "bidirectionally quillen equivalent".

   By weak equivalences, I mean the same thing as markings, sorry for the confusion.

   By biequivalent, I mean that we have left and right quillen equivalences, that is, “bidirectionally quillen equivalent”.