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## Three-dimensional supersonic flow over a spike-nosed body of revolution

Morgenstern Jr.,A.
Fonte: The Brazilian Society of Mechanical Sciences Publicador: The Brazilian Society of Mechanical Sciences
Tipo: Artigo de Revista Científica Formato: text/html
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The unsteady, viscous, supersonic flow over a spike-nosed body of revolution is numerically investigated by solving the Navier-Stokes equations. The time-accurate computations are performed employing an implicit algorithm based on the second-order time-accurate LU-SGS scheme with the incorporation of a subiteration procedure to maintain time accuracy. The characteristics of the flow field for a Mach number of 3.0, Reynolds number of 7.87 x 10(6)/m, and angles of attack of 5 and 10 degrees are described. Self-sustained asymmetric shock wave oscillations were observed in the numerical computations for these angles of attack. The main characteristic of the flow field, as well as its influence on drag coefficient is discussed.

## Neural correlates and neural computations in posterior parietal cortex during perceptual decision-making

Huk, Alexander C.; Meister, Miriam L. R.
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
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A recent line of work has found remarkable success in relating perceptual decision-making and the spiking activity in the macaque lateral intraparietal area (LIP). In this review, we focus on questions about the neural computations in LIP that are not answered by demonstrations of neural correlates of psychological processes. We highlight three areas of limitations in our current understanding of the precise neural computations that might underlie neural correlates of decisions: (1) empirical questions not yet answered by existing data; (2) implementation issues related to how neural circuits could actually implement the mechanisms suggested by both extracellular neurophysiology and psychophysics; and (3) ecological constraints related to the use of well-controlled laboratory tasks and whether they provide an accurate window on sensorimotor computation. These issues motivate the adoption of a more general “encoding-decoding framework” that will be fruitful for more detailed contemplation of how neural computations in LIP relate to the formation of perceptual decisions.

## A GPU tool for efficient, accurate, and realistic simulation of cone beam CT projections

Jia, Xun; Yan, Hao; Cerviño, Laura; Folkerts, Michael; Jiang, Steve B.
Fonte: American Association of Physicists in Medicine Publicador: American Association of Physicists in Medicine
Tipo: Artigo de Revista Científica
EN
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Purpose: Simulation of x-ray projection images plays an important role in cone beam CT (CBCT) related research projects, such as the design of reconstruction algorithms or scanners. A projection image contains primary signal, scatter signal, and noise. It is computationally demanding to perform accurate and realistic computations for all of these components. In this work, the authors develop a package on graphics processing unit (GPU), called gDRR, for the accurate and efficient computations of x-ray projection images in CBCT under clinically realistic conditions.

## Relative stability of different DNA guanine quadruplex stem topologies derived using large-scale quantum-chemical computations

Šponer, Jiří; Mládek, Arnošt; Špačková, Nad’a; Cang, Xiaohui; Cheatham, Thomas E.; Grimme, Stefan
Tipo: Artigo de Revista Científica
EN
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We provide theoretical predictions of the intrinsic stability of different arrangements of guanine quadruplex (G-DNA) stems. Most computational studies of nucleic acids have applied Molecular Mechanics (MM) approaches using simple pairwise-additive force fields. The principle limitation of such calculations is the highly approximate nature of the force fields. In this study we for the first time apply accurate QM computations (DFT-D3 with large atomic orbital basis sets) to essentially complete DNA building blocks, namely, seven different folds of the cation-stabilized 2-quartet G-DNA stem, each having more than 250 atoms. The solvent effects are approximated by COSMO continuum solvent. We reveal sizeable differences between MM and QM descriptions of relative energies of different G-DNA stems, which apparently reflect approximations of the DNA force field. Using the QM energy data, we propose correction to earlier free energy estimates of relative stabilities of different parallel, hybrid and antiparallel G-stem folds based on classical simulations. The new energy ranking visibly improves the agreement between theory and experiment. We predict the 5′-anti-anti-3′ GpG dinucleotide step to be the most stable one, closely followed by the 5′-syn-anti-3′ step. The results are in good agreement with known experimental structures of 2...

