3,316 research outputs found
Partition of unity interpolation using stable kernel-based techniques
In this paper we propose a new stable and accurate approximation technique
which is extremely effective for interpolating large scattered data sets. The
Partition of Unity (PU) method is performed considering Radial Basis Functions
(RBFs) as local approximants and using locally supported weights. In
particular, the approach consists in computing, for each PU subdomain, a stable
basis. Such technique, taking advantage of the local scheme, leads to a
significant benefit in terms of stability, especially for flat kernels.
Furthermore, an optimized searching procedure is applied to build the local
stable bases, thus rendering the method more efficient
Late movement of basin-edge lobate scarps on Mercury
Basin-edge lobate scarps are a sub-type of tectonic shortening structure on the surface of Mercury that have formed at the edge of volcanic units that fill or partly fill impact basins. We have performed a global survey of these features and find that they are widespread in basins across the planet. We obtained model ages from crater size–frequency distribution analysis for a subset of our surveyed basins, for both the smooth plains infill and for the last resolvable tectonic activity on the associated basin-edge scarps. Our results indicate that some of these lobate scarps were still accumulating strain in the late Mansurian (approximately 1 Ga). From a photogeological assessment, we find that the orientations of these basin-edge lobate scarps are similar to those reported for the global population of lobate scarps in earlier studies, appearing to align ∼north–south at low latitudes and ∼east–west at higher latitudes. However, reassessing these landforms’ orientation with artificially illuminated topographic data does not allow us to rule out the effect of illumination bias. We propose that these landforms, the result of crustal shortening in response to global contraction, formed along the interface between the basin floor and the smooth plains unit, which acted as a mechanical discontinuity along which shortening strains were concentrated
Greedy kernel methods for accelerating implicit integrators for parametric ODEs
We present a novel acceleration method for the solution of parametric ODEs by
single-step implicit solvers by means of greedy kernel-based surrogate models.
In an offline phase, a set of trajectories is precomputed with a high-accuracy
ODE solver for a selected set of parameter samples, and used to train a kernel
model which predicts the next point in the trajectory as a function of the last
one. This model is cheap to evaluate, and it is used in an online phase for new
parameter samples to provide a good initialization point for the nonlinear
solver of the implicit integrator. The accuracy of the surrogate reflects into
a reduction of the number of iterations until convergence of the solver, thus
providing an overall speedup of the full simulation. Interestingly, in addition
to providing an acceleration, the accuracy of the solution is maintained, since
the ODE solver is still used to guarantee the required precision. Although the
method can be applied to a large variety of solvers and different ODEs, we will
present in details its use with the Implicit Euler method for the solution of
the Burgers equation, which results to be a meaningful test case to demonstrate
the method's features
Romanesque and territory. The construction materials of Sardinian medieval churches: new approaches to the valorization, conservation and restoration
This paper is intended to illustrate a multidisciplinary research project devoted to the study of the constructive materials of the Romanesque churches in Sardinia during the “Giudicati” period (11th -13th centuries). The project focuses on the relationship between a selection of monuments and their territory, both from a historical-architectural perspective and from a more modern perspective addressing future restoration works. The methodologies of the traditional art-historical research (study of bibliographic, epigraphic and archival sources, formal reading of artifacts) are flanked by new technologies: digital surveys executed with a 3D laser-scanner, analyses of the materials (stones, mortars, bricks) with different instrumental methods: X-ray fluorescence (XRF) and inductively coupled mass spectrometry (ICP-MS) for chemical composition, X-ray diffractometer (XRD) to determine the alteration phases (e.g., soluble salts), optical microscopy and electronic (SEM) to study textures, mineral assemblages and microstructures, termogravimetric/differential scanning, calorimetric analysis (TG/DTA) for the composition of the binder mortars.
This multidisciplinary approach allows the achieving of important results in an archaeometric context: 1) from a historical point of view, with the possible identification of ancient traffics, trade routes, sources of raw materials, construction phases, wall textures; 2) from a conservative point of view, by studying chemical and physical weathering processes of stone materials compatible for replacement in case of future restoration works.
