Pfaffian Orientations, Flat Embeddings, and Steinberg’s Conjecture

Dissertation Defense
Tuesday, April 15, 2014 - 12:00
1 hour (actually 50 minutes)
Skiles 005
Georgia Institute of Technology
The first result of this thesis is a partial result in the direction of Steinberg's Conjecture.  Steinberg's Conjecture states that any planar graph without cycles of length four or five is three colorable.  Borodin, Glebov, Montassier, and Raspaud showed that planar graphs without cycles of length four, five, or seven are three colorable and Borodin and Glebov showed that planar graphs without five cycles or triangles at distance at most two apart are three colorable.  We prove a statement that implies the first of these theorems and is incomparable with the second: that any planar graph with no cycles of length four through six or cycles of length seven with incident triangles distance exactly two apart are three colorable. We are next concerned with the study of Pfaffian orientations.  A theorem proved by William McCuaig and, independently, Neil Robertson, Paul Seymour, and Robin Thomas provides a good characterization for whether or not a bipartite graph has a Pfaffian orientation as well as a polynomial time algorithm for that problem.  We reprove this characterization and provide a new algorithm for this problem.  First, we generalize a preliminary result needed to reprove this theorem.  Specifically, we show that any internally 4-connected, non-planar bipartite graph contains a subdivision of K3,3 in which each path has odd length.  We then make use of this result to provide a much shorter proof of this characterization using elementary methods. In the final piece of the thesis we investigate flat embeddings.  A piecewise-linear embedding of a graph in 3-space is flat if every cycle of the graph bounds a disk disjoint from the rest of the graph.  We first provide a structural theorem for flat embeddings that indicates how to build them from small pieces.  We then present a class of flat graphs that are highly non-planar in the sense that, for any fixed k, there are an infinite number of members of the class such that deleting k vertices leaves the graph non-planar.