Articles de revista
http://hdl.handle.net/2117/3918
2018-01-20T17:12:19ZMulti-TID detection and characterization in a dense Global Navigation Satellite System receiver network
http://hdl.handle.net/2117/112900
Multi-TID detection and characterization in a dense Global Navigation Satellite System receiver network
Yang, Heng; Monte Moreno, Enrique; Hernández Pajares, Manuel
The medium-scale traveling ionospheric disturbances (MSTIDs) constitute the most frequent ionospheric wave signatures. We propose a method for detecting the number of simultaneous MSTIDs from a time series of high-pass-filtered Vertical Total Electron Content (VTEC) maps and their parameters. The method is tested on the VTEC map corresponding to a simulated realistic scenario and on actual data from dual-frequency Global Positioning System (GPS) measurements gathered by +1200 GPS receivers of the GPS Earth Observation Network (GEONET) in Japan. The contribution consists of the detection of the number of independent MSTIDs from a nonuniform sampling of the ionospheric pierce points. The problem is set as a sparse decomposition on elements of a dictionary of atoms that span a linear space of possible MSTIDs. These atoms consist of plane waves characterized by a wavelength, direction, and phase on a surface defined, the part of the ionosphere sounded by the GEONET (i.e., 25°N to 50°N of latitude and 125°E to 155°E of longitude). The technique is related to the atomic decomposition and least absolute shrinkage and selection operator. The geophysical contribution of this paper is showing (a) the detection of several simultaneous MSTIDs of different characteristics, with a continuous change in the velocity; (b) detection of circular MSTID waves compatible by time and center with a specific earthquake; (c) simultaneous superposition of two distinct MSTIDs, with almost the same azimuth; and (d) the presence at nighttime of MSTIDs with velocities in the range 400–600 m/s.
2018-01-17T14:33:10ZYang, HengMonte Moreno, EnriqueHernández Pajares, ManuelThe medium-scale traveling ionospheric disturbances (MSTIDs) constitute the most frequent ionospheric wave signatures. We propose a method for detecting the number of simultaneous MSTIDs from a time series of high-pass-filtered Vertical Total Electron Content (VTEC) maps and their parameters. The method is tested on the VTEC map corresponding to a simulated realistic scenario and on actual data from dual-frequency Global Positioning System (GPS) measurements gathered by +1200 GPS receivers of the GPS Earth Observation Network (GEONET) in Japan. The contribution consists of the detection of the number of independent MSTIDs from a nonuniform sampling of the ionospheric pierce points. The problem is set as a sparse decomposition on elements of a dictionary of atoms that span a linear space of possible MSTIDs. These atoms consist of plane waves characterized by a wavelength, direction, and phase on a surface defined, the part of the ionosphere sounded by the GEONET (i.e., 25°N to 50°N of latitude and 125°E to 155°E of longitude). The technique is related to the atomic decomposition and least absolute shrinkage and selection operator. The geophysical contribution of this paper is showing (a) the detection of several simultaneous MSTIDs of different characteristics, with a continuous change in the velocity; (b) detection of circular MSTID waves compatible by time and center with a specific earthquake; (c) simultaneous superposition of two distinct MSTIDs, with almost the same azimuth; and (d) the presence at nighttime of MSTIDs with velocities in the range 400–600 m/s.Linear buckling analysis of perforated cold-formed steel storage rack columns by means of the generalised beam theory
http://hdl.handle.net/2117/112892
Linear buckling analysis of perforated cold-formed steel storage rack columns by means of the generalised beam theory
Casafont Ribera, Miguel; Bonada Bo, Jordi; Pastor Artigues, María Magdalena; Roure Fernández, Francisco; Susín Sánchez, Antonio
The investigation attempts to adapt a be am finite element procedure based on the Generalised Beam Theory (GBT) to the analysis of perforated columns. The presence of perforations is taken into account through the use of two beam elements with different properties, for the non- perforated and perforated parts of the member. Each part is meshed with its corresponding finite element and, afterwards, they are linked by means of constraint equations . Linear buckling analyses on steel storage rack columns are carried out to demonstrate how the proposed procedure should be applied. Some practical issues are discussed, such as the GBT deformation modes to be included in the analyses, or the optimum finite element discretiz ation. The resulting buckling loads are validated by comparison with the values obtained in anal yses performed using shell finite element models. Finally, it is verified that the buck ling loads produced with the propo sed method are rather accurate
