33,387 research outputs found
First- and second-order phase transitions in Ising models on small world networks, simulations and comparison with an effective field theory
We perform simulations of random Ising models defined over small-world
networks and we check the validity and the level of approximation of a recently
proposed effective field theory. Simulations confirm a rich scenario with the
presence of multicritical points with first- or second-order phase transitions.
In particular, for second-order phase transitions, independent of the dimension
d_0 of the underlying lattice, the exact predictions of the theory in the
paramagnetic regions, such as the location of critical surfaces and correlation
functions, are verified. Quite interestingly, we verify that the
Edwards-Anderson model with d_0=2 is not thermodynamically stable under graph
noise.Comment: 12 pages, 12 figures, 1 tabl
On Describing Multivariate Skewness: A Directional Approach
Most multivariate measures of skewness in the literature measure the overall skewness of a distribution. While these measures are perfectly adequate for testing the hypothesis of distributional symmetry, their relevance for describing skewed distributions is less obvious. In this article, we consider the problem of characterising the skewness of multivariate distributions. We define directional skewness as the skewness along a direction and analyse parametric classes of skewed distributions using measures based on directional skewness. The analysis brings further insight into the classes, allowing for a more informed selection of particular classes for particular applications. In the context of Bayesian linear regression under skewed error we use the concept of directional skewness twice. First in the elicitation of a prior on the parameters of the error distribution, and then in the analysis of the skewness of the posterior distribution of the regression residuals.Bayesian methods, Multivariate distribution, Multivariate regression, Prior elicitation, Skewness.
Instance Space of the Number Partitioning Problem
Within the replica framework we study analytically the instance space of the
number partitioning problem. This classic integer programming problem consists
of partitioning a sequence of N positive real numbers \{a_1, a_2,..., a_N}
(the instance) into two sets such that the absolute value of the difference of
the sums of over the two sets is minimized. We show that there is an
upper bound to the number of perfect partitions (i.e. partitions
for which that difference is zero) and characterize the statistical properties
of the instances for which those partitions exist. In particular, in the case
that the two sets have the same cardinality (balanced partitions) we find
. Moreover, we show that the disordered model resulting from hte
instance space approach can be viewed as a model of replicators where the
random interactions are given by the Hebb rule.Comment: 7 page
Structure of potentials with Higgs doublets
Extensions of the Standard Model with Higgs doublets are simple
extensions presenting a rich mathematical structure. An underlying Minkowski
structure emerges from the study of both variable space and parameter space.
The former can be completely parametrized in terms of two future lightlike
Minkowski vectors with spatial parts forming an angle whose cosine is
. For the parameter space, the Minkowski parametrization enables
one to impose sufficient conditions for bounded below potentials, characterize
certain classes of local minima and distinguish charge breaking vacua from
neutral vacua. A particular class of neutral minima presents a degenerate mass
spectrum for the physical charged Higgs bosons.Comment: 11 pages. Revtex4. Typos corrected. Few comments adde
Integral Inequalities and their Applications to the Calculus of Variations on Time Scales
We discuss the use of inequalities to obtain the solution of certain
variational problems on time scales.Comment: To appear in Mathematical Inequalities & Applications
(http://mia.ele-math.com). Accepted: 14.01.201
To be or not to be digital, that's the question! Implications for firm innovation capability and performance
Digital transformation emerges today as a process for attaining competitive advantages and company differentiation. However, what are the implications of these digital processes for the innovative capability and performance of companies? This study seeks to contribute towards a better understanding of this framework, analysing the factors that lead companies to adopt new digital processes and their consequences in terms of innovation capability and performance. Using a sample of 940 companies and recourse to multivariate statistical analysis, we conclude that the profile of the owner/manager and the adoption of new digital processes reflect in the greater competitiveness of these (digital) companies.info:eu-repo/semantics/acceptedVersio
Minkowski space structure of the Higgs potential in 2HDM: II. Minima, symmetries, and topology
We continue to explore the consequences of the recently discovered Minkowski
space structure of the Higgs potential in the two-Higgs-doublet model. Here, we
focus on the vacuum properties. The search for extrema of the Higgs potential
is reformulated in terms of 3-quadrics in the 3+1-dimensional Minkowski space.
We prove that 2HDM cannot have more than two local minima in the orbit space
and that a twice-degenerate minimum can arise only via spontaneous violation of
a discrete symmetry of the Higgs potential. Investigating topology of the
3-quadrics, we give concise criteria for existence of non-contractible paths in
the Higgs orbit space. We also study explicit symmetries of the Higgs
potential/lagrangian and their spontaneous violation from a wider perspective
than usual.Comment: 27 pages, 5 figure
Magnetized Accretion-Ejection Structures: 2.5D MHD simulations of continuous Ideal Jet launching from resistive accretion disks
We present numerical magnetohydrodynamic (MHD) simulations of a magnetized
accretion disk launching trans-Alfvenic jets. These simulations, performed in a
2.5 dimensional time-dependent polytropic resistive MHD framework, model a
resistive accretion disk threaded by an initial vertical magnetic field. The
resistivity is only important inside the disk, and is prescribed as eta =
alpha_m V_AH exp(-2Z^2/H^2), where V_A stands for Alfven speed, H is the disk
scale height and the coefficient alpha_m is smaller than unity. By performing
the simulations over several tens of dynamical disk timescales, we show that
the launching of a collimated outflow occurs self-consistently and the ejection
of matter is continuous and quasi-stationary. These are the first ever
simulations of resistive accretion disks launching non-transient ideal MHD
jets. Roughly 15% of accreted mass is persistently ejected. This outflow is
safely characterized as a jet since the flow becomes super-fastmagnetosonic,
well-collimated and reaches a quasi-stationary state. We present a complete
illustration and explanation of the `accretion-ejection' mechanism that leads
to jet formation from a magnetized accretion disk. In particular, the magnetic
torque inside the disk brakes the matter azimuthally and allows for accretion,
while it is responsible for an effective magneto-centrifugal acceleration in
the jet. As such, the magnetic field channels the disk angular momentum and
powers the jet acceleration and collimation. The jet originates from the inner
disk region where equipartition between thermal and magnetic forces is
achieved. A hollow, super-fastmagnetosonic shell of dense material is the
natural outcome of the inwards advection of a primordial field.Comment: ApJ (in press), 32 pages, Higher quality version available at
http://www-laog.obs.ujf-grenoble.fr/~fcass
- …