The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
Nathaniel M Indik - One of the best experts on this subject based on the ideXlab platform.
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spectral approach to the relativistic inverse stellar Structure Problem ii
Physical Review D, 2012Co-Authors: Lee Lindblom, Nathaniel M IndikAbstract:A new method for solving the relativistic inverse stellar Structure Problem is presented. This method determines a spectral representation of the unknown high-density portion of the stellar equation of state from a knowledge of the total masses M and radii R of the stars. Spectral representations of the equation of state are very efficient, generally requiring only a few spectral parameters to achieve good accuracy. This new method is able, therefore, to determine the high-density equation of state quite accurately from only a few accurately measured [M,R] data points. This method is tested here by determining the equations of state from mock [M,R] data computed from tabulated realistic neutron-star equations of state. The spectral equations of state obtained from these mock data are shown to agree, on average, with the originals to within a few percent (over the entire high-density range of the neutron-star interior) using only two [M,R] data points. Higher accuracies are achieved when more data are used. The accuracies of the equations of state determined in these examples are shown to be nearly optimal, in the sense that their errors are comparable to the errors of the best-fit spectral representations of these realistic equations of state.
Lee Lindblom - One of the best experts on this subject based on the ideXlab platform.
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The relativistic inverse stellar Structure Problem
arXiv: High Energy Astrophysical Phenomena, 2014Co-Authors: Lee LindblomAbstract:The observable macroscopic properties of relativistic stars (whose equations of state are known) can be predicted by solving the stellar Structure equations that follow from Einstein’s equation. For neutron stars, however, our knowledge of the equation of state is poor, so the direct stellar Structure Problem can not be solved without modeling the highest density part of the equation of state in some way. This talk will describe recent work on developing a model independent approach to determining the high-density neutron-star equation of state by solving an inverse stellar Structure Problem. This method uses the fact that Einstein’s equation provides a deterministic relationship between the equation of state and the macroscopic observables of the stars which are composed of that material. This talk illustrates how this method will be able to determine the high-density part of the neutron-star equation of state with few percent accuracy when high quality measurements of the masses and radii of just two or three neutron stars become available. This talk will also show that this method can be used with measurements of other macroscopic observables, like the masses and tidal deformabilities, which can (in principle) be measured by gravitational wave observations of binary neutron-star mergers.
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spectral approach to the relativistic inverse stellar Structure Problem ii
Physical Review D, 2012Co-Authors: Lee Lindblom, Nathaniel M IndikAbstract:A new method for solving the relativistic inverse stellar Structure Problem is presented. This method determines a spectral representation of the unknown high-density portion of the stellar equation of state from a knowledge of the total masses M and radii R of the stars. Spectral representations of the equation of state are very efficient, generally requiring only a few spectral parameters to achieve good accuracy. This new method is able, therefore, to determine the high-density equation of state quite accurately from only a few accurately measured [M,R] data points. This method is tested here by determining the equations of state from mock [M,R] data computed from tabulated realistic neutron-star equations of state. The spectral equations of state obtained from these mock data are shown to agree, on average, with the originals to within a few percent (over the entire high-density range of the neutron-star interior) using only two [M,R] data points. Higher accuracies are achieved when more data are used. The accuracies of the equations of state determined in these examples are shown to be nearly optimal, in the sense that their errors are comparable to the errors of the best-fit spectral representations of these realistic equations of state.
Artur F Izmaylov - One of the best experts on this subject based on the ideXlab platform.
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exact and approximate symmetry projectors for the electronic Structure Problem on a quantum computer
Journal of Chemical Physics, 2019Co-Authors: Robert A Lang, Artur F IzmaylovAbstract:Solving the electronic Structure Problem on a universal-gate quantum computer within the variational quantum eigensolver (VQE) methodology requires constraining the search procedure to a subspace defined by relevant physical symmetries. Ignoring symmetries results in convergence to the lowest eigenstate of the Fock space for the second quantized electronic Hamiltonian. Moreover, this eigenstate can be symmetry broken due to limitations of the wavefunction ansatz. To address this VQE Problem, we introduce and assess methods of exact and approximate projectors to irreducible eigensubspaces of available physical symmetries. Feasibility of symmetry projectors in the VQE framework is discussed, and their efficiency is compared with symmetry constraint optimization procedures. Generally, projectors introduce a higher number of terms for VQE measurement compared to the constraint approach. On the other hand, the projection formalism improves accuracy of the variational wavefunction ansatz without introducing additional unitary transformations, which is beneficial for reducing depths of quantum circuits.Solving the electronic Structure Problem on a universal-gate quantum computer within the variational quantum eigensolver (VQE) methodology requires constraining the search procedure to a subspace defined by relevant physical symmetries. Ignoring symmetries results in convergence to the lowest eigenstate of the Fock space for the second quantized electronic Hamiltonian. Moreover, this eigenstate can be symmetry broken due to limitations of the wavefunction ansatz. To address this VQE Problem, we introduce and assess methods of exact and approximate projectors to irreducible eigensubspaces of available physical symmetries. Feasibility of symmetry projectors in the VQE framework is discussed, and their efficiency is compared with symmetry constraint optimization procedures. Generally, projectors introduce a higher number of terms for VQE measurement compared to the constraint approach. On the other hand, the projection formalism improves accuracy of the variational wavefunction ansatz without introducing add...
