Shortness exponent


In graph theory, the shortness exponent is a numerical parameter of a family of graphs that measures how far from Hamiltonian the graphs in the family can be. Intuitively, if is the shortness exponent of a graph family, then every -vertex graph in the family has a cycle of length near but some graphs do not have longer cycles. More precisely, for any ordering of the graphs in into a sequence, with defined to be the length of the longest cycle in graph, the shortness exponent is defined as
This number is always in the interval from 0 to 1; it is 1 for families of graphs that always contain a Hamiltonian or near-Hamiltonian cycle, and 0 for families of graphs in which the longest cycle length can be smaller than any constant power of the number of vertices.
The shortness exponent of the polyhedral graphs is. A construction based on kleetopes shows that some polyhedral graphs have longest cycle length, while it has also been proven that every polyhedral graph contains a cycle of length. The polyhedral graphs are the graphs that are simultaneously planar and 3-vertex-connected; the assumption of 3-vertex-connectivity is necessary for these results, as there exist sets of 2-vertex-connected planar graphs with shortness exponent 0. There are many additional known results on shortness exponents of restricted subclasses of planar and polyhedral graphs.
The 3-vertex-connected cubic graphs also have a shortness exponent that has been proven to lie strictly between 0 and 1.