1,004 research outputs found
Cayley graphs of order kp are hamiltonian for k < 48
We provide a computer-assisted proof that if G is any finite group of order
kp, where k < 48 and p is prime, then every connected Cayley graph on G is
hamiltonian (unless kp = 2). As part of the proof, it is verified that every
connected Cayley graph of order less than 48 is either hamiltonian connected or
hamiltonian laceable (or has valence less than three).Comment: 16 pages. GAP source code is available in the ancillary file
Tangle-tree duality in abstract separation systems
We prove a general width duality theorem for combinatorial structures with
well-defined notions of cohesion and separation. These might be graphs and
matroids, but can be much more general or quite different. The theorem asserts
a duality between the existence of high cohesiveness somewhere local and a
global overall tree structure.
We describe cohesive substructures in a unified way in the format of tangles:
as orientations of low-order separations satisfying certain consistency axioms.
These axioms can be expressed without reference to the underlying structure,
such as a graph or matroid, but just in terms of the poset of the separations
themselves. This makes it possible to identify tangles, and apply our
tangle-tree duality theorem, in very diverse settings.
Our result implies all the classical duality theorems for width parameters in
graph minor theory, such as path-width, tree-width, branch-width or rank-width.
It yields new, tangle-type, duality theorems for tree-width and path-width. It
implies the existence of width parameters dual to cohesive substructures such
as -blocks, edge-tangles, or given subsets of tangles, for which no width
duality theorems were previously known.
Abstract separation systems can be found also in structures quite unlike
graphs and matroids. For example, our theorem can be applied to image analysis
by capturing the regions of an image as tangles of separations defined as
natural partitions of its set of pixels. It can be applied in big data contexts
by capturing clusters as tangles. It can be applied in the social sciences,
e.g. by capturing as tangles the few typical mindsets of individuals found by a
survey. It could also be applied in pure mathematics, e.g. to separations of
compact manifolds.Comment: We have expanded Section 2 on terminology for better readability,
adding explanatory text, examples, and figures. This paper replaces the first
half of our earlier paper arXiv:1406.379
Exploiting -Closure in Kernelization Algorithms for Graph Problems
A graph is c-closed if every pair of vertices with at least c common
neighbors is adjacent. The c-closure of a graph G is the smallest number such
that G is c-closed. Fox et al. [ICALP '18] defined c-closure and investigated
it in the context of clique enumeration. We show that c-closure can be applied
in kernelization algorithms for several classic graph problems. We show that
Dominating Set admits a kernel of size k^O(c), that Induced Matching admits a
kernel with O(c^7*k^8) vertices, and that Irredundant Set admits a kernel with
O(c^(5/2)*k^3) vertices. Our kernelization exploits the fact that c-closed
graphs have polynomially-bounded Ramsey numbers, as we show
A conjecture on critical graphs and connections to the persistence of associated primes
We introduce a conjecture about constructing critically (s+1)-chromatic
graphs from critically s-chromatic graphs. We then show how this conjecture
implies that any unmixed height two square-free monomial ideal I, i.e., the
cover ideal of a finite simple graph, has the persistence property, that is,
Ass(R/I^s) \subseteq Ass(R/I^{s+1}) for all s >= 1. To support our conjecture,
we prove that the statement is true if we also assume that \chi_f(G), the
fractional chromatic number of the graph G, satisfies \chi(G) -1 < \chi_f(G) <=
\chi(G). We give an algebraic proof of this result.Comment: 11 pages; Minor changes throughout the paper; to appear in Discrete
Math
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