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Assignment 9
MATH 239 A-9-1. Let d be a non-negative integer. A graph G is d-degenerate if every subgraph of G contains a vertex of degree at most d. Let G be a graph on n vertices. (a) {2 marks} Prove that G is a forest if and only if G is 1-degenerate. (b) {4 marks} Prove that G is d-degenerate if and only if there exists an ordering v1, . . . , vn of V (G) such that |NG(vi) ∩ {v1, . . . , vi−1}| ≤ d for all i ∈ {2, . . . , n}. A-9-2. {4 marks} Let d be a non-negative integer. Prove that if G is a d-degenerate graph, then G is (d+ 1)-colourable. A-9-3. {4 marks} Let G be a graph and let E1, . . . Er be a partition of E(G). For each i ∈ {1, . . . , r}, let Gi be the graph with V (Gi) = V (G) and E(Gi) = Ei. For each i ∈ {1, . . . , r}, let ki be a positive integer such that Gi is ki-colourable. Let k = ∏r i=1 ki. Prove that G is k-colourable. A-9-4. (a) {4 marks} Let G be a graph and letM be a maximal matching of G (that is, a matching that is not a proper subset of any bigger matching of G). Let N be a maximum matching of G. Prove that |N | ≤ 2|M |. (b) {2 marks} For each positive integer k, provide an example of a graph G, a maximal matchingM of G and a maximum matching N of G such that |M | = k and |N | = 2k.