25 research outputs found
Stability of Influence Maximization
The present article serves as an erratum to our paper of the same title,
which was presented and published in the KDD 2014 conference. In that article,
we claimed falsely that the objective function defined in Section 1.4 is
non-monotone submodular. We are deeply indebted to Debmalya Mandal, Jean
Pouget-Abadie and Yaron Singer for bringing to our attention a counter-example
to that claim.
Subsequent to becoming aware of the counter-example, we have shown that the
objective function is in fact NP-hard to approximate to within a factor of
for any .
In an attempt to fix the record, the present article combines the problem
motivation, models, and experimental results sections from the original
incorrect article with the new hardness result. We would like readers to only
cite and use this version (which will remain an unpublished note) instead of
the incorrect conference version.Comment: Erratum of Paper "Stability of Influence Maximization" which was
presented and published in the KDD1
Availability of food resources and habitat structure shape the individualâresource network of a Neotropical marsupial
1. Spatial and temporal variation in networks has been reported in different studies.
However, the many effects of habitat structure and food resource availability variation
on network structures have remained poorly investigated, especially in individualâ
based networks. This approach can shed light on individual specialization
of resource use and how habitat variations shape trophic interactions.
2. To test hypotheses related to habitat variability on trophic interactions, we investigated
seasonal and spatial variation in network structure of four populations of
the marsupial Gracilinanus agilis in the highly seasonal tropical savannas of the
Brazilian Cerrado.
3. We evaluated such variation with network nestedness and modularity considering
both coolâdry and warmâwet seasons, and related such variations with food resource
availability and habitat structure (considered in the present study as environmental
variation) in four sites of savanna woodland forest.
4. Network analyses showed that modularity (but not nestedness) was consistently
lower during the coolâdry season in all G. agilis populations. Our results indicated
that nestedness is related to habitat structure, showing that this metric increases
in sites with thick and spaced trees. On the other hand, modularity was positively
related to diversity of arthropods and abundance of fruits.
5. We propose that the relationship between nestedness and habitat structure is an
outcome of individual variation in the vertical space and food resource use by
G. agilis in sites with thick and spaced trees. Moreover, individual specialization in
resourceârich and populationâdense periods possibly increased the network modularity
of G. agilis. Therefore, our study reveals that environment variability considering
spatial and temporal components is important for shaping network
structure of populations
Qualitative Comparison of Community Detection Algorithms
Community detection is a very active field in complex networks analysis,
consisting in identifying groups of nodes more densely interconnected
relatively to the rest of the network. The existing algorithms are usually
tested and compared on real-world and artificial networks, their performance
being assessed through some partition similarity measure. However, artificial
networks realism can be questioned, and the appropriateness of those measures
is not obvious. In this study, we take advantage of recent advances concerning
the characterization of community structures to tackle these questions. We
first generate networks thanks to the most realistic model available to date.
Their analysis reveals they display only some of the properties observed in
real-world community structures. We then apply five community detection
algorithms on these networks and find out the performance assessed
quantitatively does not necessarily agree with a qualitative analysis of the
identified communities. It therefore seems both approaches should be applied to
perform a relevant comparison of the algorithms.Comment: DICTAP 2011, The International Conference on Digital Information and
Communication Technology and its Applications, Dijon : France (2011
Micro-behaviors and structural properties of knowledge networks: toward a 'one size fits one'cluster policy
The economic returns of cluster policies have been recently called into question. Based on a âone size fits allâ approach consisting in boosting R&D collaborations and reinforcing network density, cluster policies are suspected to have failed in reaching their objectives. The paper proposes to go back to the micro foundations of clusters in order to disentangle the links between the long run performance of clusters and their structural properties. We use a simple agent-based model to shed light on how individual motives to build knowledge relationships can give rise to emerging structures with different properties, which imply different innovation and renewal capacities. The simulation results are discussed in a micro-macro perspective, and motivate suggestions to reorient cluster policy guidelines towards more targeted public-funded incentives for R&D collaboration
Untangling a Polygon
The following problem was raised by M. Watanabe. Let P be a self-intersecting closed polygon with n vertices in general position. How manys steps does it take to untangle P, i.e., to turn it into a simple polygon, if in each step we can arbitrarily relocate one of its vertices. It is shown that in some cases one has to move all but at most O((n log n)2/3) vertices. On the other hand, every polygon can be untangled in at most n â Ω(ân) steps. Some related questions are also considered