11,452 research outputs found
Different approaches to community detection
A precise definition of what constitutes a community in networks has remained
elusive. Consequently, network scientists have compared community detection
algorithms on benchmark networks with a particular form of community structure
and classified them based on the mathematical techniques they employ. However,
this comparison can be misleading because apparent similarities in their
mathematical machinery can disguise different reasons for why we would want to
employ community detection in the first place. Here we provide a focused review
of these different motivations that underpin community detection. This
problem-driven classification is useful in applied network science, where it is
important to select an appropriate algorithm for the given purpose. Moreover,
highlighting the different approaches to community detection also delineates
the many lines of research and points out open directions and avenues for
future research.Comment: 14 pages, 2 figures. Written as a chapter for forthcoming Advances in
network clustering and blockmodeling, and based on an extended version of The
many facets of community detection in complex networks, Appl. Netw. Sci. 2: 4
(2017) by the same author
Self-adjoint symmetry operators connected with the magnetic Heisenberg ring
We consider symmetry operators a from the group ring C[S_N] which act on the
Hilbert space H of the 1D spin-1/2 Heisenberg magnetic ring with N sites. We
investigate such symmetry operators a which are self-adjoint (in a sence
defined in the paper) and which yield consequently observables of the
Heisenberg model. We prove the following results: (i) One can construct a
self-adjoint idempotent symmetry operator from every irreducible character of
every subgroup of S_N. This leads to a big manifold of observables. In
particular every commutation symmetry yields such an idempotent. (ii) The set
of all generating idempotents of a minimal right ideal R of C[S_N] contains one
and only one idempotent which ist self-adjoint. (iii) Every self-adjoint
idempotent e can be decomposed into primitive idempotents e = f_1 + ... + f_k
which are also self-adjoint and pairwise orthogonal. We give a computer
algorithm for the calculation of such decompositions. Furthermore we present 3
additional algorithms which are helpful for the calculation of self-adjoint
operators by means of discrete Fourier transforms of S_N. In our investigations
we use computer calculations by means of our Mathematica packages PERMS and
HRing.Comment: 13 page
The soft function for color octet production at threshold
We evaluate the next-to-next-to-leading order soft function for the
production of a massive color octet state at rest in the collision of two
massless colored partons in either the fundamental or the adjoint
representation. The main application of our result is the determination of the
threshold expansion of the heavy-quark pair-production cross sections in the
quark annihilation and gluon fusion channels. We discuss the factorization
necessary for this purpose and explain the relationship between hard functions
and virtual amplitudes.Comment: 18 pages, 5 figures, references added, matches published versio
Generalized modularity matrices
Various modularity matrices appeared in the recent literature on network
analysis and algebraic graph theory. Their purpose is to allow writing as
quadratic forms certain combinatorial functions appearing in the framework of
graph clustering problems. In this paper we put in evidence certain common
traits of various modularity matrices and shed light on their spectral
properties that are at the basis of various theoretical results and practical
spectral-type algorithms for community detection
Virtual amplitudes and threshold behaviour of hadronic top-quark pair-production cross sections
We present the two-loop virtual amplitudes for the production of a top-quark
pair in gluon fusion. The evaluation method is based on a numerical solution of
differential equations for master integrals in function of the quark velocity
and scattering angle starting from a boundary at high-energy. The results are
given for the renormalized infrared finite remainders on a large grid and have
recently been used in the calculation of the total cross sections at the
next-to-next-to-leading order. For convenience, we also give the known results
for the quark annihilation case on the same grid. Outside of the kinematical
range covered by the grid, we provide threshold and high-energy expansions.
