494 research outputs found
Special issue: Fractal functions and applications
This volume gathers some important advances in the fields of fractional calculus and fractal curves and functions. Fractional derivatives and integrals play an increasingly important role in applied science, and these types of models are ubiquitous in the current scientific literature. The references [1, 2] are devoted to fractional calculus and an application of it to a coronavirus spreading model. The first one studies three procedures of inverse Laplace Transforms: A Sinc–Thiele approximation, a Sinc and a Sinc–Gaussian (SG) method. Both Sinc versions are exact methods of inverse Laplace Transforms. The author proves that SG-based transformations present some advantages over the pure Sinc version regarding stability and convergence properties. The convergence is of exponential type. All the methods presented are applied to Mittag-Leffler functions depending on one, two and three parameters, and the author proves that the representation of this kind of functions is very effective. The author concludes that even for variable-order fractional differential or integral equations, the Sinc–Gaussian method is a powerful procedure..
VOSA: Virtual Observatory SED Analyzer. An application to the Collinder 69 open cluster
The physical properties of almost any kind of astronomical object can be
derived by fitting synthetic spectra or photometry extracted from theoretical
models to observational data.
We want to develop an automatic procedure to perform this kind of fittings to
a relatively large sample of members of a stellar association and apply this
methodology to the case of Collinder 69.
We combine the multiwavelength data of our sources and follow a work-flow to
derive the physical parameters of the sources. The key step of the work-flow is
performed by a new VO-tool, VOSA. All the steps in this process are done in a
VO environment.
We present this new tool, and provide physical parameters such as T, gravity, luminosity, etc. for 170 candidate members to Collinder
69, and an upper-limit for the age of this stellar association.
This kind of studies of star forming regions, clusters, etc. produces a huge
amount of data, very tedious to analyse using the traditional methodology.
Thus, they are excellent examples where to apply the VO capabilities.Comment: Accepted for publication in A&
A physical approach to Tsirelson's problem
Tsirelson's problem deals with how to model separate measurements in quantum
mechanics. In addition to its theoretical importance, the resolution of
Tsirelson's problem could have great consequences for device independent
quantum key distribution and certified randomness. Unfortunately, understanding
present literature on the subject requires a heavy mathematical background. In
this paper, we introduce quansality, a new theoretical concept that allows to
reinterpret Tsirelson's problem from a foundational point of view. Using
quansality as a guide, we recover all known results on Tsirelson's problem in a
clear and intuitive way.Comment: 11 pages, 2 figure
The young, wide and very low mass visual binary LOri167
We look for wide, faint companions around members of the 5 Myr Lambda Orionis
open cluster. We used optical, near-infrared, and Spitzer/IRAC photometry. We
report the discovery of a very wide very low mass visual binary, LOri167,
formed by a brown dwarf and a planetary-mass candidate located at 5 arcsec,
which seems to belong to the cluster. We derive Teff of 2125 and 1750 K. If
they are members, comparisons with theoretical models indicate masses of 17
(20-15) Mjup and 8 (13-7) Mjup, with a projected separation of 2000 AU. Such a
binary system would be difficult to explain in most models, particularly those
where substellar objects form in the disks surrounding higher mass stars.Comment: Astronomy & Astrophysics Letters, in pres
Unitary evolution and uniqueness of the Fock representation of Dirac fields in cosmological spacetimes
We present a privileged Fock quantization of a massive Dirac field in a
closed Friedmann-Robertson-Walker cosmology, partially selected by the criteria
of invariance of the vacuum under the symmetries of the field equations, and
unitary implementation of the dynamics. When quantizing free scalar fields in
homogeneous and isotropic spacetimes with compact spatial sections, these
criteria have been shown to pick out a unique Fock representation (up to
unitary equivalence). Here, we employ the same criteria for fermion fields and
explore whether that uniqueness result can be extended to the case of the Fock
quantization of fermions. For the massive Dirac field, we start by introducing
a specific choice of the complex structure that determines the Fock
representation. Such structure is invariant under the symmetries of the
equations of motion. We then prove that the corresponding representation of the
canonical anticommutation relations admits a unitary implementation of the
dynamics. Moreover, we construct a rather general class of representations that
satisfy the above criteria, and we demonstrate that they are all unitarily
equivalent to our previous choice. The complex structures in this class are
restricted only by certain conditions on their asymptotic behavior for modes in
