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This entry describes special methods for the construction of fibrant resolutions in the classical model structure on simplicial sets.
In as far as we may think of simplicial sets having some suitable properties as a simplicial model for weak ∞-categories (for instance for quasi-categories) and of a simplicial set that has the property of being a Kan complex as an ∞-groupoid, Kan fibrant replacement of simplicial sets is the operation of $\infty$-groupoidification in that it sends an $\infty$-category to the $\infty$-groupoid obtained by freely inverting all its non-invertible k-morphisms.
Technically, the terminology comes from the fact that with respect to the standard model structure on simplicial sets the Kan complexes are precisely the fibrant objects.
There are several methods to actually construct the Kan fibrant replacement. One convenient one, called the $Ex$ functor – described below – constructs an $\infty$-groupoid from (the nerve of) an $\infty$-category $C$ by
taking its 2-morphisms to be cospan-of-cospan multispans in $C$:
taking its 3-morphisms to be cospan-of-cospan-of-cospan multispans in $C$:
etc.
For $\Delta^k$ the simplicial $k$-simplex let $sd \Delta^k$ be its barycentric subdivision : this is the simplicial set that is the nerve of the poset of non-degenerate sub-simplicies in $\Delta^k$.
Notice that this simplicial set $sd \Delta^k$ encodes the shape of a $k$-fold cospan of cospans.
For instance,
is the ordinary cospan.
These multi-cospan simplicial sets define a functor $Ex : SSet \to SSet$ by setting
So this functor reads in a simplicial set $X$ and spits out the simplicial set whose 1-cells are cospans in $X$.
This comes with a natural map
Iterating this construction indefinitely defines a simplicial set $Ex^\infty X$ to be the colimit over
The 1–cells in $Ex^\infty X$ are zig-zags in $X$.
Then
Proposition
$Ex^\infty X$ is a Kan complex;
$X \to Ex^\infty X$ is a natural weak equivalence, in fact, an acyclic cofibration, even more strongly, it is a strong anodyne extension, i.e., a transfinite composition of cobase changes of horn inclusions (without retracts involved).
$Ex^\infty$ preserves all 5 classes of maps: weak equivalences, (acyclic) cofibrations, and (acyclic) fibrations, as well as strong anodyne extensions and simplicial homotopy equivalences.
$Ex^\infty$ preserves finite limits and filtered colimits.
$Ex^\infty(f)$ is a simplicial homotopy equivalence if and only if $f$ is a simplicial weak equivalence.
For now, see here:
An original reference is
A standard textbook reference is
174, Birkhäuser, 1999.
A summary of the basics is in
Discussion in the context of simplicial presheaves is section 3 of
See also
Last revised on May 1, 2023 at 17:51:03. See the history of this page for a list of all contributions to it.