2,124 research outputs found
Detecting degree symmetries in networks
The surrounding of a vertex in a network can be more or less symmetric. We
derive measures of a specific kind of symmetry of a vertex which we call degree
symmetry -- the property that many paths going out from a vertex have
overlapping degree sequences. These measures are evaluated on artificial and
real networks. Specifically we consider vertices in the human metabolic
network. We also measure the average degree-symmetry coefficient for different
classes of real-world network. We find that most studied examples are weakly
positively degree-symmetric. The exceptions are an airport network (having a
negative degree-symmetry coefficient) and one-mode projections of social
affiliation networks that are rather strongly degree-symmetric
Network reachability of real-world contact sequences
We use real-world contact sequences, time-ordered lists of contacts from one
person to another, to study how fast information or disease can spread across
network of contacts. Specifically we measure the reachability time -- the
average shortest time for a series of contacts to spread information between a
reachable pair of vertices (a pair where a chain of contacts exists leading
from one person to the other) -- and the reachability ratio -- the fraction of
reachable vertex pairs. These measures are studied using conditional uniform
graph tests. We conclude, among other things, that the network reachability
depends much on a core where the path lengths are short and communication
frequent, that clustering of the contacts of an edge in time tend to decrease
the reachability, and that the order of the contacts really do make sense for
dynamical spreading processes.Comment: (v2: fig. 1 fixed
Discrete concavity and the half-plane property
Murota et al. have recently developed a theory of discrete convex analysis
which concerns M-convex functions on jump systems. We introduce here a family
of M-concave functions arising naturally from polynomials (over a field of
generalized Puiseux series) with prescribed non-vanishing properties. This
family contains several of the most studied M-concave functions in the
literature. In the language of tropical geometry we study the tropicalization
of the space of polynomials with the half-plane property, and show that it is
strictly contained in the space of M-concave functions. We also provide a short
proof of Speyer's hive theorem which he used to give a new proof of Horn's
conjecture on eigenvalues of sums of Hermitian matrices.Comment: 14 pages. The proof of Theorem 4 is corrected
Neutral theory of chemical reaction networks
To what extent do the characteristic features of a chemical reaction network
reflect its purpose and function? In general, one argues that correlations
between specific features and specific functions are key to understanding a
complex structure. However, specific features may sometimes be neutral and
uncorrelated with any system-specific purpose, function or causal chain. Such
neutral features are caused by chance and randomness. Here we compare two
classes of chemical networks: one that has been subjected to biological
evolution (the chemical reaction network of metabolism in living cells) and one
that has not (the atmospheric planetary chemical reaction networks). Their
degree distributions are shown to share the very same neutral
system-independent features. The shape of the broad distributions is to a large
extent controlled by a single parameter, the network size. From this
perspective, there is little difference between atmospheric and metabolic
networks; they are just different sizes of the same random assembling network.
In other words, the shape of the degree distribution is a neutral
characteristic feature and has no functional or evolutionary implications in
itself; it is not a matter of life and death.Comment: 13 pages, 8 figure
Higher order corrections to the Newtonian potential in the Randall-Sundrum model
The general formalism for calculating the Newtonian potential in fine-tuned
or critical Randall-Sundrum braneworlds is outlined. It is based on using the
full tensor structure of the graviton propagator. This approach avoids the
brane-bending effect arising from calculating the potential for a point source.
For a single brane, this gives a clear understanding of the disputed overall
factor 4/3 entering the correction. The result can be written on a compact form
which is evaluated to high accuracy for both short and large distances.Comment: 12 pages, LaTeX2e with RevTeX4, 3 postscript figures; Minor
corrections, references update
Majority-vote model on hyperbolic lattices
We study the critical properties of a non-equilibrium statistical model, the
majority-vote model, on heptagonal and dual heptagonal lattices. Such lattices
have the special feature that they only can be embedded in negatively curved
surfaces. We find, by using Monte Carlo simulations and finite-size analysis,
that the critical exponents , and are different
from those of the majority-vote model on regular lattices with periodic
boundary condition, which belongs to the same universality class as the
equilibrium Ising model. The exponents are also from those of the Ising model
on a hyperbolic lattice. We argue that the disagreement is caused by the
effective dimensionality of the hyperbolic lattices. By comparative studies, we
find that the critical exponents of the majority-vote model on hyperbolic
lattices satisfy the hyperscaling relation
, where is an
effective dimension of the lattice. We also investigate the effect of boundary
nodes on the ordering process of the model.Comment: 8 pages, 9 figure
Relativistic Images in Randall-Sundrum II Braneworld Lensing
In this paper, we explore the properties of gravitational lensing by black
holes in the Randall-Sundrum II braneworld. We use numerical techniques to
calculate lensing observables using the Tidal Reissner-Nordstrom (TRN) and
Garriga-Tanaka metrics to examine supermassive black holes and primordial black
holes. We introduce a new way tp parameterize tidal charge in the TRN metric
which results in a large increase in image magnifications for braneworld
primordial black holes compared to their 4 dimensional analogues. Finally, we
offer a mathematical analysis that allows us to analyze the validity of the
logarithmic approximation of the bending angle for any static, spherically
symmetric metric. We apply this to the TRN metric and show that it is valid for
any amount of tidal charge.Comment: 13 pages, 3 figures; Accepted for Publication in Physical Review
Core-periphery organization of complex networks
Networks may, or may not, be wired to have a core that is both itself densely
connected and central in terms of graph distance. In this study we propose a
coefficient to measure if the network has such a clear-cut core-periphery
dichotomy. We measure this coefficient for a number of real-world and model
networks and find that different classes of networks have their characteristic
values. For example do geographical networks have a strong core-periphery
structure, while the core-periphery structure of social networks (despite their
positive degree-degree correlations) is rather weak. We proceed to study radial
statistics of the core, i.e. properties of the n-neighborhoods of the core
vertices for increasing n. We find that almost all networks have unexpectedly
many edges within n-neighborhoods at a certain distance from the core
suggesting an effective radius for non-trivial network processes
Exploring the assortativity-clustering space of a network's degree sequence
Nowadays there is a multitude of measures designed to capture different
aspects of network structure. To be able to say if the structure of certain
network is expected or not, one needs a reference model (null model). One
frequently used null model is the ensemble of graphs with the same set of
degrees as the original network. In this paper we argue that this ensemble can
be more than just a null model -- it also carries information about the
original network and factors that affect its evolution. By mapping out this
ensemble in the space of some low-level network structure -- in our case those
measured by the assortativity and clustering coefficients -- one can for
example study how close to the valid region of the parameter space the observed
networks are. Such analysis suggests which quantities are actively optimized
during the evolution of the network. We use four very different biological
networks to exemplify our method. Among other things, we find that high
clustering might be a force in the evolution of protein interaction networks.
We also find that all four networks are conspicuously robust to both random
errors and targeted attacks
On the half-plane property and the Tutte group of a matroid
A multivariate polynomial is stable if it is non-vanishing whenever all
variables have positive imaginary parts. A matroid has the weak half-plane
property (WHPP) if there exists a stable polynomial with support equal to the
set of bases of the matroid. If the polynomial can be chosen with all of its
nonzero coefficients equal to one then the matroid has the half-plane property
(HPP). We describe a systematic method that allows us to reduce the WHPP to the
HPP for large families of matroids. This method makes use of the Tutte group of
a matroid. We prove that no projective geometry has the WHPP and that a binary
matroid has the WHPP if and only if it is regular. We also prove that T_8 and
R_9 fail to have the WHPP.Comment: 8 pages. To appear in J. Combin. Theory Ser.
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