11,474 research outputs found
Parallel O(log(n)) time edge-colouring of trees and Halin graphs
We present parallel O(log(n))-time algorithms for optimal edge colouring of trees and Halin graphs with n processors on a a parallel random access machine without write conflicts (P-RAM). In the case of Halin graphs with a maximum degree of three, the colouring algorithm automatically finds every Hamiltonian cycle of the graph
On-line list colouring of random graphs
In this paper, the on-line list colouring of binomial random graphs G(n,p) is
studied. We show that the on-line choice number of G(n,p) is asymptotically
almost surely asymptotic to the chromatic number of G(n,p), provided that the
average degree d=p(n-1) tends to infinity faster than (log log n)^1/3(log
n)^2n^(2/3). For sparser graphs, we are slightly less successful; we show that
if d>(log n)^(2+epsilon) for some epsilon>0, then the on-line choice number is
larger than the chromatic number by at most a multiplicative factor of C, where
C in [2,4], depending on the range of d. Also, for d=O(1), the on-line choice
number is by at most a multiplicative constant factor larger than the chromatic
number
Colouring random graphs and maximising local diversity
We study a variation of the graph colouring problem on random graphs of
finite average connectivity. Given the number of colours, we aim to maximise
the number of different colours at neighbouring vertices (i.e. one edge
distance) of any vertex. Two efficient algorithms, belief propagation and
Walksat are adapted to carry out this task. We present experimental results
based on two types of random graphs for different system sizes and identify the
critical value of the connectivity for the algorithms to find a perfect
solution. The problem and the suggested algorithms have practical relevance
since various applications, such as distributed storage, can be mapped onto
this problem.Comment: 10 pages, 10 figure
Graph properties, graph limits and entropy
We study the relation between the growth rate of a graph property and the
entropy of the graph limits that arise from graphs with that property. In
particular, for hereditary classes we obtain a new description of the colouring
number, which by well-known results describes the rate of growth.
We study also random graphs and their entropies. We show, for example, that
if a hereditary property has a unique limiting graphon with maximal entropy,
then a random graph with this property, selected uniformly at random from all
such graphs with a given order, converges to this maximizing graphon as the
order tends to infinity.Comment: 24 page
Colouring random graphs: Tame colourings
Given a graph G, a colouring is an assignment of colours to the vertices of G
so that no two adjacent vertices are coloured the same. If all colour classes
have size at most t, then we call the colouring t-bounded, and the t-bounded
chromatic number of G, denoted by , is the minimum number of colours
in such a colouring. Every colouring of G is then -bounded, where
denotes the size of a largest independent set.
We study colourings of the random graph G(n, 1/2) and of the corresponding
uniform random graph G(n,m) with . We show that is maximally concentrated on at most
two explicit values for . This behaviour stands in stark
contrast to that of the normal chromatic number, which was recently shown not
to be concentrated on any sequence of intervals of length .
Moreover, when and if the expected number of
independent sets of size is not too small, we determine an explicit
interval of length that contains with high
probability. Both results have profound consequences: the former is at the core
of the intriguing Zigzag Conjecture on the distribution of
and justifies one of its main hypotheses, while the latter is an important
ingredient in the proof of a non-concentration result for
that is conjectured to be optimal.
These two results are consequences of a more general statement. We consider a
class of colourings that we call tame, and provide tight bounds for the
probability of existence of such colourings via a delicate second moment
argument. We then apply those bounds to the two aforementioned cases. As a
further consequence of our main result, we prove two-point concentration of the
equitable chromatic number of G(n,m).Comment: 75 page
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