2018-01-17T12:54:19ZCasafont Ribera, MiguelBonada Bo, JordiPastor Artigues, María MagdalenaRoure Fernández, FranciscoSusín Sánchez, AntonioThe investigation attempts to adapt a be am finite element procedure based on the Generalised Beam Theory (GBT) to the analysis of perforated columns. The presence of perforations is taken into account through the use of two beam elements with different properties, for the non- perforated and perforated parts of the member. Each part is meshed with its corresponding finite element and, afterwards, they are linked by means of constraint equations . Linear buckling analyses on steel storage rack columns are carried out to demonstrate how the proposed procedure should be applied. Some practical issues are discussed, such as the GBT deformation modes to be included in the analyses, or the optimum finite element discretiz ation. The resulting buckling loads are validated by comparison with the values obtained in anal yses performed using shell finite element models. Finally, it is verified that the buck ling loads produced with the propo sed method are rather accurateGemelli-obturator complex in the deep gluteal space: an anatomic and dynamic study
http://hdl.handle.net/2117/112882
Gemelli-obturator complex in the deep gluteal space: an anatomic and dynamic study
Balius, Ramon; Susín Sánchez, Antonio; Morros, Carles; Pujol, Montse; Pérez Cuenca, Dolores; Sala Blanch, Xavier
Objective: To investigate the behavior of the sciatic nerve during hip rotation at subgluteal space. Materials and methods: Sonographic examination (high-resolution ultrasound machine at 5.0–14 MHZ) of the gemelli-obturator internus complex following two approaches: (1) a study on cadavers and (2) a study on healthy volunteers. The cadavers were examined in pronation, pelvis-fixed position by forcing internal and external rotations of the hip with the knee in 90° flexion. Healthy volunteers were examined during passive internal and external hip rotation (prone position; lumbar and pelvic regions fixed). Subjects with a history of major trauma, surgery or pathologies affecting the examined regions were excluded. Results: The analysis included eight hemipelvis from six fresh cadavers and 31 healthy volunteers. The anatomical study revealed the presence of connective tissue attaching the sciatic nerve to the structures of the gemellus-obturator system at deep subgluteal space. The amplitude of the nerve curvature during rotating position was significantly greater than during resting position. During passive internal rotation, the sciatic nerve of both cadavers and healthy volunteers transformed from a straight structure to a curved structure tethered at two points as the tendon of the obturator internus contracted downwards. Conversely, external hip rotation caused the nerve to relax. Conclusion: Anatomically, the sciatic nerve is closely related to the gemelli-obturator internus complex. This relationship results in a reproducible dynamic behavior of the sciatic nerve during passive hip rotation, which may contribute to explain the pathological mechanisms of the obturator internal gemellus syndrome.
2018-01-17T10:34:34ZBalius, RamonSusín Sánchez, AntonioMorros, CarlesPujol, MontsePérez Cuenca, DoloresSala Blanch, XavierObjective: To investigate the behavior of the sciatic nerve during hip rotation at subgluteal space. Materials and methods: Sonographic examination (high-resolution ultrasound machine at 5.0–14 MHZ) of the gemelli-obturator internus complex following two approaches: (1) a study on cadavers and (2) a study on healthy volunteers. The cadavers were examined in pronation, pelvis-fixed position by forcing internal and external rotations of the hip with the knee in 90° flexion. Healthy volunteers were examined during passive internal and external hip rotation (prone position; lumbar and pelvic regions fixed). Subjects with a history of major trauma, surgery or pathologies affecting the examined regions were excluded. Results: The analysis included eight hemipelvis from six fresh cadavers and 31 healthy volunteers. The anatomical study revealed the presence of connective tissue attaching the sciatic nerve to the structures of the gemellus-obturator system at deep subgluteal space. The amplitude of the nerve curvature during rotating position was significantly greater than during resting position. During passive internal rotation, the sciatic nerve of both cadavers and healthy volunteers transformed from a straight structure to a curved structure tethered at two points as the tendon of the obturator internus contracted downwards. Conversely, external hip rotation caused the nerve to relax. Conclusion: Anatomically, the sciatic nerve is closely related to the gemelli-obturator internus complex. This relationship results in a reproducible dynamic behavior of the sciatic nerve during passive hip rotation, which may contribute to explain the pathological mechanisms of the obturator internal gemellus syndrome.Quintessential inflation at low reheating temperatures
http://hdl.handle.net/2117/112876
Quintessential inflation at low reheating temperatures
Aresté Saló, Llibert; Haro Cases, Jaume
We have tested some simple quintessential inflation models, imposing the requirement that they match with the recent observational data provided by the BICEP and Planck team and leading to a reheating temperature, which is obtained via gravitational particle production after inflation, supporting the nucleosynthesis success. Moreover, for the models coming from supergravity one needs to demand low temperatures in order to avoid problems such as the gravitino overproduction or the gravitational production of moduli fields, which are obtained only when the reheating temperature is due to the production of massless particles with a coupling constant very close to its conformal value.
