Letter graphs and geometric grid classes of permutations: characterization and recognition
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In this paper, we reveal an intriguing relationship between two seemingly unrelated notions: letter graphs and geometric grid classes of permutations. An important property common for both of them is well-quasi-orderability, implying, in a non-constructive way, a polynomial-time recognition of geometric grid classes of permutations and k-letter graphs for a fixed k. However, constructive algorithms are available only for k=2. In this paper, we present the first constructive polynomial-time algorithm for the recognition of 3-letter graphs. It is based on a structural characterization of graphs in this class.
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