2,100 research outputs found
Improving Transient Performance of Adaptive Control Architectures using Frequency-Limited System Error Dynamics
We develop an adaptive control architecture to achieve stabilization and
command following of uncertain dynamical systems with improved transient
performance. Our framework consists of a new reference system and an adaptive
controller. The proposed reference system captures a desired closed-loop
dynamical system behavior modified by a mismatch term representing the
high-frequency content between the uncertain dynamical system and this
reference system, i.e., the system error. In particular, this mismatch term
allows to limit the frequency content of the system error dynamics, which is
used to drive the adaptive controller. It is shown that this key feature of our
framework yields fast adaptation with- out incurring high-frequency
oscillations in the transient performance. We further show the effects of
design parameters on the system performance, analyze closeness of the uncertain
dynamical system to the unmodified (ideal) reference system, discuss robustness
of the proposed approach with respect to time-varying uncertainties and
disturbances, and make connections to gradient minimization and classical
control theory.Comment: 27 pages, 7 figure
Analysis of the Performance of Mexican Pension Funds: Evidence from a Stationary Bootstrap Application
This paper assesses the performance of Mexican pension funds (AFORES) by using an asset pricing model that includes macroeconomic factors and benchmark portfolios to explain returns. We apply a bootstrap statistical technique to obtain the cross-sectional distribution of performance measures (alphas) across all pension funds. This is done to determine whether a pension fund manager adds value to the portfolio before commissions charges, or if the performance observed, after controlling for the relevant factors, is simply explained by luck. Moreover, by comparing pension fund alphas to the distributions of alphas corresponding to lower rankings, we can find out if a particular fund statistically distinguishes itself from others. Our results provide evidence that pension funds managers do not add value to the portfolio and that funds are not distinguishable from each other.Pension funds, Performance evaluation, Stationary bootstrap
Hydrothermal stability of Ru/SiO2-C: A promising catalyst for biomass processing through liquid-phase reactions
In this work, structural and morphological properties of SiO2-C composite material to be used as support for catalysts in the conversion of biomass-derived oxygenated hydrocarbons, such as glycerol, were investigated in liquid water under various temperatures conditions. The results show that this material does not lose surface area, and the hot liquid water does not generate changes in the structure. Neither change in relative concentrations of oxygen functional groups nor in Si/C ratio due to hydrothermal treatment was revealed by X-ray photoelectron spectroscopy (XPS) analysis. Raman analysis showed that the material is made of a disordered graphitic structure in an amorphous silica matrix, which remains stable after hydrothermal treatment. Results of the hydrogenolysis of glycerol using a Ru/SiO2-C catalyst indicate that the support gives more stability to the active phase than a Ru/SiO2 consisting of commercial silica
Prevention of neurological injuries during mandibular third molar surgery: technical notes
Surgery to the mandibular third molar is common, and injuries to the inferior alveolar nerve and the lingual nerve are well-recognized complications of this procedure. The aim of these technical notes is to describe operative measures for reducing neurological complications during mandibular third molar surgery. The following procedure should be used to prevent damage to the inferior alveolar nerve: a well-designed mucoperiosteal flap, to obtain appropriate access to the surgical area; a conservative ostectomy on the distal and distal-lingual side; tooth sectioning, to facilitate its removal by decreasing the retention zones; tooth dislocation in the path of withdrawal imposed by the curvature of the root apex; and careful socket debridement, when the roots of the extracted tooth are in intimate contact with the mandibular canal. To prevent injury to the lingual nerve, it is important (I) to assess the integrity of the mandibular inner cortex and exclude the presence of fenestration, which could cause the dislocation of the tooth or its fragment into the sublingual or submandibular space; (II) to avoid inappropriate or excessive dislocation proceedings, in order to prevent lingual cortex fracture; (III) to perform horizontal mesial-distal crown sectioning of the lingually inclined tooth; (IV) to protect the lingual flap with a retractor showing the cortical ridge; and (V) to pass the suture not too apically and from the inner side in a buccal-lingual direction in the retromolar are
