Motivated by constructions in topological data analysis and algebraic combinatorics, we study homotopy theory on the category of Cech closure spaces $\mathbf{Cl}$, the category whose objects are sets endowed with a Cech closure operator and whose morphisms are the continuous maps between them. We introduce new classes of Cech closure structures on metric spaces, graphs, and simplicial complexes, and we show how each of these cases gives rise to an interesting homotopy theory. In particular, we show that there exists a natural family of Cech closure structures on metric spaces which produces a non-trivial homotopy theory for finite metric spaces, i.e. point clouds, the spaces of interest in topological data analysis. We then give a Cech closure structure to graphs and simplicial complexes which may be used to construct a new combinatorial (as opposed to topological) homotopy theory for each skeleton of those spaces. We further show that there is a Seifert-van Kampen theorem for closure spaces, a well-defined notion of persistent homotopy, and an associated interleaving distance. As an illustration of the difference with the topological setting, we calculate the fundamental group for the circle, `circular graphs', and the wedge of circles endowed with different closure structures. Finally, we produce a continuous map from the topological circle to `circular graphs' which, given the appropriate closure structures, induces an isomorphism on the fundamental groups.
A comparison of spectral sequences computing unstable homotopy groups of $p$-complete, nilpotent spaces
本文的重点是比较两个不稳定同伦谱序列--不稳定mod p Adams谱序列和Goerss-Hopkins谱序列，前者计算p-完备空间的不稳定同伦，后者计算H k-代数之间E-无穷映射空间的不稳定同伦，其中k是p元域的代数闭包，p是奇素数.利用p-完备幂零空间与Hk-代数子集之间的一个连接，证明了这个连接的单位提供了这些谱序列之间的同构.
