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