13 research outputs found
Coagulation--fragmentation duality, Poisson--Dirichlet distributions and random recursive trees
In this paper we give a new example of duality between fragmentation and
coagulation operators. Consider the space of partitions of mass (i.e.,
decreasing sequences of nonnegative real numbers whose sum is 1) and the
two-parameter family of Poisson--Dirichlet distributions that take values in this space. We introduce families of
random fragmentation and coagulation operators and
, respectively, with the following property: if
the input to has
distribution, then the output has
distribution, while the reverse is true for .
This result may be proved using a subordinator representation and it provides a
companion set of relations to those of Pitman between and . Repeated
application of the operators gives rise to a family
of fragmentation chains. We show that these Markov chains can be encoded
naturally by certain random recursive trees, and use this representation to
give an alternative and more concrete proof of the coagulation--fragmentation
duality.Comment: Published at http://dx.doi.org/10.1214/105051606000000655 in the
Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute
of Mathematical Statistics (http://www.imstat.org
Normal limit laws for vertex degrees in randomly grown hooking networks and bipolar networks
We consider two types of random networks grown in blocks. Hooking networks
are grown from a set of graphs as blocks, each with a labelled vertex called a
hook. At each step in the growth of the network, a vertex called a latch is
chosen from the hooking network and a copy of one of the blocks is attached by
fusing its hook with the latch. Bipolar networks are grown from a set of
directed graphs as blocks, each with a single source and a single sink. At each
step in the growth of the network, an arc is chosen and is replaced with a copy
of one of the blocks. Using P\'olya urns, we prove normal limit laws for the
degree distributions of both networks. We extend previous results by allowing
for more than one block in the growth of the networks and by studying
arbitrarily large degrees.Comment: 28 pages, 6 figure
A Family of Tractable Graph Distances
Important data mining problems such as nearest-neighbor search and clustering
admit theoretical guarantees when restricted to objects embedded in a metric
space. Graphs are ubiquitous, and clustering and classification over graphs
arise in diverse areas, including, e.g., image processing and social networks.
Unfortunately, popular distance scores used in these applications, that scale
over large graphs, are not metrics and thus come with no guarantees. Classic
graph distances such as, e.g., the chemical and the CKS distance are arguably
natural and intuitive, and are indeed also metrics, but they are intractable:
as such, their computation does not scale to large graphs. We define a broad
family of graph distances, that includes both the chemical and the CKS
distance, and prove that these are all metrics. Crucially, we show that our
family includes metrics that are tractable. Moreover, we extend these distances
by incorporating auxiliary node attributes, which is important in practice,
while maintaining both the metric property and tractability.Comment: Extended version of paper appearing in SDM 201
Fringe trees, Crump-Mode-Jagers branching processes and -ary search trees
This survey studies asymptotics of random fringe trees and extended fringe
trees in random trees that can be constructed as family trees of a
Crump-Mode-Jagers branching process, stopped at a suitable time. This includes
random recursive trees, preferential attachment trees, fragmentation trees,
binary search trees and (more generally) -ary search trees, as well as some
other classes of random trees.
We begin with general results, mainly due to Aldous (1991) and Jagers and
Nerman (1984). The general results are applied to fringe trees and extended
fringe trees for several particular types of random trees, where the theory is
developed in detail. In particular, we consider fringe trees of -ary search
trees in detail; this seems to be new.
Various applications are given, including degree distribution, protected
nodes and maximal clades for various types of random trees. Again, we emphasise
results for -ary search trees, and give for example new results on protected
nodes in -ary search trees.
A separate section surveys results on height, saturation level, typical depth
and total path length, due to Devroye (1986), Biggins (1995, 1997) and others.
This survey contains well-known basic results together with some additional
general results as well as many new examples and applications for various
classes of random trees