What Is... Seminar
A quasi-tree is a space which is quasi-isometric (coarsely bilipschitz) to a tree. Such a space may locally look very strange, but has the large-scale structure of a tree (usually an infinite tree). Such spaces arise naturally from group actions on spaces with some negative curvature properties. I'll say a bit about how they arise and what they are good for, as well as a characterization by a "Bottleneck Property".