A retracting-free bidirectional circuit in a graph G is a closed walk which traverses every edge exactly once in each direction and such that no edge is succeeded by the same edge in the opposite direction. Such a circuit revisits each vertex only in a number of steps. Studying the class Ω of all graphs admitting at least one retracting-free bidirectional circuit was proposed by Ore (1951) and is by now of practical use to nanotechnology. The latter needs in various molecular polyhedra that are constructed from a single chain molecule in the retracting-free way. Some earlier results for simple graphs, obtained by Thomassen and, then, by other authors, are specially refined by us for a cubic graph Q. Most of such refinements depend only on the number n of vertices of Q.Keywords: cubic graph, spanning tree, cotree, retracting-free bidirectional circuit Mathematics Subject Classification : 05C10, 05C30, 05C45, 94C15 DOI:10.5614/ejgta.2017.5.1.13
PreliminariesLet Q = (V, E) be a simple cubic graph with the vertex set V and edge set E |V | = n, |E| = m = 3n/2 . A spanning tree T of Q is a subtree covering all the vertices of Q |V (T )| = n; |E(T )| = n − 1 . Its cotree Q − E(T ) V Q − E(T ) = n; E Q − E(T ) = (n + 2)/2 is a graph Q less all edges belonging to T .