23,785 research outputs found
Evaluating links through spectral decomposition
Spectral decomposition has been rarely used to investigate complex networks.
In this work we apply this concept in order to define two types of
link-directed attacks while quantifying their respective effects on the
topology. Several other types of more traditional attacks are also adopted and
compared. These attacks had substantially diverse effects, depending on each
specific network (models and real-world structures). It is also showed that the
spectral-based attacks have special effect in affecting the transitivity of the
networks
Evaluating Overfit and Underfit in Models of Network Community Structure
A common data mining task on networks is community detection, which seeks an
unsupervised decomposition of a network into structural groups based on
statistical regularities in the network's connectivity. Although many methods
exist, the No Free Lunch theorem for community detection implies that each
makes some kind of tradeoff, and no algorithm can be optimal on all inputs.
Thus, different algorithms will over or underfit on different inputs, finding
more, fewer, or just different communities than is optimal, and evaluation
methods that use a metadata partition as a ground truth will produce misleading
conclusions about general accuracy. Here, we present a broad evaluation of over
and underfitting in community detection, comparing the behavior of 16
state-of-the-art community detection algorithms on a novel and structurally
diverse corpus of 406 real-world networks. We find that (i) algorithms vary
widely both in the number of communities they find and in their corresponding
composition, given the same input, (ii) algorithms can be clustered into
distinct high-level groups based on similarities of their outputs on real-world
networks, and (iii) these differences induce wide variation in accuracy on link
prediction and link description tasks. We introduce a new diagnostic for
evaluating overfitting and underfitting in practice, and use it to roughly
divide community detection methods into general and specialized learning
algorithms. Across methods and inputs, Bayesian techniques based on the
stochastic block model and a minimum description length approach to
regularization represent the best general learning approach, but can be
outperformed under specific circumstances. These results introduce both a
theoretically principled approach to evaluate over and underfitting in models
of network community structure and a realistic benchmark by which new methods
may be evaluated and compared.Comment: 22 pages, 13 figures, 3 table
Detecting communities of triangles in complex networks using spectral optimization
The study of the sub-structure of complex networks is of major importance to
relate topology and functionality. Many efforts have been devoted to the
analysis of the modular structure of networks using the quality function known
as modularity. However, generally speaking, the relation between topological
modules and functional groups is still unknown, and depends on the semantic of
the links. Sometimes, we know in advance that many connections are transitive
and, as a consequence, triangles have a specific meaning. Here we propose the
study of the modular structure of networks considering triangles as the
building blocks of modules. The method generalizes the standard modularity and
uses spectral optimization to find its maximum. We compare the partitions
obtained with those resulting from the optimization of the standard modularity
in several real networks. The results show that the information reported by the
analysis of modules of triangles complements the information of the classical
modularity analysis.Comment: Computer Communications (in press
Semiclassical approach to discrete symmetries in quantum chaos
We use semiclassical methods to evaluate the spectral two-point correlation
function of quantum chaotic systems with discrete geometrical symmetries. The
energy spectra of these systems can be divided into subspectra that are
associated to irreducible representations of the corresponding symmetry group.
We show that for (spinless) time reversal invariant systems the statistics
inside these subspectra depend on the type of irreducible representation. For
real representations the spectral statistics agree with those of the Gaussian
Orthogonal Ensemble (GOE) of Random Matrix Theory (RMT), whereas complex
representations correspond to the Gaussian Unitary Ensemble (GUE). For systems
without time reversal invariance all subspectra show GUE statistics. There are
no correlations between non-degenerate subspectra. Our techniques generalize
recent developments in the semiclassical approach to quantum chaos allowing one
to obtain full agreement with the two-point correlation function predicted by
RMT, including oscillatory contributions.Comment: 26 pages, 8 Figure
Improving Entity Retrieval on Structured Data
The increasing amount of data on the Web, in particular of Linked Data, has
led to a diverse landscape of datasets, which make entity retrieval a
challenging task. Explicit cross-dataset links, for instance to indicate
co-references or related entities can significantly improve entity retrieval.
However, only a small fraction of entities are interlinked through explicit
statements. In this paper, we propose a two-fold entity retrieval approach. In
a first, offline preprocessing step, we cluster entities based on the
\emph{x--means} and \emph{spectral} clustering algorithms. In the second step,
we propose an optimized retrieval model which takes advantage of our
precomputed clusters. For a given set of entities retrieved by the BM25F
retrieval approach and a given user query, we further expand the result set
with relevant entities by considering features of the queries, entities and the
precomputed clusters. Finally, we re-rank the expanded result set with respect
to the relevance to the query. We perform a thorough experimental evaluation on
the Billions Triple Challenge (BTC12) dataset. The proposed approach shows
significant improvements compared to the baseline and state of the art
approaches
Mayer expansion of the Nekrasov pre potential: the subleading -order
The Mayer cluster expansion technique is applied to the Nekrasov instanton
partition function of super Yang-Mills. The
subleading small -correction to the Nekrasov-Shatashvili limiting
value of the prepotential is determined by a detailed analysis of all the
one-loop diagrams. Indeed, several types of contributions can be distinguished
according to their origin: long range interaction or potential expansion,
clusters self-energy, internal structure, one-loop cyclic diagrams, etc.. The
field theory result derived more efficiently in [1], under some minor technical
assumptions, receives here definite confirmation thanks to several remarkable
cancellations: in this way, we may infer the validity of these assumptions for
further computations in the field theoretical approach.Comment: 29 pages, 9 figure
Opportunistic Third-Party Backhaul for Cellular Wireless Networks
With high capacity air interfaces and large numbers of small cells, backhaul
-- the wired connectivity to base stations -- is increasingly becoming the cost
driver in cellular wireless networks. One reason for the high cost of backhaul
is that capacity is often purchased on leased lines with guaranteed rates
provisioned to peak loads. In this paper, we present an alternate
\emph{opportunistic backhaul} model where third parties provide base stations
and backhaul connections and lease out excess capacity in their networks to the
cellular provider when available, presumably at significantly lower costs than
guaranteed connections. We describe a scalable architecture for such
deployments using open access femtocells, which are small plug-and-play base
stations that operate in the carrier's spectrum but can connect directly into
the third party provider's wired network. Within the proposed architecture, we
present a general user association optimization algorithm that enables the
cellular provider to dynamically determine which mobiles should be assigned to
the third-party femtocells based on the traffic demands, interference and
channel conditions and third-party access pricing. Although the optimization is
non-convex, the algorithm uses a computationally efficient method for finding
approximate solutions via dual decomposition. Simulations of the deployment
model based on actual base station locations are presented that show that large
capacity gains are achievable if adoption of third-party, open access
femtocells can reach even a small fraction of the current market penetration of
WiFi access points.Comment: 9 pages, 6 figure
Eigenvalus of Casimir Invariants for Type-I Quantum Superalgebras
We present the eigenvalues of the Casimir invariants for the type I quantum
superalgebras on any irreducible highest weight module.Comment: 13 pages, AmsTex file; to appear in Lett. Math. Phy
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