Pfaffian orientations and the elusive Heawood graph

Author(s)

Alberto Alexandre Assis Miranda

• Document type: Master's Dissertation
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Computação
• Defense date: 2006-07-08

Examining board members

Cláudio Leonardo Lucchesi; Marcelo Henriques de Carvalho; Orlando Lee

• Advisor: Cláudio Leonardo Lucchesi

Abstract

A graph G that contains a perfect matching is Pfaffiano if there is an orientation D of the edges of G, such that every conformal circuit of G is oddly oriented in D. A subgraph H of G is conformal if G - V (H) has a perfect matching. A circuit with an even number of edges is oddly oriented if the number of edges whose orientation in D agrees with any sense of traversal of the circuit is odd. Counting the number of perfect matchings of a graph is known to be NP-hard [11, page 307]. However, when restricted to Pfaffiano graphs, this problem is solvable in polynomial time [11, page 319]. The characterisation of Pfaffiano bipartite graphs, achieved by Charles Little, is almost thirty years old [9]. However, only recently, polynomial time algorithms for determining whether a bipartite graph is Pfaffiano were discovered, by McCuaig [13] and independently by Robertson, Seymour and Thomas [14]. This problem's solution solves a lot of problems, some of them are quite famous, in graph theory, economy and chemistry, as described in McCuaig's article [13, pages 16 to 35]. On this dissertation, we present a new proof of the correctness of this algorithm distinct from the two previously known proofs (AU)