A space-fractional Monodomain model for cardiac electrophysiology combining anisotropy and heterogeneity on realistic geometries
Classical models of electrophysiology do not typically account for the effects of high structural heterogeneity in the spatio-temporal description of excitation waves propagation. We consider a modification of the Monodomain model obtained by replacing the diffusive term of the classical formulation with a fractional power of the operator, defined in the spectral sense. The resulting nonlocal model describes different levels of tissue heterogeneity as the fractional exponent is varied. The numerical method for the solution of the fractional Monodomain relies on an integral representation of the nonlocal operator combined with a finite element discretisation in space, allowing to handle in a natural way bounded domains in more than one spatial dimension. Numerical tests in two spatial dimensions illustrate the features of the model. Activation times, action potential duration and its dispersion throughout the domain are studied as a function of the fractional parameter: the expected peculiar behaviour driven by tissue heterogeneities is recovered
Learning mathematics and competences: Bringing together three theoretical perspectives
The symposium aims to bring into analysis and discussion a possibility of
articulating and integrating theoretical concepts from Activity Theory, Situated
Learning and Critical Mathematics Education in order to contribute to
understanding learning in practices where mathematics and technology seem to
be relevant
A twisted conformal field theory description of the Quantum Hall Effect
We construct an effective conformal field theory by using a procedure which
induces twisted boundary conditions for the fundamental scalar fields. That
allows to describe a quantum Hall fluid at Jain hierarchical filling,
nu=m/(2pm+1), in terms of one charged scalar field and m-1 neutral ones. Then
the resulting algebra of the chiral primary fields is U(1)xW_m. Finally the
ground state wave functions are given as correlators of appropriate composite
fields (a-electrons).Comment: 11 pages, plain Late
Meta-modeling on detailed geography for accurate prediction of invasive alien species dispersal
Invasive species are recognized as a significant threat to biodiversity. The mathematical modeling of their spatio-temporal dynamics can provide significant help to environmental managers in devising suitable control strategies. Several mathematical approaches have been proposed in recent decades to efficiently model the dispersal of invasive species. Relying on the assumption that the dispersal of an individual is random, but the density of individuals at the scale of the population can be considered smooth, reaction-diffusion models are a good trade-off between model complexity and flexibility for use in different situations. In this paper we present a continuous reaction-diffusion model coupled with arbitrary Polynomial Chaos (aPC) to assess the impact of uncertainties in the model parameters. We show how the finite elements framework is well-suited to handle important landscape heterogeneities as elevation and the complex geometries associated with the boundaries of an actual geographical region. We demonstrate the main capabilities of the proposed coupled model by assessing the uncertainties in the invasion of an alien species invading the Basque Country region in Northern Spain
Optimal receptor-cluster size determined by intrinsic and extrinsic noise
Biological cells sense external chemical stimuli in their environment using
cell-surface receptors. To increase the sensitivity of sensing, receptors often
cluster, most noticeably in bacterial chemotaxis, a paradigm for signaling and
sensing in general. While amplification of weak stimuli is useful in absence of
noise, its usefulness is less clear in presence of extrinsic input noise and
intrinsic signaling noise. Here, exemplified on bacterial chemotaxis, we
combine the allosteric Monod-Wyman- Changeux model for signal amplification by
receptor complexes with calculations of noise to study their
interconnectedness. Importantly, we calculate the signal-to-noise ratio,
describing the balance of beneficial and detrimental effects of clustering for
the cell. Interestingly, we find that there is no advantage for the cell to
build receptor complexes for noisy input stimuli in absence of intrinsic
signaling noise. However, with intrinsic noise, an optimal complex size arises
in line with estimates of the sizes of chemoreceptor complexes in bacteria and
protein aggregates in lipid rafts of eukaryotic cells.Comment: 15 pages, 12 figures,accepted for publication on Physical Review
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