Date of Award
Doctor of Philosophy in Mathematics
Several conjectures concerning planar graph colorings are still unsolved to this day. One of the more famous ones is Steinberg's Conjecture (first stated in 1976), which we work towards in this dissertation. Steinberg's Conjecture states that all planar graphs without cycles of length 4 or 5 are 3-colorable, that is, we can color the vertices of such a graph using three colors in a way that leaves no adjacent vertices colored the same.
We use defective colorings to move closer to the affirmation or nullification of this conjecture. A defective coloring is any non-proper coloring, that is, some adjacent vertices may end up colored the same color. We use (d1, d2, d3)-colorings, which are 3-colorings where the maximum degree of the ith color class is at most di, for i = 1, 2, 3. In this study, we prove that all planar graphs without cycles of length 4 or 5 are (3, 0, 0)-colorable.
We define a cycle having two triangular chords as 2-chorded and a face of size k as a k-face. Let a b8-face be an 8-face incident to a vertex of degree three, which is itself also incident to faces of size 3 and 6. Let G be the set of all planar graph without C4's, C5's, 2-chorded C8's, or 2-chorded C9's. In this paper, we prove that all graphs in G with at most eleven b8-faces is (1, 0, 0)-colorable.
We end this dissertation by examining a paper claiming to prove that all planar graphs without 5- and 8-cycles and without adjacent triangles are 3-colorable. We show some counterexamples to a claim given in the paper.
Smith, Diana L., "TOWARDS STEINBERG'S CONJECTURE" (2014). Open Access Dissertations. Paper 212.