Acyclic categories related to structures in algebraic topology - MathOverflow

Acyclic categories related to structures in algebraic topology

2012-07-26T10:49:30Z

An acyclic category (also called loopfree category or scwol (small category without loops)) is a small category where only identity morphisms have inverses, and any morphism from an object to itself is the identity.

Every poset P can be regarded as an acyclic category by identifying the set of objects with the elements of P. And saying there is a morphism from x to y, if and only if $x\le y$. Hence we can regard acyclic categories as a generalization of posets.

Posets play a crucial role in combinatorial algebraic topology, e.g. in form of intersection lattices related to hyperplane arrangements, or face posets of simplicial complexes. I'm looking for examples where we get acyclic categories as generalized posets encoding information about some structure (e.g.: Is there some kind of generalized hyperplane arrangement which yields an intersection category).

I'm aware of the face category of a polytopal complex and the salvetti categori of a complex, toric arrangement, but I'd be glad if anyone knew some further applications for acyclic categories in the field of algebraic topology

Answer by Patricia Hersh for Acyclic categories related to structures in algebraic topology

2012-07-26T22:43:11Z

One thing that has come up a little bit in my work and I believe also e.g. in work of Dmitry Feichtner-Kozlov is such an extension of the widely used fact (popularized by Gian-Carlo Rota) that the Moebius function $\mu_P(x,y) $ of a poset $P$ may be interpreted as the reduced Euler characteristic of a simplicial complex called the order complex associated to a subposet of $P$, namely associated to the open interval from $x$ to $y$ (i.e. the subposet comprised of those $z\in P$ satisfying $x < z < y$); the faces in the order complex are the chains of comparable elements in the open interval, so e.g. vertices are individual elements.

There can be situations where one may wish to use a deformation of the usual Moebius function in an inclusion-exclusion counting formula and/or where a variant of the usual Moebius function could have more pleasant formulas, including ones where it is possible to interpret this as the reduced Euler characteristic of the nerve of a small category obtained from a poset by letting each cover relation $u\prec v$ have a multiplicity counting the number of different maps from $u$ to $v$. The deformed Moebius function is the inverse function in the incidence algebra to this weighted incidence relation. This hasn't been explored very widely though, at least to my knowledge. For me this sort of weighted inclusion-exclusion arose naturally in calculating a symmetric function by an inclusion-exclusion of quasisymmetric functions.

As one example, the usual Moebius function for the poset of set partitions of a multiset gets quite messy, resulting from the fact that it is not multiplicative in terms of decomposition into blocks in a partition; 10 or 15 years ago, Robert Kleinberg and I looked at a multiplicative deformation of it which satisfied much nicer formulas than the traditional Moebius function in this case and interpreted this deformed Moebius function as such a reduced Euler characteristic for the nerve of a small category. Some references in this general direction are:

E. Babson and D. Kozlov, Group actions on posets, J. Algebra, 285 (2005), no. 2, 439--450.
P. Hersh, Chain decomposition and the flag f-vector, J. Combin. Theory Ser. A, 103 (2003), 27--52
P. Hersh and R. Kleinberg, A multiplicative deformation of the Moebius function for the poset of partitions of a multiset, Communicating Mathematics (special volume in honor of Joe Gallian's 65th birthday), 113--118, Contemp. Math., 479, Amer. Math. Soc., Providence, RI, 2009.

Answer by Peter May for Acyclic categories related to structures in algebraic topology

2012-07-27T02:23:21Z

This is not a generalization of posets, but an illustration of how they come up unexpectedly. There is a notion of subdivision of a (small) category, and the second subdivision of any category is a poset. This was folklore in the early 1960's. It plays a role in Thomason's Quillen equivalence between the category of small categories and the category of simplicial sets.

Answer by Dai Tamaki for Acyclic categories related to structures in algebraic topology

2012-08-24T14:33:30Z

Ralph Cohen, John Jones, and Graeme Segal found an interesting "construction" of a topological acyclic category $C(f)$ from a Morse-Smale function $f : M\to \mathbb{R}$ in a preprint in early 90's. Objects are critical points and morphism spaces are given by moduli spaces of gradient flows.