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1. • CommentRowNumber1.
   • CommentAuthorDmitri Pavlov
   • CommentTimeJul 18th 2015
   • (edited Jul 18th 2015)
   • PermaLink
   Author: Dmitri Pavlov
   Format: MarkdownItexI created the article [[multisimplicial set]]. It seems that I triggered some bug in Instiki that prevents it from rendering displayed formulas correctly. The article compiles just fine in TeX, so this is clearly a bug. In fact, Instiki seems to insert some new text: > definedasthechunk29161400wikichunklinkchunkofthetautologicalfunctor All this “chunk” and “wikichunklink” stuff is clearly a bug.

   I created the article multisimplicial set.

   It seems that I triggered some bug in Instiki that prevents it from rendering displayed formulas correctly.

   The article compiles just fine in TeX, so this is clearly a bug.

   In fact, Instiki seems to insert some new text:

   definedasthechunk29161400wikichunklinkchunkofthetautologicalfunctor

   All this “chunk” and “wikichunklink” stuff is clearly a bug.

2. • CommentRowNumber2.
   • CommentAuthorRodMcGuire
   • CommentTimeJul 19th 2015
   • PermaLink
   Author: RodMcGuire
   Format: MarkdownItexhmm, what you typed now generates > An important operation on multisimplicial sets is the _exterior product_ $$Fun((\Delta^m)^{op},Set)\times Fun((\Delta^n)^{op},Set)\to Fun((\Delta^{m+n})^{op},Set)$$ defined as the [[left Kan extension]] of the tautological functor $$\Delta^m\times\Delta^n\to\Delta^{m+n}.$$ The nLab/nForum likes double dollar expressions to be on their own line, giving the following. (I've updated the nLab page) > An important operation on multisimplicial sets is the _exterior product_ > $$Fun((\Delta^m)^{op},Set)\times Fun((\Delta^n)^{op},Set)\to Fun((\Delta^{m+n})^{op},Set)$$ > defined as the [[left Kan extension]] of the tautological functor > $$\Delta^m\times\Delta^n\to\Delta^{m+n}.$$

   hmm, what you typed now generates

   An important operation on multisimplicial sets is the exterior product $$Fun((\Delta^m)^{op},Set)\times Fun((\Delta^n)^{op},Set)\to Fun((\Delta^{m+n})^{op},Set)$$defined as the [[left Kan extension]] of the tautological functor$$\Delta^m\times\Delta^n\to\Delta^{m+n}.$$

   The nLab/nForum likes double dollar expressions to be on their own line, giving the following. (I’ve updated the nLab page)

   An important operation on multisimplicial sets is the exterior product

   $$Fun((\Delta^m)^{op},Set)\times Fun((\Delta^n)^{op},Set)\to Fun((\Delta^{m+n})^{op},Set)$$

   defined as the left Kan extension of the tautological functor

   $$\Delta^m\times\Delta^n\to\Delta^{m+n}.$$

3. • CommentRowNumber3.
   • CommentAuthorDmitri Pavlov
   • CommentTimeJul 19th 2015
   • PermaLink
   Author: Dmitri Pavlov
   Format: MarkdownItexSo one can work around the bug via subtle changes in whitespace. Hardly a permanent solution, though. Is Instiki still being maintained? How does one report a bug in it?

   So one can work around the bug via subtle changes in whitespace. Hardly a permanent solution, though. Is Instiki still being maintained? How does one report a bug in it?

