310 research outputs found
Electric routing and concurrent flow cutting
We investigate an oblivious routing scheme, amenable to distributed
computation and resilient to graph changes, based on electrical flow. Our main
technical contribution is a new rounding method which we use to obtain a bound
on the L1->L1 operator norm of the inverse graph Laplacian. We show how this
norm reflects both latency and congestion of electric routing.Comment: 21 pages, 0 figures. To be published in Springer LNCS Book No. 5878,
Proceedings of The 20th International Symposium on Algorithms and Computation
(ISAAC'09
A simpler and more efficient algorithm for the next-to-shortest path problem
Given an undirected graph with positive edge lengths and two
vertices and , the next-to-shortest path problem is to find an -path
which length is minimum amongst all -paths strictly longer than the
shortest path length. In this paper we show that the problem can be solved in
linear time if the distances from and to all other vertices are given.
Particularly our new algorithm runs in time for general
graphs, which improves the previous result of time for sparse
graphs, and takes only linear time for unweighted graphs, planar graphs, and
graphs with positive integer edge lengths.Comment: Partial result appeared in COCOA201
Three-in-a-Tree in Near Linear Time
The three-in-a-tree problem is to determine if a simple undirected graph
contains an induced subgraph which is a tree connecting three given vertices.
Based on a beautiful characterization that is proved in more than twenty pages,
Chudnovsky and Seymour [Combinatorica 2010] gave the previously only known
polynomial-time algorithm, running in time, to solve the
three-in-a-tree problem on an -vertex -edge graph. Their three-in-a-tree
algorithm has become a critical subroutine in several state-of-the-art graph
recognition and detection algorithms.
In this paper we solve the three-in-a-tree problem in time,
leading to improved algorithms for recognizing perfect graphs and detecting
thetas, pyramids, beetles, and odd and even holes. Our result is based on a new
and more constructive characterization than that of Chudnovsky and Seymour. Our
new characterization is stronger than the original, and our proof implies a new
simpler proof for the original characterization. The improved characterization
gains the first factor in speed. The remaining improvement is based on
dynamic graph algorithms.Comment: 46 pages, 12 figures, accepted to STOC 202
Separating Hierarchical and General Hub Labelings
In the context of distance oracles, a labeling algorithm computes vertex
labels during preprocessing. An query computes the corresponding distance
from the labels of and only, without looking at the input graph. Hub
labels is a class of labels that has been extensively studied. Performance of
the hub label query depends on the label size. Hierarchical labels are a
natural special kind of hub labels. These labels are related to other problems
and can be computed more efficiently. This brings up a natural question of the
quality of hierarchical labels. We show that there is a gap: optimal
hierarchical labels can be polynomially bigger than the general hub labels. To
prove this result, we give tight upper and lower bounds on the size of
hierarchical and general labels for hypercubes.Comment: 11 pages, minor corrections, MFCS 201
Insights into the migration of the European Roller from ring recoveries
AbstractDespite recent advances in avian tracking technology, archival devices still present several limitations. Traditional ring recoveries provide a complementary method for studying migratory movements, particularly for cohorts of birds with a low return rate to the breeding site. Here we provide the first international analysis of ring recovery data in the European Roller Coracias garrulus, a long-distance migrant of conservation concern. Our data comprise 58 records of Rollers ringed during the breeding season and recovered during the non-breeding season. Most records come from Eastern Europe, half are of juveniles and over three quarters are of dead birds. Thus, ring recoveries provide migration data for cohorts of Rollersâjuveniles and unsuccessful migrantsâfor which no information currently exists, complementing recent tracking studies. Qualitatively, our results are consistent with direct tracking studies, illustrating a broad-front migration across the Mediterranean Basin in autumn and the use of the Arabian Peninsula by Rollers from eastern populations in spring. Autumn movements were, on average, in a more southerly direction for juveniles than adults, which were more easterly. Juvenile autumn recovery direction also appeared to be more variable than in adults, though this difference was not statistically significant. This is consistent with juveniles following a naĂŻve vector-based orientation program, and perhaps explains the âmoderateâ migratory connectivity previously described for the Roller. In the first (qualitative) analysis of Roller non-breeding season mortality, we highlight the high prevalence of shooting. The recovery age ratio was juvenile-biased in autumn but adult-biased in spring. Although not statistically significant, this difference points towards a higher non-breeding season mortality of juveniles than adults. Our study demonstrates the complementarity of ring recoveries to direct tracking, providing an insight into the migration of juvenile Rollers and non-breeding season mortality