2018-01-17T07:07:14ZAresté Saló, LlibertHaro Cases, JaumeWe have tested some simple quintessential inflation models, imposing the requirement that they match with the recent observational data provided by the BICEP and Planck team and leading to a reheating temperature, which is obtained via gravitational particle production after inflation, supporting the nucleosynthesis success. Moreover, for the models coming from supergravity one needs to demand low temperatures in order to avoid problems such as the gravitino overproduction or the gravitational production of moduli fields, which are obtained only when the reheating temperature is due to the production of massless particles with a coupling constant very close to its conformal value.Homotopy linear algebra
http://hdl.handle.net/2117/112830
Homotopy linear algebra
Gálvez Carrillo, Maria Immaculada; Kock, Joachim; Tonks, Andrew
By homotopy linear algebra we mean the study of linear functors between slices of the 8-category of 8-groupoids, subject to certain finiteness conditions. After some standard definitions and results, we assemble said slices into 8-categories to model the duality between vector spaces and profinite-dimensional vector spaces, and set up a global notion of homotopy cardinality à la Baez, Hoffnung and Walker compatible with this duality. We needed these results to support our work on incidence algebras and Möbius inversion over 8-groupoids; we hope that they can also be of independent interest.
2018-01-16T09:24:19ZGálvez Carrillo, Maria ImmaculadaKock, JoachimTonks, AndrewBy homotopy linear algebra we mean the study of linear functors between slices of the 8-category of 8-groupoids, subject to certain finiteness conditions. After some standard definitions and results, we assemble said slices into 8-categories to model the duality between vector spaces and profinite-dimensional vector spaces, and set up a global notion of homotopy cardinality à la Baez, Hoffnung and Walker compatible with this duality. We needed these results to support our work on incidence algebras and Möbius inversion over 8-groupoids; we hope that they can also be of independent interest.Transformation and decomposition of clutters into matroids
http://hdl.handle.net/2117/112728
Transformation and decomposition of clutters into matroids
Martí Farré, Jaume; Mier Vinué, Anna de
A clutter is a family of mutually incomparable sets. The set of circuits of a matroid, its set of bases, and its set of hyperplanes are examples of clutters arising from matroids. In this paper we address the question of determining which are the matroidal clutters that best approximate an arbitrary clutter ¿. For this, we first define two orders under which to compare clutters, which give a total of four possibilities for approximating ¿ (i.e., above or below with respect to each order); in fact, we actually consider the problem of approximating ¿ with clutters from any collection of clutters S, not necessarily arising from matroids. We show that, under some mild conditions, there is a finite non-empty set of clutters from S that are the closest to ¿ and, moreover, that ¿ is uniquely determined by them, in the sense that it can be recovered using a suitable clutter operation. We then particularize these results to the case where S is a collection of matroidal clutters and give algorithmic procedures to compute these clutters.
2018-01-12T13:52:45ZMartí Farré, JaumeMier Vinué, Anna deA clutter is a family of mutually incomparable sets. The set of circuits of a matroid, its set of bases, and its set of hyperplanes are examples of clutters arising from matroids. In this paper we address the question of determining which are the matroidal clutters that best approximate an arbitrary clutter ¿. For this, we first define two orders under which to compare clutters, which give a total of four possibilities for approximating ¿ (i.e., above or below with respect to each order); in fact, we actually consider the problem of approximating ¿ with clutters from any collection of clutters S, not necessarily arising from matroids. We show that, under some mild conditions, there is a finite non-empty set of clutters from S that are the closest to ¿ and, moreover, that ¿ is uniquely determined by them, in the sense that it can be recovered using a suitable clutter operation. We then particularize these results to the case where S is a collection of matroidal clutters and give algorithmic procedures to compute these clutters.Second order linear difference equations
http://hdl.handle.net/2117/112719
Second order linear difference equations
Encinas Bachiller, Andrés Marcos; Jiménez Jiménez, M. Jose
We provide the explicit solution of a general second order linear difference equation via the computation of its associated Green function. This Green function is completely characterized and we obtain a closed expression for it using functions of two–variables, that we have called Chebyshev functions due to its intimate relation with the usual one–variable Chebyshev polynomials. In fact, we show that Chebyshev functions become Chebyshev polynomials if constant coefficients are considered.