Sardinian Romanesque architectural heritage is particularly remarkable: about 200 churches of different types and sizes, with the almost exclusive use of cut stones. Bi- or poly-chromy, deriving from the use of different building materials, characterizes many of these monuments, becoming also a vehicle for political and cultural meanings. The paper will present some case studies aimed to illustrate the progress of the project and the results achieved
Stable interpolation with exponential-polynomial splines and node selection via greedy algorithms
In this work we extend some ideas about greedy algorithms, which are well-established tools for, e.g., kernel bases, and exponential-polynomial splines whose main drawback consists in possible overfitting and consequent oscillations of the approximant. To partially overcome this issue, we develop some results on theoretically optimal interpolation points. Moreover, we introduce two algorithms which perform an adaptive selection of the spline interpolation points based on the minimization either of the sample residuals (f-greedy), or of an upper bound for the approximation error based on the spline Lebesgue function (λ-greedy). Both methods allow us to obtain an adaptive selection of the sampling points, i.e., the spline nodes. While the f-greedy selection is tailored to one specific target function, the λ-greedy algorithm enables us to define target-data-independent interpolation nodes
Variable stars in the open cluster NGC 6791 and its surrounding field
Aims: This work presents a high--precision variability survey in the field of
the old, super metal-rich open cluster NGC 6791.
Methods: The data sample consists of more than 75,000 high-precision CCD time
series measurements in the V band obtained mainly at the Canada-France-Hawaii
Telescope, with additional data from S. Pedro Martir and Loiano observatories,
over a time span of ten nights. The field covers an area of 42x28 arcmin^2.
Results: We have discovered 260 new variables and re-determined periods and
amplitudes of 70 known variable stars. By means of a photometric evaluation of
the membership in NGC 6791, and a preliminary membership based on the proper
motions, we give a full description of the variable content of the cluster and
surrounding field in the range 16<V<23.5. Accurate periods can be given for the
variables with P<4.0 d, while for ones with longer periods the limited
time-baseline hampered precise determinations. We categorized the entire sample
as follows: 6 pulsating, 3 irregular, 3 cataclysmic, 89 rotational variables
and 61 eclipsing systems; moreover, we detected 168 candidate variables for
which we cannot give a variability class since their periods are much longer
than our time baseline.
Conclusions: On the basis of photometric considerations, and of the positions
of the stars with respect to the center of the cluster, we inferred that 11 new
variable stars are likely members of the cluster, for 22 stars the membership
is doubtful and 137 are likely non-members. We also detected an outburst of
about 3 mag in the light curve of a very faint blue star belonging to the
cluster and we suggest that this star could be a new U Gem (dwarf nova)
cataclysmic variable.Comment: 24 pages, 19 Figures, A&A accepte
Optimizing the Earth-LISA "rendez-vous"
We present a general survey of heliocentric LISA orbits, hoping it might help
in the exercise of rescoping the mission. We try to semi-analytically optimize
the orbital parameters in order to minimize the disturbances coming from the
Earth-LISA interaction. In a set of numerical simulations we include
nonautonomous perturbations and provide an estimate of Doppler shift and
breathing as a function of the trailing angle.Comment: 18 pages, 16 figures. Submitted on CQ
Detecting olive grove abandonment with Sentinel-2 and machine learning: The development of a web-based tool for land management
Linking the proximal tibiofibular joint to hominid locomotion: A morphometric study of extant species
Objectives: We perform a comparative assessment of shape variation of the proximal fibula in extant humans and great apes, intending to investigate the possible link between proximal fibular shape and locomotor patterns.Methods: Our sample includes 94 fibulae of 37 Homo sapiens, 15 Gorilla, 17 Pongo, and 25 Pan. Fibular morphology was investigated through three-dimensional (semi)landmark-based geometric morphometric methods.Results: We found unique features of the human fibular head compared to that of great apes (i.e., oblique articular surface, the presence of the styloid process, specific morphology of muscle attachment sites), supporting the functional role of this bone in relation to human obligate bipedalism. Great apes also showed distinctive traits in their proximal fibula morphology, in agreement with differences in locomotor behavior.Conclusion: The morphology of the proximal fibula in extant humans and great apes is indicative of locomotor behavior, offering the potential for the comparative analysis of fossil hominin remains
The Low End of the Initial Mass Function in Young LMC Clusters: I. The Case of R136
We report the result of a study in which we have used very deep broadband V
and I WFPC2 images of the R136 cluster in the Large Magellanic Cloud from the
HST archive, to sample the luminosity function below the detection limit of 2.8
Mo previously reached. In these new deeper images, we detect stars down to a
limiting magnitude of m_F555W = 24.7 (~ 1 magnitude deeper than previous
works), and identify a population of red stars evenly distributed in the
surrounding of the R136 cluster. A comparison of our color-magnitude diagram
with recentely computed evolutionary tracks indicates that these red objects
are pre-main sequence stars in the mass range 0.6 - 3 Mo. We construct the
initial mass function (IMF) in the 1.35 - 6.5 Mo range and find that, after
correcting for incompleteness, the IMF shows a definite flattening below ~ 2
Mo. We discuss the implications of this result for the R136 cluster and for our
understanding of starburst galaxies formation and evolution in general.Comment: 29 pages, 6 tables, 11 figures included + 3 external files, accepted
for publication by Ap.
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