4. • CommentRowNumber4.
   • CommentAuthorDmitri Pavlov
   • CommentTimeDec 4th 2021
   • PermaLink
   Author: Dmitri Pavlov
   Format: MarkdownItexAdded a new section: ## Relation to simplicial sets The category of $n$-fold multisimplicial sets can be equipped with a [[model structure]] that turns it into a [[model category]] that is Quillen equivalent to the standard [[Kan–Quillen model structure]] on [[simplicial sets]]. Cofibrations of multisimplicial sets are precisely [[monomorphisms]]. Weak equivalences are induced from [[simplicial sets]] by the diagonal functor. The corresponding [[Quillen adjunction]] is constructed as as a [[nerve-realization adjunction]] for the functor $$\Delta^n\to sSet$$ that sends a multisimplex $(m_1,\ldots,m_n)$ to $\Delta^{m_1}\times\cdots\times\Delta^{m_n}$. The left adjoint is given by a [[left Kan extension]]. The right adjoint sends a [[simplicial set]] $X$ to its multisimplicial nerve, which sends a multisimplex $(m_1,\ldots,m_n)$ to $sSet(\Delta^{m_1}\times\cdots\times\Delta^{m_n},X)$. To see that this [[Quillen adjunction]] is a [[Quillen equivalence]], one can either argue directly, by computing the derived [[unit]] and derived [[counit]] and showing that they are [[weak equivalences]], or, much more elegantly, invoke a theorem by [[Grothendieck]] and [[Maltsiniotis]]. The latter theorem (\cite{Maltsiniotis}, Proposition 1.6.8) states that a totally aspherical [[small category]] $C$ with a separating aspherical interval is a [[strict test category]], and, therefore, the category of presheaves of sets on $C$ admits a [[model structure]] whose cofibrations are monomorphisms and weak equivalences are induced by the [[category of elements]] functor from the [[Thomason model structure]] on the category of [[small categories]]. Furthermore, the resulting model category of presheaves of sets on $C$ is a cartesian [[combinatorial model category]] that is Quillen equivalent to the [[Kan–Quillen model structure]] on the category of [[simplicial sets]]. We now verify the conditions of the theorem: * The category of multisimplices is nonempty. * The categorical product of two representable multisimplicial sets is acyclic, i.e., weakly equivalent to the terminal object. * The multisimplex $([1],[1],\ldots,[1])$ together with the two canonical inclusions of $([0],\ldots,[0])$ is a separating aspherical interval. <a href="https://ncatlab.org/nlab/revision/diff/multisimplicial+set/4">diff</a>, <a href="https://ncatlab.org/nlab/revision/multisimplicial+set/4">v4</a>, <a href="https://ncatlab.org/nlab/show/multisimplicial+set">current</a>

   Added a new section:

   Relation to simplicial sets

   The category of $n$-fold multisimplicial sets can be equipped with a model structure that turns it into a model category that is Quillen equivalent to the standard Kan–Quillen model structure on simplicial sets.

   Cofibrations of multisimplicial sets are precisely monomorphisms. Weak equivalences are induced from simplicial sets by the diagonal functor.

   The corresponding Quillen adjunction is constructed as as a nerve-realization adjunction for the functor

   $$\Delta^n\to sSet$$

   that sends a multisimplex $(m_1,\ldots,m_n)$ to $\Delta^{m_1}\times\cdots\times\Delta^{m_n}$.

   The left adjoint is given by a left Kan extension. The right adjoint sends a simplicial set $X$ to its multisimplicial nerve, which sends a multisimplex $(m_1,\ldots,m_n)$ to $sSet(\Delta^{m_1}\times\cdots\times\Delta^{m_n},X)$.

   To see that this Quillen adjunction is a Quillen equivalence, one can either argue directly, by computing the derived unit and derived counit and showing that they are weak equivalences, or, much more elegantly, invoke a theorem by Grothendieck and Maltsiniotis.

   The latter theorem (\cite{Maltsiniotis}, Proposition 1.6.8) states that a totally aspherical small category $C$ with a separating aspherical interval is a strict test category, and, therefore, the category of presheaves of sets on $C$ admits a model structure whose cofibrations are monomorphisms and weak equivalences are induced by the category of elements functor from the Thomason model structure on the category of small categories. Furthermore, the resulting model category of presheaves of sets on $C$ is a cartesian combinatorial model category that is Quillen equivalent to the Kan–Quillen model structure on the category of simplicial sets.

   We now verify the conditions of the theorem:

   • The category of multisimplices is nonempty.

   • The categorical product of two representable multisimplicial sets is acyclic, i.e., weakly equivalent to the terminal object.

   • The multisimplex $([1],[1],\ldots,[1])$ together with the two canonical inclusions of $([0],\ldots,[0])$ is a separating aspherical interval.

   diff, v4, current