This post is meant to set up all the necessary ideas to define the category of exit paths.
Now we get into new territory.
Definition: The nerve of a category C is the collection N(C) = \{N(C)_n = Fun([n],C)\}_{n\geqslant 0}, where [n] is considered as a category with objects 0,\dots,n and a single morphism in \Hom_{[n]}(s,t) iff s\leqslant t.
Note that the nerve of C is a simplicial set, as it is a functor from \Delta^{op}\to Fun(\Delta,C). Moreover, the pieces N(C)_0 are the objects of C and N(C)_1 are the morphisms of C, so all the information about C is contained in its nerve. There is more in the higher pieces N(C)_n, so the nerve (and simplicial sets in general) may be viewed as a generalization of a category.
Let \text{sSet} be the category of simplicial sets. We may consider \Delta^n = \Hom_\Delta(-,[n]) as a contravariant functor \Delta\to \text{Set}, so it is an object of \text{sSet}.
Definition: Fix n\geqslant 0 and choose 0\leqslant i\leqslant n. Then the ith n-horn of a simplicial set is the functor \Lambda^n_i\subset \Delta^n generated by all the faces \Delta^n(d_j), for j\neq i.
We purposefully do not describe what "\subset" or "generated by" mean for functors, hoping that intuition fills in the gaps. In some sense the horn feels like a partially defined functor (though it is a true simplicial set), well described by diagrams, for instance with n=2 and i=1 we have
Definition: A simplicial set S is a Kan complex whenever every map f:\Lambda^n_i\to S factors through \Delta^n. That is, when there exists a
The map \iota is the inclusion. Moreover, S is an \infty-category, or quasi-category, if the extending map f' is unique.
Example: Some basic examples of \infty-categories, for X a topological space, are
Definition: A morphism p\in \Hom_{\text{sSet}}(S,T) is a Kan fibration if for every commutative diagram (of solid arrows)
the dotted arrow exists, making the new diagram commute.
Definition: Let C,D,A be categories with functors F:C\to D and G:C\to A.
The setting for this section is constructible sheaves over a topological space X. We begin with a slightly more technical definition of a stratification.
Definition: Let (A,\leqslant) be a partially ordered set with the upset topology. That is, if x\in U is open and x\leqslant y, then y\in A. An A-stratification of X is a continuous function f:X\to A.
We now begin with a Treumann's definition of an exit path, combined with Lurie's stratified setting.
Definition: An exit path in an A-stratified space X is a continuous map \gamma:[0,1]\to X for which there exists a pair of chains a_1\leqslant \cdots \leqslant a_n in A and 0=t_0\leqslant \cdots \leqslant t_n=1 in [0,1] such that f(\gamma(t))=a_i whenever t\in (t_{i-1},t_i].
This really is a path, and so gives good intuition for what is happening. Recall that the geometric realization of the functor \Delta^n is |\Delta^n| = \{(t_0,\dots,t_n)\in \R^{n+1}\ :\ t_0+\cdots+t_n=1\}. Oserving that [0,1]\cong|\Delta^1|, Lurie's definition of an exit path is more general by instead considering maps from |\Delta^n|.
Definition: The category of exit paths in an A-stratified space X is the simplicial subset Sing^A(X)\subset Sing(X) consisting of those simplices \gamma:|\Delta^n|\to X for which there exists a chain a_0\leqslant \cdots \leqslant a_n in A such that f(\gamma(t_0,\dots,t_i,0,\dots,0))=a_i for t_i\neq 0.
Example: As with all new ideas, it is useful to have an example. Consider the space X=\Ran^{\leqslant 2}(M)\times \R_{\geqslant 0} of a closed manifold M (see post "A constructible sheaf over the Ran space" 2017-06-24 for more). With the poset (A,\leqslant) being (a\leqslant b\leqslant c) and stratifying map
\begin{array}{r c l} f\ :\ X & \to & A, \\ (P,t) & \mapsto & \begin{cases} a & \text{ if } P\in \Ran^1(M), \\ b & \text{ if } P\in \Ran^2(M), t\leqslant d(P_1,P_2), \\ c & \text{ else,} \end{cases} \end{array}
we can make a continuous map \gamma:\Delta^3\to X by
\begin{array}{r c l} (1,0,0) & \mapsto & (P\in \Ran^1(M),0), \\ (t_0,t_1\neq 0,0) & \mapsto & (P\in \Ran^2(M), d(P_1,P_2)), \\ (t_0,t_1,t_2\neq 0) & \mapsto & (P\in \Ran^2(M), t>d(P_1,P_2)). \end{array}
Then f(\gamma(t_0\neq 0,0,0))=a, and f(\gamma(t_0,t_1\neq 0,0))=b, and f(\gamma(t_0,t_1,t_2\neq 0))=c, as desired. The embedding of such a simplex \gamma is described by the diagram below.