Self-organized Emergence of Navigability on Small-World Networks
This paper mainly investigates why small-world networks are navigable and how
to navigate small-world networks. We find that the navigability can naturally
emerge from self-organization in the absence of prior knowledge about
underlying reference frames of networks. Through a process of information
exchange and accumulation on networks, a hidden metric space for navigation on
networks is constructed. Navigation based on distances between vertices in the
hidden metric space can efficiently deliver messages on small-world networks,
in which long range connections play an important role. Numerical simulations
further suggest that high cluster coefficient and low diameter are both
necessary for navigability. These interesting results provide profound insights
into scalable routing on the Internet due to its distributed and localized
requirements.Comment: 3 figure
Triangle Counting in Dynamic Graph Streams
Estimating the number of triangles in graph streams using a limited amount of
memory has become a popular topic in the last decade. Different variations of
the problem have been studied, depending on whether the graph edges are
provided in an arbitrary order or as incidence lists. However, with a few
exceptions, the algorithms have considered {\em insert-only} streams. We
present a new algorithm estimating the number of triangles in {\em dynamic}
graph streams where edges can be both inserted and deleted. We show that our
algorithm achieves better time and space complexity than previous solutions for
various graph classes, for example sparse graphs with a relatively small number
of triangles. Also, for graphs with constant transitivity coefficient, a common
situation in real graphs, this is the first algorithm achieving constant
processing time per edge. The result is achieved by a novel approach combining
sampling of vertex triples and sparsification of the input graph. In the course
of the analysis of the algorithm we present a lower bound on the number of
pairwise independent 2-paths in general graphs which might be of independent
interest. At the end of the paper we discuss lower bounds on the space
complexity of triangle counting algorithms that make no assumptions on the
structure of the graph.Comment: New version of a SWAT 2014 paper with improved result
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The transmission of trauma in refugee families: associations between intra-family trauma communication style, childrenâs attachment security and psychosocial adjustment
This study explores the transmission of trauma in 30 Middle Eastern refugee families in Denmark, where one or both parents were referred for treatment of PTSD symptoms and had non-traumatized children aged 4â9 years. The aim of the study was to explore potential risk and protective factors by examining the association between intra-family communication style regarding the parentsâ traumatic experiences from the past, childrenâs psychosocial adjustment and attachment security. A negative impact of parental trauma on children might be indicated, as childrenâs Total Difficulties Scores on the Strengths and Difficulties Questionnaire (SDQ) were significantly higher than the Danish norms. A negative association between childrenâs attachment security as measured by the Attachment and Traumatization Story Task and higher scores on the SDQ Total Difficulties Scale approached significance, suggesting that the transmission of trauma may be associated with disruptions in childrenâs attachment representations. Furthermore a significant association between parental trauma communication and childrenâs attachment style was found
Surface characterization and surface electronic structure of organic quasi-one-dimensional charge transfer salts
We have thoroughly characterized the surfaces of the organic charge-transfer
salts TTF-TCNQ and (TMTSF)2PF6 which are generally acknowledged as prototypical
examples of one-dimensional conductors. In particular x-ray induced
photoemission spectroscopy turns out to be a valuable non-destructive
diagnostic tool. We show that the observation of generic one-dimensional
signatures in photoemission spectra of the valence band close to the Fermi
level can be strongly affected by surface effects. Especially, great care must
be exercised taking evidence for an unusual one-dimensional many-body state
exclusively from the observation of a pseudogap.Comment: 11 pages, 12 figures, v2: minor changes in text and figure labellin
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