## Are accurate computations of the 13C′ shielding feasible at the DFT level of theory?

Vila, Jorge A.; Arnautova, Yelena A.; Martin, Osvaldo A; Scheraga, Harold A.
Tipo: Artigo de Revista Científica
EN
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The goal of this study is twofold. First, to investigate the relative influence of the main structural factors affecting the computation of the 13C′ shielding, namely, the conformation of the residue itself and the next nearest-neighbor effects. Second, to determine whether calculation of the 13C′ shielding at the DFT level of theory, with an accuracy similar to that of the 13Cα shielding, is feasible with the existing computational resources. The DFT calculations, carried out for a large number of possible conformations of the tripeptide Ac-GXY-NMe, with different combinations of X and Y residues, enable us to conclude that the accurate computation of the 13C′ shielding for a given residue X depends on the: (i) (φ,ψ) backbone torsional angles of X; (ii) side-chain conformation of X; (iii) (φ,ψ) torsional angles of Y; and (iv) identity of residue Y. Consequently, DFT-based quantum mechanical calculations of the 13C′ shielding, with all these factors taken into account, are two orders of magnitude more CPU demanding than the computation, with similar accuracy, of the 13Cα shielding. Despite not considering the effect of the possible hydrogen bond interaction of the carbonyl oxygen, this work contributes to our general understanding of the main structural factors affecting the accurate computation of the 13C′ shielding in proteins and may spur significant progress in effort to develop new validation methods for protein structures.

## Accurate solution of structured least squares problems via rank-revealing decompositions

Castro González, Nieves; Ceballos Cañón, Johan Armando; Martínez Dopico, Froilán C.; Molera, Juan M.
Fonte: Society for Industrial and Applied Mathematics Publicador: Society for Industrial and Applied Mathematics
Tipo: info:eu-repo/semantics/publishedVersion; info:eu-repo/semantics/article
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Least squares problems min(x) parallel to b - Ax parallel to(2) where the matrix A is an element of C-mXn (m >= n) has some particular structure arise frequently in applications. Polynomial data fitting is a well-known instance of problems that yield highly structured matrices, but many other examples exist. Very often, structured matrices have huge condition numbers kappa(2)(A) = parallel to A parallel to(2) parallel to A(dagger)parallel to(2) (A(dagger) is the Moore-Penrose pseudoinverse of A) and therefore standard algorithms fail to compute accurate minimum 2-norm solutions of least squares problems. In this work, we introduce a framework that allows us to compute minimum 2-norm solutions of many classes of structured least squares problems accurately, i.e., with errors parallel to(x) over cap (0) - x(0)parallel to(2)/parallel to x(0)parallel to(2) = O(u), where u is the unit roundoff, independently of the magnitude of kappa(2)(A) for most vectors b. The cost of these accurate computations is O(n(2)m) flops, i.e., roughly the same cost as standard algorithms for least squares problems. The approach in this work relies in computing first an accurate rank-revealing decomposition of A, an idea that has been widely used in recent decades to compute...

## Accurate Computations of Eigenvalues of Extremely Ill-conditioned Matrices and Differential Operators

Ye, Qiang
Tipo: Artigo de Revista Científica
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This paper is concerned with computations of a few smaller eigenvalues (in absolute value) of a large extremely ill-conditioned matrix. It is shown that smaller eigenvalues can be accurately computed for a diagonally dominant matrix or a product of diagonally dominant matrices by combining a standard iterative method with the accurate inversion algorithms that have been developed for such matrices. Applications to the finite difference discretization of differential operators are discussed. In particular, a new discretization is derived for the 1-dimensional biharmonic operator that can be written as a product of diagonally dominant matrices. Numerical examples are presented to demonstrate the accuracy achieved by the new algorithms.

## Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra

Baring, Matthew G.; Harding, Alice K.
Tipo: Artigo de Revista Científica
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The splitting of photons in the presence of an intense magnetic field has recently found astrophysical applications in polar cap models of gamma-ray pulsars and in magnetar scenarios for soft gamma repeaters. Numerical computation of the polarization-dependent rates of this third order QED process for arbitrary field strengths and energies below pair creation threshold is difficult: thus early analyses focused on analytic developments and simpler asymptotic forms. The recent astrophysical interest spurred the use of the S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In this paper, we present numerical computations of a full proper-time expression for the rate of splitting that was obtained by Stoneham, and is exact up to the pair creation threshold. While the numerical results derived here are in accord with the earlier asymptotic forms due to Adler, our computed rates still differ by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner, reflecting the extreme difficulty of generating accurate S-matrix numerics for fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time rates appear very accurate, and exceed Adler's asymptotic specializations significantly only for photon energies just below pair threshold and for supercritical fields...

