LINKED TREE-DECOMPOSITIONS OF INFINITE REPRESENTED MATROIDS By

It is natural to try to extend the results of Robertson and Seymour’s Graph Minors Project to other objects. As linked tree-decompositions (ltds) of graphs played a key role in the Graph Minors Project, establishing the existence of ltds of other objects is a useful step towards such extensions. There has been progress in this direction for both infinite graphs and matroids. Kříž and Thomas proved that infinite graphs of finite tree-width have ltds. More recently, Geelen, Gerards and Whittle proved that matroids have linked branch-decompositions, which are similar to ltds. These results suggest that infinite matroids of finite treewidth should have ltds. We answer this conjecture affirmatively for the representable case. Specifically, an independence space is an infinite matroid, and a point configuration (hereafter configuration) is a represented independence space. It is shown that every configuration having tree-width k ∈ ω has an ltd of width at most 2k. Configuration analogues for bridges of X (also called connected components modulo X) and chordality in graphs are introduced to prove this result. A correspondence is established between chordal configurations only containing subspaces of dimension at most k ∈ ω and configuration tree-decompositions having width at most k. This correspondence is used to characterise finite-width ltds of configurations by their local structure, enabling the proof of the existence result. The theory developed is also used to show compactness of configuration tree-width: a configuration has tree-width at most k ∈ ω if and only if each of its finite subconfigurations has tree-width at most k ∈ ω. The existence of ltds for configurations having finite tree-width opens the possibility of well-quasi-ordering (or even better-quasi-ordering) by minors those independence spaces representable over a fixed finite field and having bounded tree-width.