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Unlike minors, the induced subgraph obstructions to bounded treewidth come in a large variety, including, for every t ∈ N, the t-basic obstructions: the graphs Kt+1 and Kt,t, along with the subdivisions of the t-by-t wall and their line graphs. But this list is far from complete. The simplest example of a “non-basic” obstruction is due to Pohoata and Davies (independently). For every n ∈ N, they construct certain graphs of treewidth n and with no 3-basic obstruction as an induced subgraph, which we call n-arrays. Let us say a graph class G is clean if the only obstructions to bounded treewidth in G are in fact the basic ones. It follows that a full description of the induced subgraph obstructions to bounded treewidth is equivalent to a characterization of all families H of graphs for which the class of all H-free graphs is clean (a graph G is H-free if no induced subgraph of G is isomorphic to any graph in H). This remains elusive, but there is an immediate necessary condition: if H-free graphs are clean, then there are only finitely many n ∈ N such that there is an n-array which is H-free. The above necessary condition is not sufficient in general. However, the situation turns out to be different if H is finite: we prove that for every finite set H of graphs, the class of all H-free graphs is clean if and only if there is no H-free n-array except possibly for finitely many values of n.
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A Note on the Gyárfás–Sumner Conjecture
The Gyárfás–Sumner conjecture says that for every tree T and every integer t ≥ 1, if G is a graph with no clique of size t and with sufficiently large chromatic number, then G contains an induced subgraph isomorphic to T. This remains open, but we prove that under the same hypotheses, G contains a subgraph H isomorphic to T that is “path-induced”; that is, for some distinguished vertex r, every path of H with one end r is an induced path of G.
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- Award ID(s):
- 2154169
- PAR ID:
- 10517604
- Publisher / Repository:
- Springer Link
- Date Published:
- Journal Name:
- Graphs and Combinatorics
- Volume:
- 40
- Issue:
- 2
- ISSN:
- 0911-0119
- Subject(s) / Keyword(s):
- induced subgraphs chromatic number
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s00373-024-02754-z
