| 21. | In the planar undirected graph with 3 vertices and 3 edges, in the form of a triangle.
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| 22. | The Robertson Seymour theorem states that finite undirected graphs and graph minors form a well-quasi-ordering.
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| 23. | In an undirected graph, this means that each new loop increases the degree of a vertex by two.
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| 24. | It is not known how to test efficiently whether an undirected graph can be represented as a visibility graph.
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| 25. | Undirected graphs often use the latter convention of counting loops twice, whereas directed graphs typically use the former convention.
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| 26. | Simplicial sets should not be confused with abstract simplicial complexes, which generalize simple undirected graphs rather than directed multigraphs.
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| 27. | On an undirected graph, a walker takes a step from the current node, j, to node i.
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| 28. | An undirected graph has the property that e _ { ij } and e _ { ji } are considered identical.
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| 29. | In the regular undirected graph with 70 vertices and 140 edges discovered by Guy H . J . Meredith in 1973.
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| 30. | It unifies the ideas of incidence chromatic number of a graph and game chromatic number in case of an undirected graph.
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