The continuous gradability of the cut-point orders of R-trees
Logic and Set Theory Seminar
19th May 2021, 4:30 pm – 6:00 pm
An R-tree is a metric space tree in which every point can be branching. Favre and Jonsson posed the following problem in 2004: can the class of orders underlying R-trees be characterised by the fact that every branch is order-isomorphic to a real interval? In the first part of the talk, I answer this question in the negative: there is a branchwise-real tree order which is not continuously gradable. In the second part, I show that a branchwise-real tree order is continuously gradable if and only if every embedded well-stratified (i.e. set-theoretic) tree is R-gradable. This tighter link with set theory is put to work in the third part answering a number of refinements of the main question, yielding several independence results.