2018-01-12T11:42:21ZEncinas Bachiller, Andrés MarcosJiménez Jiménez, M. JoseWe provide the explicit solution of a general second order linear difference equation via the computation of its associated Green function. This Green function is completely characterized and we obtain a closed expression for it using functions of two–variables, that we have called Chebyshev functions due to its intimate relation with the usual one–variable Chebyshev polynomials. In fact, we show that Chebyshev functions become Chebyshev polynomials if constant coefficients are considered.Parrondo's dynamic paradox for the stability of non-hyperbolic fixed points
http://hdl.handle.net/2117/112706
Parrondo's dynamic paradox for the stability of non-hyperbolic fixed points
Cima Mollet, Anna; Gasull Embid, Armengol; Mañosa Fernández, Víctor
We show that for periodic non-autonomous discrete dynamical systems, even when a common fixed point for each of the autonomous associated dynamical systems is repeller, this fixed point can became a local attractor for the whole system, giving rise to a Parrondo's dynamic type paradox
2018-01-12T09:40:39ZCima Mollet, AnnaGasull Embid, ArmengolMañosa Fernández, VíctorWe show that for periodic non-autonomous discrete dynamical systems, even when a common fixed point for each of the autonomous associated dynamical systems is repeller, this fixed point can became a local attractor for the whole system, giving rise to a Parrondo's dynamic type paradoxImage segmentation based on statistical confidence intervals
http://hdl.handle.net/2117/112700
Image segmentation based on statistical confidence intervals
Buenestado Caballero, Pablo; Acho Zuppa, Leonardo
Image segmentation is defined as a partition realized to an image into homogeneous regions to modify it into something that is more meaningful and softer to examine. Although several segmentation approaches have been proposed recently, in this paper, we develop a new image segmentation method based on the statistical confidence interval tool along with the well-known Otsu algorithm. According to our numerical experiments, our method has a dissimilar performance in comparison to the standard Otsu algorithm to specially process images with speckle noise perturbation. Actually, the effect of the speckle noise entropy is almost filtered out by our algorithm. Furthermore, our approach is validated by employing some image samples.
2018-01-12T07:20:47ZBuenestado Caballero, PabloAcho Zuppa, LeonardoImage segmentation is defined as a partition realized to an image into homogeneous regions to modify it into something that is more meaningful and softer to examine. Although several segmentation approaches have been proposed recently, in this paper, we develop a new image segmentation method based on the statistical confidence interval tool along with the well-known Otsu algorithm. According to our numerical experiments, our method has a dissimilar performance in comparison to the standard Otsu algorithm to specially process images with speckle noise perturbation. Actually, the effect of the speckle noise entropy is almost filtered out by our algorithm. Furthermore, our approach is validated by employing some image samples.On trees with the same restricted U-polynomial and the Prouhet–Tarry–Escott problem
http://hdl.handle.net/2117/112678
On trees with the same restricted U-polynomial and the Prouhet–Tarry–Escott problem
Aliste Prieto, José; Mier Vinué, Anna de; Zamora, José
This paper focuses on the well-known problem due to Stanley of whether two non-isomorphic trees can have the same U-polynomial (or, equivalently, the same chromatic symmetric function). We consider the Uk-polynomial, which is a restricted version of U-polynomial, and construct, for any given kk, non-isomorphic trees with the same Uk-polynomial. These trees are constructed by encoding solutions of the Prouhet–Tarry–Escott problem. As a consequence, we find a new class of trees that are distinguished by the U-polynomial up to isomorphism.
2018-01-11T14:04:18ZAliste Prieto, JoséMier Vinué, Anna deZamora, JoséThis paper focuses on the well-known problem due to Stanley of whether two non-isomorphic trees can have the same U-polynomial (or, equivalently, the same chromatic symmetric function). We consider the Uk-polynomial, which is a restricted version of U-polynomial, and construct, for any given kk, non-isomorphic trees with the same Uk-polynomial. These trees are constructed by encoding solutions of the Prouhet–Tarry–Escott problem. As a consequence, we find a new class of trees that are distinguished by the U-polynomial up to isomorphism.