Combinatorial Theory

Robertson and Seymour proved two fundamental theorems about tangles in graphs: the tree-of-tangles theorem, which says that every graph has a tree-decomposition such that distinguishable tangles live in different nodes of the tree, and the tangle-tree duality theorem, which says that graphs without a \(k\)-tangle have a tree-decomposition that witnesses the non-existence of such tangles, in that \(k\)-tangles would have to live in a node but no node is large enough to accommodate one.

Erde combined these two fundamental theorems into one, by constructing a single tree-decomposition such that every node either accommodates a single \(k\)-tangle or is too small to accommodate one. Such a tree-decomposition thus shows at a glance how many \(k\)-tangles a graph has and where they are.

The two fundamental theorems have since been extended to abstract separation systems, which support tangles in more general discrete structures. In this paper we extend Erde's unified theorem to such general systems.

Mathematics Subject Classifications: 05C83, 05C40, 06A07

Keywords: Tree of tangles, tangle-tree duality, abstract separation system, submodularity, canonical