## Compact and accurate variational wave functions of three-electron atomic systems constructed from semi-exponential radial basis functions

Frolov, Alexei M
Tipo: Artigo de Revista Científica
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The semi-exponential basis set of radial functions (A.M. Frolov, Physics Letters A {\bf 374}, 2361 (2010)) is used for variational computations of bound states in three-electron atomic systems. It appears that semi-exponential basis set has a substantially greater potential for accurate variational computations of bound states in three-electron atomic systems than it was originally anticipated. In particular, the 40-term Larson's wave function improved with the use of semi-exponential radial basis functions now produces the total energy \linebreak -7.47805413551 $a.u.$ for the ground $1^2S-$state in the ${}^{\infty}$Li atom (only one spin function $\chi_1 = \alpha \beta \alpha - \beta \alpha \alpha$ was used in these calculations). This variational energy is very close to the exact ground state energy of the ${}^{\infty}$Li atom and it substantially lower than the total energy obtained with the original Larson's 40-term wave function (-7.477944869 $a.u.$).

## Highly accurate calculations of the rotationally excited bound states in three-body systems

Frolov, Alexei M.; Wardlaw, David M.
Tipo: Artigo de Revista Científica
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An effective optimization strategy has been developed to construct highly accurate bound state wave functions in various three-body systems. Our procedure appears to be very effective for computations of weakly bound states and various excited states, including rotationally excited states, i.e. states with $L \ge 1$. The efficiency of our procedure is illustrated by computations of the excited $P^{*}(L = 1)-$states in the $dd\mu, dt\mu$ and $tt\mu$ muonic molecular ions, $P(L = 1)-$states in the non-symmetric $pd\mu, pt\mu$ and $dt\mu$ ions and $2^{1}P(L = 1)-$ and $2^{3}P(L = 1)-$states in He atom(s).

## Accurate estimate of the relic density and the kinetic decoupling in non-thermal dark matter models

Tipo: Artigo de Revista Científica
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35.94%
Non-thermal dark matter generation is an appealing alternative to the standard paradigm of thermal WIMP dark matter. We reconsider non-thermal production mechanisms in a systematic way, and develop a numerical code for accurate computations of the dark matter relic density. We discuss in particular scenarios with long-lived massive states decaying into dark matter particles, appearing naturally in several beyond the standard model theories, such as supergravity and superstring frameworks. Since non-thermal production favors dark matter candidates with large pair annihilation rates, we analyze the possible connection with the anomalies detected in the lepton cosmic-ray flux by Pamela and Fermi. Concentrating on supersymmetric models, we consider the effect of these non-standard cosmologies in selecting a preferred mass scale for the lightest supersymmetric particle as dark matter candidate, and the consequent impact on the interpretation of new physics discovered or excluded at the LHC. Finally, we examine a rather predictive model, the G2-MSSM, investigating some of the standard assumptions usually implemented in the solution of the Boltzmann equation for the dark matter component, including coannihilations. We question the hypothesis that kinetic equilibrium holds along the whole phase of dark matter generation...

## An Efficient Pseudospectral Method for the Computation of the Self-force on a Charged Particle: Circular Geodesics around a Schwarzschild Black Hole

Canizares, Priscilla; Sopuerta, Carlos F.
Tipo: Artigo de Revista Científica
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The description of the inspiral of a stellar-mass compact object into a massive black hole sitting at a galactic centre is a problem of major relevance for the future space-based gravitational-wave observatory LISA (Laser Interferometer Space Antenna), as the signals from these systems will be buried in the data stream and accurate gravitational-wave templates will be needed to extract them. The main difficulty in describing these systems lies in the estimation of the gravitational effects of the stellar-mass compact object on his own trajectory around the massive black hole, which can be modeled as the action of a local force, the self-force. In this paper, we present a new time-domain numerical method for the computation of the self-force in a simplified model consisting of a charged scalar particle orbiting a nonrotating black hole. We use a multi-domain framework in such a way that the particle is located at the interface between two domains so that the presence of the particle and its physical effects appear only through appropriate boundary conditions. In this way we eliminate completely the presence of a small length scale associated with the need of resolving the particle. This technique also avoids the problems associated with the impact of a low differentiability of the solution in the accuracy of the numerical computations. The spatial discretization of the field equations is done by using the pseudospectral collocation method and the time evolution...

