171 research outputs found
Few Long Lists for Edge Choosability of Planar Cubic Graphs
It is known that every loopless cubic graph is 4-edge choosable. We prove the
following strengthened result.
Let G be a planar cubic graph having b cut-edges. There exists a set F of at
most 5b/2 edges of G with the following property. For any function L which
assigns to each edge of F a set of 4 colours and which assigns to each edge in
E(G)-F a set of 3 colours, the graph G has a proper edge colouring where the
colour of each edge e belongs to L(e).Comment: 14 pages, 1 figur
A quadratic lower bound for subset sums
Let A be a finite nonempty subset of an additive abelian group G, and let
\Sigma(A) denote the set of all group elements representable as a sum of some
subset of A. We prove that |\Sigma(A)| >= |H| + 1/64 |A H|^2 where H is the
stabilizer of \Sigma(A). Our result implies that \Sigma(A) = Z/nZ for every set
A of units of Z/nZ with |A| >= 8 \sqrt{n}. This consequence was first proved by
Erd\H{o}s and Heilbronn for n prime, and by Vu (with a weaker constant) for
general n.Comment: 12 page
There are only a finite number of excluded minors for the class of bicircular matroids
We show that the class of bicircular matroids has only a finite number of
excluded minors. Key tools used in our proof include representations of
matroids by biased graphs and the recently introduced class of quasi-graphic
matroids. We show that if is an excluded minor of rank at least ten, then
is quasi-graphic. Several small excluded minors are quasi-graphic. Using
biased-graphic representations, we find that already contains one of these.
We also provide an upper bound, in terms of rank, on the number of elements in
an excluded minor, so the result follows.Comment: Added an appendix describing all known excluded minors. Added Gordon
Royle as author. Some proofs revised and correcte
Cayley sum graphs and eigenvalues of -fullerenes
We determine the spectra of cubic plane graphs whose faces have sizes 3 and
6. Such graphs, "(3,6)-fullerenes", have been studied by chemists who are
interested in their energy spectra. In particular we prove a conjecture of
Fowler, which asserts that all their eigenvalues come in pairs of the form
except for the four eigenvalues . We
exhibit other families of graphs which are "spectrally nearly bipartite" in
this sense. Our proof utilizes a geometric representation to recognize the
algebraic structure of these graphs, which turn out to be examples of Cayley
sum graphs
Generation and Properties of Snarks
For many of the unsolved problems concerning cycles and matchings in graphs
it is known that it is sufficient to prove them for \emph{snarks}, the class of
nontrivial 3-regular graphs which cannot be 3-edge coloured. In the first part
of this paper we present a new algorithm for generating all non-isomorphic
snarks of a given order. Our implementation of the new algorithm is 14 times
faster than previous programs for generating snarks, and 29 times faster for
generating weak snarks. Using this program we have generated all non-isomorphic
snarks on vertices. Previously lists up to vertices have been
published. In the second part of the paper we analyze the sets of generated
snarks with respect to a number of properties and conjectures. We find that
some of the strongest versions of the cycle double cover conjecture hold for
all snarks of these orders, as does Jaeger's Petersen colouring conjecture,
which in turn implies that Fulkerson's conjecture has no small counterexamples.
In contrast to these positive results we also find counterexamples to eight
previously published conjectures concerning cycle coverings and the general
cycle structure of cubic graphs.Comment: Submitted for publication V2: various corrections V3: Figures updated
and typos corrected. This version differs from the published one in that the
Arxiv-version has data about the automorphisms of snarks; Journal of
Combinatorial Theory. Series B. 201
A fractal set from the binary reflected Gray code
The permutation associated with the decimal expression of the binary reflected Gray code with N bits is considered. Its cycle structure is studied. Considered as a set of points, its self-similarity is pointed out. As a fractal, it is shown to be the attractor of an IFS. For large values of N the set is examined from the point of view of time series analysis
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