129 research outputs found

### A simple and elementary proof of Whitney's unique embedding theorem

In this note we give a short and elementary proof of a more general version
of Whitney's theorem that 3-connected planar graphs have a unique embedding in
the plane. A consequence of the theorem is that cubic plane graphs cannot be
embedded in a higher genus with a simple dual. The aim of this paper is to
promote a simple and elementary proof, which is especially well suited for
lectures presenting Whitney's theorem

### Generation of cubic graphs and snarks with large girth

We describe two new algorithms for the generation of all non-isomorphic cubic
graphs with girth at least $k\ge 5$ which are very efficient for $5\le k \le 7$
and show how these algorithms can be efficiently restricted to generate snarks
with girth at least $k$.
Our implementation of these algorithms is more than 30, respectively 40 times
faster than the previously fastest generator for cubic graphs with girth at
least 6 and 7, respectively.
Using these generators we have also generated all non-isomorphic snarks with
girth at least 6 up to 38 vertices and show that there are no snarks with girth
at least 7 up to 42 vertices. We present and analyse the new list of snarks
with girth 6.Comment: 27 pages (including appendix

### Polyhedra with few 3-cuts are hamiltonian

In 1956, Tutte showed that every planar 4-connected graph is hamiltonian. In
this article, we will generalize this result and prove that polyhedra with at
most three 3-cuts are hamiltonian. In 2002 Jackson and Yu have shown this
result for the subclass of triangulations. We also prove that polyhedra with at
most four 3-cuts have a hamiltonian path. It is well known that for each $k \ge
6$ non-hamiltonian polyhedra with $k$ 3-cuts exist. We give computational
results on lower bounds on the order of a possible non-hamiltonian polyhedron
for the remaining open cases of polyhedra with four or five 3-cuts.Comment: 21 pages; changed titl

### Goldberg, Fuller, Caspar, Klug and Coxeter and a general approach to local symmetry-preserving operations

Cubic polyhedra with icosahedral symmetry where all faces are pentagons or
hexagons have been studied in chemistry and biology as well as mathematics. In
chemistry one of these is buckminsterfullerene, a pure carbon cage with maximal
symmetry, whereas in biology they describe the structure of spherical viruses.
Parameterized operations to construct all such polyhedra were first described
by Goldberg in 1937 in a mathematical context and later by Caspar and Klug --
not knowing about Goldberg's work -- in 1962 in a biological context. In the
meantime Buckminster Fuller also used subdivided icosahedral structures for the
construction of his geodesic domes. In 1971 Coxeter published a survey article
that refers to these constructions. Subsequently, the literature often refers
to the Goldberg-Coxeter construction. This construction is actually that of
Caspar and Klug. Moreover, there are essential differences between this
(Caspar/Klug/Coxeter) approach and the approaches of Fuller and of Goldberg. We
will sketch the different approaches and generalize Goldberg's approach to a
systematic one encompassing all local symmetry-preserving operations on
polyhedra

### Computing the maximal canonical form for trees in polynomial time

Known algorithms computing a canonical form for trees in linear time use specialized canonical forms for trees and no canonical forms defined for all graphs. For a graph the maximal canonical form is obtained by relabelling the vertices with in a way that the binary number with bits that is the result of concatenating the rows of the adjacency matrix is maximal. This maximal canonical form is not only defined for all graphs but even plays a special role among the canonical forms for graphs due to some nesting properties allowing orderly algorithms. We give an algorithm to compute the maximal canonical form of a tree

### Uniquely hamiltonian graphs for many sets of degrees

We give constructive proofs for the existence of uniquely hamiltonian graphs
for various sets of degrees. We give constructions for all sets with minimum 2
(a
trivial case), all sets with minimum 3 that contain an even number (for sets
without an even number it is known that no uniquely hamiltonian graphs exist),
and
all sets with at least two elements and minimum 4 where all other elements
are at least 10. For minimum degree 3 and 4, the constructions also give
3-connected graphs

### Generation of cubic graphs

We describe a new algorithm for the efficient generation of all non-isomorphic connected cubic graphs. Our implementation of this algorithm is more than 4 times faster than previous generators. The generation can also be efficiently restricted to cubic graphs with girth at least 4 or 5

### Types of triangle in plane Hamiltonian triangulations and applications to domination and k-walks

We investigate the minimum number t(0)(G) of faces in a Hamiltonian triangulation G so that any Hamiltonian cycle C of G has at least t(0)(G) faces that do not contain an edge of C. We prove upper and lower bounds on the maximum of these numbers for all triangulations with a fixed number of facial triangles. Such triangles play an important role when Hamiltonian cycles in triangulations with 3-cuts are constructed from smaller Hamiltonian cycles of 4-connected subgraphs. We also present results linking the number of these triangles to the length of 3-walks in a class of triangulation and to the domination number

### Ramsey numbers R(K3,G) for graphs of order 10

In this article we give the generalized triangle Ramsey numbers R(K3,G) of 12
005 158 of the 12 005 168 graphs of order 10. There are 10 graphs remaining for
which we could not determine the Ramsey number. Most likely these graphs need
approaches focusing on each individual graph in order to determine their
triangle Ramsey number. The results were obtained by combining new
computational and theoretical results. We also describe an optimized algorithm
for the generation of all maximal triangle-free graphs and triangle Ramsey
graphs. All Ramsey numbers up to 30 were computed by our implementation of this
algorithm. We also prove some theoretical results that are applied to determine
several triangle Ramsey numbers larger than 30. As not only the number of
graphs is increasing very fast, but also the difficulty to determine Ramsey
numbers, we consider it very likely that the table of all triangle Ramsey
numbers for graphs of order 10 is the last complete table that can possibly be
determined for a very long time.Comment: 24 pages, submitted for publication; added some comment

### House of Graphs: a database of interesting graphs

In this note we present House of Graphs (http://hog.grinvin.org) which is a
new database of graphs. The key principle is to have a searchable database and
offer -- next to complete lists of some graph classes -- also a list of special
graphs that already turned out to be interesting and relevant in the study of
graph theoretic problems or as counterexamples to conjectures. This list can be
extended by users of the database.Comment: 8 pages; added a figur

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