References: Lurie (Higher algebra, Appendix A), Lurie (What is... an \infty-category?), Groth (A short course on \infty-categories, Section 1), Joyal (Quasi-categories and Kan complexes), Goerss and Jardine (Simplicial homotopy theory, Chapter 1), Treumann (Exit paths and constructible stacks)
Preliminaries
Let X be a topological space and C a category. Recall the following terms:- Δ: The category whose objects are finite ordered sets [n]=(1,…,n) and whose morphisms are non-decreasing maps. It has several full subcategories, including
- Δs, comprising the same objects of Δ and only injective morphisms, and
- Δ⩽, comprising only the objects [0],\dots,[n] with the same morphisms.
- equalizer: An object E and a universal map e:E\to X, with respect to two maps f,g:X\to Y. It is universal in the sense that all maps into X whose compositions with f,g are equal factor through e. Equalizers and coequalizers are described by the diagram below, with universality given by existence of the dotted maps.
- fibered product or pullback: The universal object X\times_Z Y with maps to X and Y, with respect to maps X\to Z and Y\to Z.
- fully faithful: A functor F whose morphism restriction \Hom(X,Y)\to \Hom(F(X),F(Y)) is surjective (full) and injective (faithful).
- locally constant sheaf: A sheaf \mathcal F over X for which every x\in X has a neighborhood U such that \mathcal F|_U is a constant sheaf. For example, constructible sheaves are locally constant on every stratum.
- simplicial object: A contravariant functor from \Delta to any other category. When the target category is \text{Set}, it is called a simplicial set. They may also be viewed as a collection S = \{S_n\}_{\geqslant 0} for S_n=S([n]) the value of the functor on each [n]. Simplicial sets come with two natural maps:
- face maps d_i:S_n\to S_{n-1} induced by the map [n-1]\to [n] which skips the ith piece, and
- degeneracy maps s_i:S_n\to S_{n+1} induced by the map [n+1]\to[n] which repeats the ith piece.
- stratification: A property of a cover \{U_i\} of X for which consecutive differences U_{i+1}\setminus U_i have ``nicer" properties than all of X. For example, E_i\to U_{i+1}\setminus U_i is a rank i vector bundle, but there is no vector bundle E\to X that restricts to every E_i.
Now we get into new territory.
Definition: The nerve of a category C is the collection N(C) = \{N(C)_n = Fun([n],C)\}_{n\geqslant 0}, where [n] is considered as a category with objects 0,\dots,n and a single morphism in \Hom_{[n]}(s,t) iff s\leqslant t.
Note that the nerve of C is a simplicial set, as it is a functor from \Delta^{op}\to Fun(\Delta,C). Moreover, the pieces N(C)_0 are the objects of C and N(C)_1 are the morphisms of C, so all the information about C is contained in its nerve. There is more in the higher pieces N(C)_n, so the nerve (and simplicial sets in general) may be viewed as a generalization of a category.
Kan structures
Let \text{sSet} be the category of simplicial sets. We may consider \Delta^n = \Hom_\Delta(-,[n]) as a contravariant functor \Delta\to \text{Set}, so it is an object of \text{sSet}.
Definition: Fix n\geqslant 0 and choose 0\leqslant i\leqslant n. Then the ith n-horn of a simplicial set is the functor \Lambda^n_i\subset \Delta^n generated by all the faces \Delta^n(d_j), for j\neq i.
We purposefully do not describe what "\subset" or "generated by" mean for functors, hoping that intuition fills in the gaps. In some sense the horn feels like a partially defined functor (though it is a true simplicial set), well described by diagrams, for instance with n=2 and i=1 we have
Definition: A simplicial set S is a Kan complex whenever every map f:\Lambda^n_i\to S factors through \Delta^n. That is, when there exists a
The map \iota is the inclusion. Moreover, S is an \infty-category, or quasi-category, if the extending map f' is unique.
