43,219 research outputs found
Knot invariants and the Bollobas-Riordan polynomial of embedded graphs
For a graph G embedded in an orientable surface \Sigma, we consider
associated links L(G) in the thickened surface \Sigma \times I. We relate the
HOMFLY polynomial of L(G) to the recently defined Bollobas-Riordan polynomial
of a ribbon graph. This generalizes celebrated results of Jaeger and Traldi. We
use knot theory to prove results about graph polynomials and, after discussing
questions of equivalence of the polynomials, we go on to use our formulae to
prove a duality relation for the Bollobas-Riordan polynomial. We then consider
the specialization to the Jones polynomial and recent results of Chmutov and
Pak to relate the Bollobas-Riordan polynomials of an embedded graph and its
tensor product with a cycle.Comment: v2: minor corrections, to appear in European Journal of Combinatoric
The Go polynomials of a graph
Abstract This paper introduces graph polynomials based on a concept from the game of Go. Suppose that, for each vertex of a graph, we either leave it uncoloured or choose a colour uniformly at random from a set of available colours, with the choices for the vertices being independent and identically distributed. We ask for the probability that the resulting partial assignment of colours has the following property: for every colour class, each component of the subgraph it induces has a vertex that is adjacent to an uncoloured vertex. In Go terms, we are requiring that every group is uncaptured. This deÿnition leads to Go polynomials for a graph. Although these polynomials are based on properties that are less "local" in nature than those used to deÿne more traditional graph polynomials such as the chromatic polynomial, we show that they satisfy recursive relations based on local modiÿcations similar in spirit to the deletion-contraction relation for the chromatic polynomial. We then show that they are #P-hard to compute in general, using a result on linear forms in logarithms from transcendental number theory. We also brie y record some correlation inequalities
Eigenvalue interlacing and weight parameters of graphs
Eigenvalue interlacing is a versatile technique for deriving results in
algebraic combinatorics. In particular, it has been successfully used for
proving a number of results about the relation between the (adjacency matrix or
Laplacian) spectrum of a graph and some of its properties. For instance, some
characterizations of regular partitions, and bounds for some parameters, such
as the independence and chromatic numbers, the diameter, the bandwidth, etc.,
have been obtained. For each parameter of a graph involving the cardinality of
some vertex sets, we can define its corresponding weight parameter by giving
some "weights" (that is, the entries of the positive eigenvector) to the
vertices and replacing cardinalities by square norms. The key point is that
such weights "regularize" the graph, and hence allow us to define a kind of
regular partition, called "pseudo-regular," intended for general graphs. Here
we show how to use interlacing for proving results about some weight parameters
and pseudo-regular partitions of a graph. For instance, generalizing a
well-known result of Lov\'asz, it is shown that the weight Shannon capacity
of a connected graph \G, with vertices and (adjacency matrix)
eigenvalues , satisfies \Theta\le
\Theta^* \le \frac{\|\vecnu\|^2}{1-\frac{\lambda_1}{\lambda_n}} where
is the (standard) Shannon capacity and \vecnu is the positive
eigenvector normalized to have smallest entry 1. In the special case of regular
graphs, the results obtained have some interesting corollaries, such as an
upper bound for some of the multiplicities of the eigenvalues of a
distance-regular graph. Finally, some results involving the Laplacian spectrum
are derived. spectrum are derived
Computing Node Polynomials for Plane Curves
According to the G\"ottsche conjecture (now a theorem), the degree N^{d,
delta} of the Severi variety of plane curves of degree d with delta nodes is
given by a polynomial in d, provided d is large enough. These "node
polynomials" N_delta(d) were determined by Vainsencher and Kleiman-Piene for
delta <= 6 and delta <= 8, respectively. Building on ideas of Fomin and
Mikhalkin, we develop an explicit algorithm for computing all node polynomials,
and use it to compute N_delta(d) for delta <= 14. Furthermore, we improve the
threshold of polynomiality and verify G\"ottsche's conjecture on the optimal
threshold up to delta <= 14. We also determine the first 9 coefficients of
N_delta(d), for general delta, settling and extending a 1994 conjecture of Di
Francesco and Itzykson.Comment: 23 pages; to appear in Mathematical Research Letter
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