Abstract

Roughly, a graph has small “tree-width” if it can be constructed by piecing small graphs together in a tree structure. Here we study the obstructions to the existence of such a tree structure. We find, for instance: 1. (i) a minimax formula relating tree-width with the largest such obstructions 2. (ii) an association between such obstructions and large grid minors of the graph 3. (iii) a “tree-decomposition” of the graph into pieces corresponding with the obstructions. These results will be of use in later papers.

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