Example: Some basic examples of \infty-categories, for X a topological space, are
- Sing(X), made up of pieces Sing(X)_n = \Hom(\Delta^n,X), and
- LCS(X), the category of locally constant sheaves over X. Here LCS(X)_n over an object A, whose objects are B\to A and morphisms are the appropriate commutative diagrams
Definition: A morphism p\in \Hom_{\text{sSet}}(S,T) is a Kan fibration if for every commutative diagram (of solid arrows)
the dotted arrow exists, making the new diagram commute.
Definition: Let C,D,A be categories with functors F:C\to D and G:C\to A.
- The left Kan extension of F along G is a functor A\xrightarrow L D and a universal natural transformation F\stackrel \lambda \rightsquigarrow L\circ G.
- The right Kan extension of F along G is a functor A\xrightarrow R D and a universal natural transformation R\circ G \stackrel \rho\rightsquigarrow F.
Exit paths
The setting for this section is constructible sheaves over a topological space X. We begin with a slightly more technical definition of a stratification.
Definition: Let (A,\leqslant) be a partially ordered set with the upset topology. That is, if x\in U is open and x\leqslant y, then y\in A. An A-stratification of X is a continuous function f:X\to A.
We now begin with a Treumann's definition of an exit path, combined with Lurie's stratified setting.
Definition: An exit path in an A-stratified space X is a continuous map \gamma:[0,1]\to X for which there exists a pair of chains a_1\leqslant \cdots \leqslant a_n in A and 0=t_0\leqslant \cdots \leqslant t_n=1 in [0,1] such that f(\gamma(t))=a_i whenever t\in (t_{i-1},t_i].
This really is a path, and so gives good intuition for what is happening. Recall that the geometric realization of the functor \Delta^n is |\Delta^n| = \{(t_0,\dots,t_n)\in \R^{n+1}\ :\ t_0+\cdots+t_n=1\}. Oserving that [0,1]\cong|\Delta^1|, Lurie's definition of an exit path is more general by instead considering maps from |\Delta^n|.
Definition: The category of exit paths in an A-stratified space X is the simplicial subset Sing^A(X)\subset Sing(X) consisting of those simplices \gamma:|\Delta^n|\to X for which there exists a chain a_0\leqslant \cdots \leqslant a_n in A such that f(\gamma(t_0,\dots,t_i,0,\dots,0))=a_i for t_i\neq 0.
Example: As with all new ideas, it is useful to have an example. Consider the space X=\Ran^{\leqslant 2}(M)\times \R_{\geqslant 0} of a closed manifold M (see post "A constructible sheaf over the Ran space" 2017-06-24 for more). With the poset (A,\leqslant) being (a\leqslant b\leqslant c) and stratifying map
\begin{array}{r c l} f\ :\ X & \to & A, \\ (P,t) & \mapsto & \begin{cases} a & \text{ if } P\in \Ran^1(M), \\ b & \text{ if } P\in \Ran^2(M), t\leqslant d(P_1,P_2), \\ c & \text{ else,} \end{cases} \end{array}
we can make a continuous map \gamma:\Delta^3\to X by
\begin{array}{r c l} (1,0,0) & \mapsto & (P\in \Ran^1(M),0), \\ (t_0,t_1\neq 0,0) & \mapsto & (P\in \Ran^2(M), d(P_1,P_2)), \\ (t_0,t_1,t_2\neq 0) & \mapsto & (P\in \Ran^2(M), t>d(P_1,P_2)). \end{array}
Then f(\gamma(t_0\neq 0,0,0))=a, and f(\gamma(t_0,t_1\neq 0,0))=b, and f(\gamma(t_0,t_1,t_2\neq 0))=c, as desired. The embedding of such a simplex \gamma is described by the diagram below.
Both the image of (1,0,0) and the 1-simplex from (1,0,0) to (0,1,0) lie in the singularity set of \Ran^{\leqslant 2}(M)\times \R_{\geqslant 0}, which is pairs (P,t) where t=d(P_i,P_j) for some i,j. The idea that the simplex "exits" a stratum is hopefully made clear by this image.
References: Lurie (Higher algebra, Appendix A), Lurie (What is... an \infty-category?), Groth (A short course on \infty-categories, Section 1), Joyal (Quasi-categories and Kan complexes), Goerss and Jardine (Simplicial homotopy theory, Chapter 1), Treumann (Exit paths and constructible stacks)