3,508 research outputs found
Uniform sampling of steady states in metabolic networks: heterogeneous scales and rounding
The uniform sampling of convex polytopes is an interesting computational
problem with many applications in inference from linear constraints, but the
performances of sampling algorithms can be affected by ill-conditioning. This
is the case of inferring the feasible steady states in models of metabolic
networks, since they can show heterogeneous time scales . In this work we focus
on rounding procedures based on building an ellipsoid that closely matches the
sampling space, that can be used to define an efficient hit-and-run (HR) Markov
Chain Monte Carlo. In this way the uniformity of the sampling of the convex
space of interest is rigorously guaranteed, at odds with non markovian methods.
We analyze and compare three rounding methods in order to sample the feasible
steady states of metabolic networks of three models of growing size up to
genomic scale. The first is based on principal component analysis (PCA), the
second on linear programming (LP) and finally we employ the lovasz ellipsoid
method (LEM). Our results show that a rounding procedure is mandatory for the
application of the HR in these inference problem and suggest that a combination
of LEM or LP with a subsequent PCA perform the best. We finally compare the
distributions of the HR with that of two heuristics based on the Artificially
Centered hit-and-run (ACHR), gpSampler and optGpSampler. They show a good
agreement with the results of the HR for the small network, while on genome
scale models present inconsistencies.Comment: Replacement with major revision
The Interrupted Power Law and The Size of Shadow Banking
Using public data (Forbes Global 2000) we show that the asset sizes for the
largest global firms follow a Pareto distribution in an intermediate range,
that is ``interrupted'' by a sharp cut-off in its upper tail, where it is
totally dominated by financial firms. This flattening of the distribution
contrasts with a large body of empirical literature which finds a Pareto
distribution for firm sizes both across countries and over time. Pareto
distributions are generally traced back to a mechanism of proportional random
growth, based on a regime of constant returns to scale. This makes our findings
of an ``interrupted'' Pareto distribution all the more puzzling, because we
provide evidence that financial firms in our sample should operate in such a
regime. We claim that the missing mass from the upper tail of the asset size
distribution is a consequence of shadow banking activity and that it provides
an (upper) estimate of the size of the shadow banking system. This estimate --
which we propose as a shadow banking index -- compares well with estimates of
the Financial Stability Board until 2009, but it shows a sharper rise in shadow
banking activity after 2010. Finally, we propose a proportional random growth
model that reproduces the observed distribution, thereby providing a
quantitative estimate of the intensity of shadow banking activity.Comment: 12 pages, 5 figures, 2 tables. To appear in Plos ONE 201
Reading entanglement in terms of spin configurations in quantum magnets
We consider a quantum many-body system made of interacting
spins on a lattice, and develop a formalism which allows to extract, out of
conventional magnetic observables, the quantum probabilities for any selected
spin pair to be in maximally entangled or factorized two-spin states. This
result is used in order to capture the meaning of entanglement properties in
terms of magnetic behavior. In particular, we consider the concurrence between
two spins and show how its expression extracts information on the presence of
bipartite entanglement out of the probability distributions relative to
specific sets of two-spin quantum states. We apply the above findings to the
antiferromagnetic Heisenberg model in a uniform magnetic field, both on a chain
and on a two-leg ladder. Using Quantum Monte Carlo simulations, we obtain the
above probability distributions and the associated entanglement, discussing
their evolution under application of the field.Comment: Final version, to appear in European Physical Journal
Planar master integrals for the two-loop light-fermion electroweak corrections to Higgs plus jet production
We present the analytic calculation of the planar master integrals which
contribute to compute the two-loop light-fermion electroweak corrections to the
production of a Higgs boson in association with a jet in gluon-gluon fusion.
The complete dependence on the electroweak-boson mass is retained. The master
integrals are evaluated by means of the differential equations method and the
analytic results are expressed in terms of multiple polylogarithms up to weight
four.Comment: 21 pages, ancillary file
Impact of urbanization on predator and parasitoid insects at multiple spatial scales
Landscapes are becoming increasingly urbanized, causing loss and fragmentation of natural habitats, with potentially negative effects on biodiversity. Insects are among the organisms with the largest diversity in urbanized environments. Here, we sampled predator (Ampulicidae, Sphecidae and Crabronidae) and parasitoid (Tachinidae) flower-visiting insects in 36 sites in the city of Rome (Italy). Although the diversity of herbivorous insects in urban areas mostly depends on the availability of flowering plants and nesting sites, predators and parasitoids generally require a larger number of resources during their life cycle, and are expected to be particularly influenced by urbanization. As flower-visitors can easily move between habitat patches, the effect of urbanization was tested at multiple spatial scales (local, landscape and sub-regional). We found that urbanization influenced predator and parasitoid flower-visitors at all three spatial scales. At the local scale, streets and buildings negatively influenced evenness of predators and species richness and abundance of parasitoids probably acting as dispersal barrier. At the landscape scale, higher percentage of urban decreased predator abundance, while increasing their evenness, suggesting an increase in generalist and highly mobile species. Area and compactness (i.e. Contiguity index) of urban green interactively influenced predator communities, whereas evenness of parasitoids increased with increasing Contiguity index. At the sub-regional scale, species richness and abundance of predators increased with increasing distance from the city center. Compared to previous studies testing the effect of urbanization, we found little variation in species richness, abundance and evenness along our urbanization gradient. The current insect fauna has been probably selected for its tolerance to habitat loss and fragmentation, being the result of the intensive anthropogenic alteration occurred in the area in the last centuries. Conservation strategies aimed at predator and parasitoid flying insects have to take in account variables at multiple spatial-scales, as well as the complementarity of resources across the landscape
Transforming nonlocality into frequency dependence: a shortcut to spectroscopy
Measurable spectra are theoretically very often derived from complicated
many-body Green's functions. In this way, one calculates much more information
than actually needed. Here we present an in principle exact approach to
construct effective potentials and kernels for the direct calculation of
electronic spectra. In particular, the potential that yields the spectral
function needed to describe photoemission turns out to be dynamical but {\it
local} and {\it real}. As example we illustrate this ``photoemission
potential'' for sodium and aluminium, modelled as homogeneous electron gas, and
discuss in particular its frequency dependence stemming from the nonlocality of
the corresponding self-energy. We also show that our approach leads to a very
short derivation of a kernel that is known to well describe absorption and
energy-loss spectra of a wide range of materials
Master Integrals for the two-loop, non-planar QCD corrections to top-quark pair production in the quark-annihilation channel
We present the analytic calculation of the Master Integrals for the two-loop,
non-planar topologies that enter the calculation of the amplitude for top-quark
pair hadroproduction in the quark-annihilation channel. Using the method of
differential equations, we expand the integrals in powers of the dimensional
regulator and determine the expansion coefficients in terms of
generalized harmonic polylogarithms of two dimensionless variables through to
weight four.Comment: 28 pages, 2 figures, ancillary files include
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