4,073 research outputs found
Structure of the flow and Yamada polynomials of cubic graphs
We establish a quadratic identity for the Yamada polynomial of ribbon cubic
graphs in 3-space, extending the Tutte golden identity for planar cubic graphs.
An application is given to the structure of the flow polynomial of cubic graphs
at zero. The golden identity for the flow polynomial is conjectured to
characterize planarity of cubic graphs, and we prove this conjecture for a
certain infinite family of non-planar graphs.
Further, we establish exponential growth of the number of chromatic
polynomials of planar triangulations, answering a question of D. Treumann and
E. Zaslow. The structure underlying these results is the chromatic algebra, and
more generally the SO(3) topological quantum field theory.Comment: 22 page
Colouring quadrangulations of projective spaces
A graph embedded in a surface with all faces of size 4 is known as a
quadrangulation. We extend the definition of quadrangulation to higher
dimensions, and prove that any graph G which embeds as a quadrangulation in the
real projective space P^n has chromatic number n+2 or higher, unless G is
bipartite. For n=2 this was proved by Youngs [J. Graph Theory 21 (1996),
219-227]. The family of quadrangulations of projective spaces includes all
complete graphs, all Mycielski graphs, and certain graphs homomorphic to
Schrijver graphs. As a corollary, we obtain a new proof of the Lovasz-Kneser
theorem
Asymptotic enumeration and limit laws for graphs of fixed genus
It is shown that the number of labelled graphs with n vertices that can be
embedded in the orientable surface S_g of genus g grows asymptotically like
where , and is the exponential growth rate of planar graphs. This generalizes the
result for the planar case g=0, obtained by Gimenez and Noy.
An analogous result for non-orientable surfaces is obtained. In addition, it
is proved that several parameters of interest behave asymptotically as in the
planar case. It follows, in particular, that a random graph embeddable in S_g
has a unique 2-connected component of linear size with high probability
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