## Triplet states in the Be atom: bound state spectrum and hyperfine structure

Frolov, Alexei M.; Ruiz, Maria Belen
Tipo: Artigo de Revista Científica
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The bound state spectrum of low-lying triplet states in the Be atom is investigated. In particular, we perform accurate computations of various bound triplet S, P, D, F, and G states in the four-electron Be atom. For the 23S(L=0) state in the Be atom we determine the hyperfine structure and a number of bound states properties by using results of highly accurate computations. The energies of the hyperfine structure levels for this state are {\epsilon}(F=12) = -13725.927(7) MHz, {\epsilon}(F=32) = -5490.371(7) MHz and {\epsilon}(F=52) = 8235.556(7) MHz, respectively. The observed hyperfine structure splittings for the the 23S(L=0) state in the 9Be atom must be {\Delta}12 = 8235.556(7) MHz and {\Delta}23 = 13725.927(7) MHz, respectively.; Comment: A substantially more complete analysis of this problem can be found in our new publication arXiv:1310.2657 which also includes many other related results for the triplet bound state spectra of the Be-atom. See updated and extended version: ArXiv:1310.2657

## Efficient Computation of Hyperspherical Bessel Functions

Kosowsky, Arthur
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Fast and accurate computations of the power spectrum of cosmic microwave background fluctuations are essential for comparing current and upcoming data sets with the large parameter space of viable cosmological models. The most efficient numerical algorithm for power spectrum calculation, recently implemented by Seljak and Zaldarriaga, involves integrating sources against spherical Bessel functions or, in the case of a non-flat universe, analogous hyperspherical Bessel functions. Evaluation of these special functions usually dominates the computation time in non-flat spatial geometries. This paper presents a highly accurate and very fast WKB approximation for computing hyperspherical Bessel functions which will greatly increase the speed of microwave background power spectrum computations in open and closed universes.; Comment: 9 pages with 4 included figures. C code available on request

## A GPU Tool for Efficient, Accurate, and Realistic Simulation of Cone Beam CT Projections

Jia, Xun; Yan, Hao; Cervino, Laura; Folkerts, Michael; Jiang, Steve B.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.23%
Simulation of x-ray projection images plays an important role in cone beam CT (CBCT) related research projects. A projection image contains primary signal, scatter signal, and noise. It is computationally demanding to perform accurate and realistic computations for all of these components. In this work, we develop a package on GPU, called gDRR, for the accurate and efficient computations of x-ray projection images in CBCT under clinically realistic conditions. The primary signal is computed by a tri-linear ray-tracing algorithm. A Monte Carlo (MC) simulation is then performed, yielding the primary signal and the scatter signal, both with noise. A denoising process is applied to obtain a smooth scatter signal. The noise component is then obtained by combining the difference between the MC primary and the ray-tracing primary signals, and the difference between the MC simulated scatter and the denoised scatter signals. Finally, a calibration step converts the calculated noise signal into a realistic one by scaling its amplitude. For a typical CBCT projection with a poly-energetic spectrum, the calculation time for the primary signal is 1.2~2.3 sec, while the MC simulations take 28.1~95.3 sec. Computation time for all other steps is negligible. The ray-tracing primary signal matches well with the primary part of the MC simulation result. The MC simulated scatter signal using gDRR is in agreement with EGSnrc results with a relative difference of 3.8%. A noise calibration process is conducted to calibrate gDRR against a real CBCT scanner. The calculated projections are accurate and realistic...

## Necessary conditions for accurate computations of three-body partial decay widths

Garrido, E.; Jensen, A. S.; Fedorov, D. V.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
35.94%
The partial width for decay of a resonance into three fragments is largely determined at distances where the energy is smaller than the effective potential producing the corresponding wave function. At short distances the many-body properties are accounted for by preformation or spectroscopic factors. We use the adiabatic expansion method combined with the WKB approximation to obtain the indispensable cluster model wave functions at intermediate and larger distances. We test the concept by deriving conditions for the minimal basis expressed in terms of partial waves and radial nodes. We compare results for different effective interactions and methods. Agreement is found with experimental values for a sufficiently large basis. We illustrate the ideas with realistic examples from $\alpha$-emission of $^{12}$C and two-proton emission of $^{17}$Ne. Basis requirements for accurate momentum distributions are briefly discussed.; Comment: To be published in Physical Review C

## Necessary conditions for accurate computations of three-body partial decay widths

Garrido, Eduardo; Jensen, A.S.; Fedorov, D.V.
Fonte: American Physical Society Publicador: American Physical Society
Tipo: Artículo Formato: 281698 bytes; application/pdf
ENG
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12 pages, 5 figures, 1 table.-- PACS number(s): 21.45.−v, 23.60.+e, 25.70.Ef, 31.15.xj; The partial width for decay of a resonance into three fragments is largely determined at distances where the energy is smaller than the effective potential producing the corresponding wave function. At short distances the many-body properties are accounted for by preformation or spectroscopic factors. We use the adiabatic expansion method combined with the WKB approximation to obtain the indispensable cluster model wave functions at intermediate and larger distances. We test the concept by deriving conditions for the minimal basis expressed in terms of partial waves and radial nodes. We compare results for different effective interactions and methods. Agreement is found with experimental values for a sufficiently large basis. We illustrate the ideas with realistic examples from alpha emission of 12C and two-proton emission of 17Ne. Basis requirements for accurate momentum distributions are briefly discussed.; Peer reviewed

