Excluding infinite minors

AbstractLet κ be an infinite cardinal, and let H be either a complete graph with κ vertices, or a tree in which every vertex has valency κ. What can we say about graphs G which (i) have no minor isomorphic to H, or (ii) contain no subgraph which is a subdivision of H?These four questions are answered for each infinite cardinal κ. In each case we find that there corresponds a necessary and sufficient structural condition (or, in some cases, several equivalent conditions) for G not to contain H in the appropriate way. We survey these results and a number of related theorems

Applications of tree decompositions and accessibility to treeability of Borel graphs

A framework to handle tree decompositions of the components of a Borel graph in a Borel fashion is introduced, along the lines of Tserunyan's Stallings Theorem for equivalence relations arXiv:1805.09506. This setting leads to a notion of accessibility for Borel graphs, together with a treeability criterion. This criterion is applied to show that, in particular, Borel equivalence relations associated to Borel graphs with accessible planar connected components are measure treeable, generalising results of Conley, Gaboriau, Marks, and Tucker-Drob arXiv:2104.07431 and Timar arXiv:1910.01307. It is also proven that uniformly locally finite Borel graphs with components of finite tree-width yield Borel treeable equivalence relations. Our results imply that p.m.p countable Borel equivalence relations with measured property (T) do not admit locally finite graphings with planar components a.s.Comment: 39 page