We show that many graphs with bounded treewidth can be described as subgraphs of the strong product of a graph with smaller treewidth and a bounded-size complete graph. To this end, define the underlying treewidth of a graph class G to be the minimum non-negative integer c such that, for some function f , for every graph G ∈ G there is a graphWe introduce disjointed coverings of graphs and show they determine the underlying treewidth of any graph class. Using this result, we prove that the class of planar graphs has underlying treewidth 3; the class of K s,t -minor-free graphs has underlying treewidth s (for t max{s, 3}); and the class of K t -minor-free graphs has underlying treewidth t − 2. In general, we prove that a monotone class has bounded underlying treewidth if and only if it excludes some fixed topological minor. We also study the underlying treewidth of graph classes defined by an excluded subgraph or excluded induced subgraph. We show that the class of graphs with no H subgraph has bounded underlying treewidth if and only if every component of H is a subdivided star, and that the class of graphs with no induced H subgraph has bounded underlying treewidth if and only if every component of H is a star.

We show that each real-representable matroid is a minor of a complex-representable excluded minor for realrepresentability. More generally, for an infinite field F 1 and a field extension F 2 , if F 1 -representability is not equivalent to F 2representability, then each F 1 -representable matroid is a minor of a F 2 -representable excluded minor for F 1 -representability.

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