TL;DR: In this paper, it was shown that every planar graph has an acyclic coloring with nine colors, provided that no circuit is bichromatic, and conjecture that five colors are sufficient.
Abstract: A coloring of the vertices of a graph byk colors is called acyclic provided that no circuit is bichromatic. We prove that every planar graph has an acyclic coloring with nine colors, and conjecture that five colors are sufficient. Other results on related types of colorings are also obtained; some of them generalize known facts about “point-arboricity”.
TL;DR: In this article, the edge coloring number of a graph composed of k copies of the totally disconnected graph with n nodes was studied and it was shown that 2n colors suffice to edge-color the graph unless n and k are both odd.
Abstract: The graph composed of k subgraphs, each the totally-disconnected graph with n nodes, arranged in a cycle, can be edge-colored with 2n colors unless n and k are both odd. The edge coloring number of a graph is the smallest number of colors for its edges so that no two adjacent edges have like color. Vizing [1] showed that the edge coloring number is either the maximum degree of a node, or one more than this number. In this note we consider the edge coloring number of a graph composed of k copies of the totally disconnected graph with n nodes. The totally disconnected graphs are arranged in a k-cycle; i.e., two nodes are joined by an edge if and only if they are in components adjacent in the k-cycle. We show that the edge coloring number of the graph is 2n unless n and k are both odd. For n and k both odd, assume that 2n colors suffice to edge-color the graph. Then each node, being of degree 2n, has an adjacent edge of each color. Each edge is adjacent to two nodes, thus the edges of a given color must be adjacent to an even number of nodes. The graph has kn nodes, an odd number, giving a contradiction. For k even a construction of the pattern of colors is given. Designate the nodes in the first totally disconnected component by AO *, A,,-,; those in the next component in the cycle by similarly subscripted B's, etc. Call the colors XO, * * *, Xn-l and YO, * * *, Yn_-. Color edge AiB3 with ,+j, the latter subscript reduced mod n. Color edge BiCj with Y+j. Continue the alternation of X and Y around the cycle of even length k. When n is even the constructive procedure developed below yields an edge coloring. Again, proceeding around the cycle, designate nodes by subscripted A's, B's, * , etc. Let the colors be W, X, Y, and Z, each letter assigned n/2 different subscripts. Describe the colorings of the AiB, Received by the editors April 14, 1972. AMS (MOS) subject classfltcations (1970). Primary 05C99.
TL;DR: It is proved that if k >= 3 and there exists a regular graph with valency k, edge connectivity k and chromatic index k + 1, then there exists such a graph of any girth g >= 4.