From expansions of the two-loop virtual amplitudes, we determine the
threshold behavior of the total cross sections at next-to-next-to-leading order
for the quark annihilation and gluon fusion channels including previously
unknown constant terms. In our analysis of the quark annihilation channel, we
uncover the presence of a velocity enhanced logarithm of Coulombic origin,
which was missed in a previous study.Comment: 28 pages, 3 figures, 4 tables, results for the virtual amplitudes
attached in Mathematica forma
Enhancing network robustness for malicious attacks
In a recent work [Proc. Natl. Acad. Sci. USA 108, 3838 (2011)], the authors
proposed a simple measure for network robustness under malicious attacks on
nodes. With a greedy algorithm, they found the optimal structure with respect
to this quantity is an onion structure in which high-degree nodes form a core
surrounded by rings of nodes with decreasing degree. However, in real networks
the failure can also occur in links such as dysfunctional power cables and
blocked airlines. Accordingly, complementary to the node-robustness measurement
(), we propose a link-robustness index (). We show that solely
enhancing cannot guarantee the improvement of . Moreover, the
structure of -optimized network is found to be entirely different from
that of onion network. In order to design robust networks resistant to more
realistic attack condition, we propose a hybrid greedy algorithm which takes
both the and into account. We validate the robustness of our
generated networks against malicious attacks mixed with both nodes and links
failure. Finally, some economical constraints for swapping the links in real
networks are considered and significant improvement in both aspects of
robustness are still achieved.Comment: 6 pages, 6 figure
The structure of algebraic covariant derivative curvature tensors
We use the Nash embedding theorem to construct generators for the space of
algebraic covariant derivative curvature tensors
On homotopies with triple points of classical knots
We consider a knot homotopy as a cylinder in 4-space. An ordinary triple
point of the cylinder is called {\em coherent} if all three branches
intersect at pairwise with the same index. A {\em triple unknotting} of a
classical knot is a homotopy which connects with the trivial knot and
which has as singularities only coherent triple points. We give a new formula
for the first Vassiliev invariant by using triple unknottings. As a
corollary we obtain a very simple proof of the fact that passing a coherent
triple point always changes the knot type. As another corollary we show that
there are triple unknottings which are not homotopic as triple unknottings even
if we allow more complicated singularities to appear in the homotopy of the
homotopy.Comment: 10 pages, 13 figures, bugs in figures correcte
Encoding dynamics for multiscale community detection: Markov time sweeping for the Map equation
The detection of community structure in networks is intimately related to
finding a concise description of the network in terms of its modules. This
notion has been recently exploited by the Map equation formalism (M. Rosvall
and C.T. Bergstrom, PNAS, 105(4), pp.1118--1123, 2008) through an
information-theoretic description of the process of coding inter- and
intra-community transitions of a random walker in the network at stationarity.
However, a thorough study of the relationship between the full Markov dynamics
and the coding mechanism is still lacking. We show here that the original Map
coding scheme, which is both block-averaged and one-step, neglects the internal
structure of the communities and introduces an upper scale, the `field-of-view'
limit, in the communities it can detect. As a consequence, Map is well tuned to
detect clique-like communities but can lead to undesirable overpartitioning
when communities are far from clique-like. We show that a signature of this
behavior is a large compression gap: the Map description length is far from its
ideal limit. To address this issue, we propose a simple dynamic approach that
introduces time explicitly into the Map coding through the analysis of the
weighted adjacency matrix of the time-dependent multistep transition matrix of
the Markov process. The resulting Markov time sweeping induces a dynamical
zooming across scales that can reveal (potentially multiscale) community
structure above the field-of-view limit, with the relevant partitions indicated
by a small compression gap.Comment: 10 pages, 6 figure
Transition from rotating waves to modulated rotating waves on the sphere
We study non-resonant and resonant Hopf bifurcation of a rotating wave in
SO(3)-equivariant reaction-diffusion systems on a sphere. We obtained reduced
differential equations on so(3), the characterization of modulated rotating
waves obtained by Hopf bifurcation of a rotating wave, as well as results
regarding the resonant case. Our main tools are the equivariant center manifold
reduction and the theory of Lie groups and Lie algebras, especially for the
group SO(3) of all rigid rotations on a sphere
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