the ultraviolet sector of the Dirac operator. We finally show that, if one
assumes that these asymptotic conditions are in fact trivial once our criteria
are fulfilled, then the time-dependent scaling in the definition of the
fermionic annihilation and creation-like variables is essentially unique.Comment: 24 page
Fractal functions on the real projective plane
Formerly the geometry was based on shapes, but since the last centuries this
founding mathematical science deals with transformations, projections and
mappings. Projective geometry identifies a line with a single point, like the
perspective on the horizon line and, due to this fact, it requires a
restructuring of the real mathematical and numerical analysis. In particular,
the problem of interpolating data must be refocused. In this paper we define a
linear structure along with a metric on a projective space, and prove that the
space thus constructed is complete. Then we consider an iterated function
system giving rise to a fractal interpolation function of a set of data.Comment: 25 pages, 18 figure
Almost quantum correlations
Quantum theory is not only successfully tested in laboratories every day but also constitutes a robust theoretical framework: small variations usually lead to implausible consequences, such as faster-than-light communication. It has even been argued that quantum theory may be special among possible theories. Here we report that, at the level of correlations among different systems, quantum theory is not so special. We define a set of correlations, dubbed 'almost quantum', and prove that it strictly contains the set of quantum correlations but satisfies all-but-one of the proposed principles to capture quantum correlations. We present numerical evidence that the remaining principle is satisfied too. © 2015 Macmillan Publishers Limited
Uniqueness of the Fock quantization of Dirac fields in 2+1 dimensions
We study the Fock quantization of a free Dirac field in 2+1-dimensional
backgrounds which are conformally ultrastatic, with a time-dependent conformal
factor. As it is typical for field theories, there is an infinite ambiguity in
the Fock representation of the canonical anticommutation relations. Different
choices may lead to unitarily inequivalent theories that describe different
physics. To remove this ambiguity one usually requires that the vacuum be
invariant under the unitary transformations that implement the symmetries of
the equations of motion. However, in non-stationary backgrounds, where time
translation is not a symmetry transformation, the requirement of vacuum
invariance is in general not enough to fix completely the Fock representation.
We show that this problem is overcome in the considered scenario by demanding,
in addition, a unitarily implementable quantum dynamics. The combined
imposition of these conditions selects a unique family of equivalent Fock
representations. Moreover, one also obtains an essentially unique splitting of
the time variation of the Dirac field into an explicit dependence on the
background scale factor and a quantum evolution of the corresponding creation
and annihilation operators.Comment: 24 pages. Document replaced to match published versio
Spectroscopy of Very Low Mass Stars and Brown Dwarfs in the Lambda Orionis Star Forming Region
Context. Most observational studies so far point towards brown dwarfs sharing
a similar formation mechanism as the one accepted for low mass stars. However,
larger databases and more systematic studies are needed before strong
conclusions can be reached. Aims. In this second paper of a series devoted to
the study of the spectroscopic properties of the members of the Lambda Orionis
Star Forming Region, we study accretion, activity and rotation for a wide set
of spectroscopically confirmed members of the central star cluster Collinder 69
to draw analogies and/or differences between the brown dwarf and stellar
populations of this cluster. Moreover, we present comparisons with other star
forming regions of similar and different ages to address environmental effects
on our conclusions. Methods. We study prominent photospheric lines to derive
rotational velocities and emission lines to distinguish between accretion
processes and chromospheric activity. In addition, we include information about
disk presence and X-ray emission. Results. We report very large differences in
the disk fractions of low mass stars and brown dwarfs (~58%) when compared to
higher mass stars (26+4-3%) with 0.6 Msun being the critical mass we find for
this dichotomy. As a byproduct, we address the implications of the spatial
distribution of disk and diskless members in the formation scenario of the
cluster itself. We have used the Halpha emission to discriminate among
accreting and non-accreting sources finding that 38+8-7% of sources harboring
disks undergo active accretion and that his percentage stays similar in the
substellar regime. For those sources we have estimated accretion rates.
Finally, regarding rotational velocities, we find a high dispersion in vsin(i)
which is even larger among the diskless population.Comment: Accepted for publication in A&A. 18 figs including the Appendix and
an online tabl
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