An obstruction to contractibility of spaces of directed paths.
Robin Belton, Robyn Brooks, Stefania Ebli, Lisbeth Fajstrup, Brittany Terese Fasy, Catherine Ray, Nicole Sanderson, Elizabeth Vidaurre · arXiv (Cornell University) · 2019
Defining homotopy or collapsibility in directed topological spaces is surprisingly difficult. In the directed setting, the spaces of directed paths between fixed initial and terminal points are the defining feature for distinguishing different directed spaces. The simplest case is when the space of directed paths is homotopy equivalent to that of a single path; we call this the trivial space of directed paths. Surprisingly, directed spaces that are trivial topologically may have non-trivial spaces of directed paths, which means that information is lost when the direction of these topological spaces is ignored. We define a notion of directed contractibility and directed collapsibility in the setting of a directed Euclidean cubical complex, using the spaces of directed paths of the underlying directed topological space, relative to an initial or a final vertex. In addition, we give a sufficient condition for a directed Euclidean cubical complex to have contractible spaces of directed paths from a fixed initial vertex.