## Extreme-mass-ratio inspirals modelling and test of an alternative theory of gravity /

Cañizares Martínez, Priscil·la
Fonte: [Barcelona] : Universitat Autònoma de Barcelona. Departament de Física, Publicador: [Barcelona] : Universitat Autònoma de Barcelona. Departament de Física,
Tipo: Tesis i dissertacions electròniques; info:eu-repo/semantics/doctoralThesis Formato: application/pdf
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Descripció del recurs: el 01 setembre 2012; Extreme-Mass-Ratio Inspirals (EMRIs) són sistemes binaris que estan compostos per Objectes Estel.lars Compactes (OECs) orbitant al voltant de Forats Negres Massius (FNMs) situats als centres galàctics. Aquests sistemes són una de les fonts pricipals d'Ones Gravitacionals (OGs) per detectors espacials com l'antena espacial LISA (Laser Interferometer Space Antenna). Un EMRI emet senyals molt llargs i complexes dintre del fort camp gravitatori del FNM. Aquests senyals porten codificada l'estructura del FNM. Per aquest motiu, les OGs procedent d'EMRIs són una font valuosa per estudiar els FNMs situats als centres galàctics i la ciencia relacionada amb ells. En aquesta tesi estudiem dos aspectes diferents dels EMRIs: El seu modelatge i l'estimació dels paràmetres del sistema a partir dels seus senyals gravitatoris. La primera part d'aquesta tesi està dedicada al modelatge d'EMRIs, necessari per obtenir les formes d'ona de les OGs que farem servir en la seva detecció. Per aquest motiu, necessitem conèixer com el camp gravitatori del OEC afecta la seva propia trajectoria i el desvia d'un moviment geodèsic. En aquest sentit, degut a la gran diferència entre les masses del sistema, podem considerar l'OEC com a una partícula sense estructura que orbita en una geodèsica del FNM. En aquesta representació...

## Matching NLO QCD computations and parton shower simulations.

Frixione, Stefano; Webber, Bryan R.
Fonte: HEP, Cavendish Laboratory, University of Cambridge Publicador: HEP, Cavendish Laboratory, University of Cambridge
Tipo: Article; not applicable
EN
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We propose a method for matching the next-to-leading order (NLO) calculation of a given QCD process with a parton shower Monte Carlo (MC) simulation. The method has the following features: fully exclusive events are generated, with hadronization according to the MC model; total exclusive rates are accurate to NLO; NLO results for distributions are recovered upon expansion in ?s; hard emissions are treated as in NLO computations while soft/collinear emissions are handled by the MC simulation, with the same logarithmic accuracy as the MC; and matching between the hard- and soft/collinear-emission regions is smooth. A fraction of events with negative weight is generated, but unweighting remains possible with reasonable efficiency. The method is clarified using a simple toy model, and illustrated by application to the hadroproduction of W+W? pairs.

## Efficient Bayesian active learning and matrix modelling

Houlsby, Neil
Fonte: University of Cambridge; Department of Engineering Publicador: University of Cambridge; Department of Engineering
Tipo: Thesis; doctoral; PhD
EN
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With the advent of the Internet and growth of storage capabilities, large collections of unlabelled data are now available. However, collecting supervised labels can be costly. Active learning addresses this by selecting, sequentially, only the most useful data in light of the information collected so far. The online nature of such algorithms often necessitates efficient computations. Thus, we present a framework for information theoretic Bayesian active learning, named Bayesian Active Learning by Disagreement, that permits efficient and accurate computations of data utility. Using this framework we develop new techniques for active Gaussian process modelling and adaptive quantum tomography. The latter has been shown, in both simulation and laboratory experiments, to yield faster learning rates than any non-adaptive design. Numerous datasets can be represented as matrices. Bayesian models of matrices are becoming increasingly popular because they can handle noisy or missing elements, and are extensible to different data-types. However, efficient inference is crucial to allow these flexible probabilistic models to scale to large real-world datasets. Binary matrices are a ubiquitous datatype, so we present a stochastic inference algorithm for fast learning in this domain. Preference judgements are a common...