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Siblings of Countable Cographs
Geňa Hahn, Maurice Pouzet and Robert Woodrow
We show that every countable cograph has either one or infinitely many siblings. This answers, very partially, a conjecture of Thomass´e. The main tools are the notion of well quasi-ordering and the correspondence between cographs and some labelled ordered trees.
Keywords: Graphs, cographs, trees, equimorphy, isomorphy, well quasi-ordering