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relation between fermionic and qubit mean fields in the electronic Structure Problem
Journal of Chemical Physics, 2018Co-Authors: Ilya G Ryabinkin, Scott N Genin, Artur F IzmaylovAbstract:For quantum computing applications, the electronic Hamiltonian for the electronic Structure Problem needs to be unitarily transformed into a qubit form. We found that mean-field procedures on the original electronic Hamiltonian and on its transformed qubit counterpart can give different results. We establish conditions of when fermionic and qubit mean fields provide the same or different energies. In cases when the fermionic mean-field (Hartree–Fock) approach provides an accurate description (electronic correlation effects are small), the choice of molecular orbitals for the electron Hamiltonian representation becomes the determining factor in whether the qubit mean-field energy will be equal to or higher than that of the fermionic counterpart. In strongly correlated cases, the qubit mean-field approach has a higher chance to undergo symmetry breaking and lower its energy below the fermionic counterpart.For quantum computing applications, the electronic Hamiltonian for the electronic Structure Problem needs to be unitarily transformed into a qubit form. We found that mean-field procedures on the original electronic Hamiltonian and on its transformed qubit counterpart can give different results. We establish conditions of when fermionic and qubit mean fields provide the same or different energies. In cases when the fermionic mean-field (Hartree–Fock) approach provides an accurate description (electronic correlation effects are small), the choice of molecular orbitals for the electron Hamiltonian representation becomes the determining factor in whether the qubit mean-field energy will be equal to or higher than that of the fermionic counterpart. In strongly correlated cases, the qubit mean-field approach has a higher chance to undergo symmetry breaking and lower its energy below the fermionic counterpart.
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relation between fermionic and qubit mean fields in the electronic Structure Problem
arXiv: Chemical Physics, 2018Co-Authors: Ilya G Ryabinkin, Scott N Genin, Artur F IzmaylovAbstract:For quantum computing applications, the electronic Hamiltonian for the electronic Structure Problem needs to be unitarily transformed to a qubit form. We found that mean-field procedures on the original electronic Hamiltonian and on its transformed qubit counterpart can give different results. We establish conditions of when fermionic and qubit mean fields provide the same or different energies. In cases when the fermionic mean-field (Hartree-Fock) approach provides an accurate description (electronic correlation effects are small), the choice of molecular orbitals for the electron Hamiltonian representation becomes the determining factor in whether the qubit mean-field energy will be equal to or higher than that of the fermionic counterpart. In strongly correlated cases, the qubit mean-field approach has a higher chance to undergo symmetry breaking and lower its energy below the fermionic counterpart.
Gang Li - One of the best experts on this subject based on the ideXlab platform.
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A comparative study of two classes of efficient lattice filters with Structure robustness consideration
Proceedings of the 31st Chinese Control Conference, 2012Co-Authors: Gang Li, Xiongxiong He, Chaogeng HuangAbstract:This paper deals with low complexity digital filter Structures with consideration of maximizing a newly defined Structure robustness measure. Two classes of efficient lattice-based filter Structures are considered. For an Nth order digital filter, the Structures in these classes all have 2N + 1 multipliers. The expression of robustness measure is derived and the optimum Structure Problem is formulated in terms of maximizing this measure with respect to the degrees of freedom for each of the two classes. A design example is presented to demonstrate the excellent finite wordlength performance of the optimized Structures.
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An improved lattice IIR digital filter Structure
2011 8th International Conference on Information Communications & Signal Processing, 2011Co-Authors: Chaogeng Huang, Gang Li, Zhixing Xu, Hong Xu, Liping ChangAbstract:In this paper, a new digital lattice filter Structure with minimum signal power ratio is derived, which is actually an improved version of the recently proposed Structure reported in [15]. Signed power-of-two (SPT), which is one of the most well known methods for reducing implementation complexity, is applied to a part of the Structure parameters. The optimum Structure Problem is formulated in terms of minimizing the signal power ratio with respect to the two sets of free parameters in the proposed Structure by using genetic algorithm. A numerical example is given as well as simulations in order to shows that the newly derived Structure can achieve much better performance than the Structure proposed in [15] in terms of finite wordlength properties.
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Two classes of efficient digital controller Structures with stability consideration
IEEE Transactions on Automatic Control, 2006Co-Authors: Gang LiAbstract:In this note, the efficient controller Structure Problem is investigated subject to minimizing finite word length (FWL) effects. Two classes of sparse controller Structures characterized with a set of free parameters are derived. The stability behavior of each class is analyzed with a newly defined stability related measure. The optimal Structure Problems are studied in terms of maximizing the stability related measure for the two classes of sparse Structures as well as the fully parametrized state-space realizations. A design example is given, which shows that the optimized sparse Structures can beat the fully parametrized optimal realizations in terms of both stability performance and computation efficiency.
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On the Structure of digital controllers with finite word length consideration
IEEE Transactions on Automatic Control, 1998Co-Authors: Gang LiAbstract:It is well known that a stable discrete-time control system may become unstable when the digital controller is actually implemented with a digital control processor due to the finite word length effects and that the stability behavior of the system depends on the Structure of the digital controller. Due to the difficulties in adopting the existing stability robustness measures, a new measure is derived, which is tractable and hence enables us to handle the digital controller Structure Problem easily. The optimal controller Structure Problem is formulated by maximizing this measure. An analytical solution is presented. An algorithm is developed for finding sparser Structures.
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On the Structure of digital controllers in sampled-data systems with FWL consideration
Proceedings of 35th IEEE Conference on Decision and Control, 1996Co-Authors: Gang Li, M. GeversAbstract:A sampled-data control system consists of the continuous-time plant, and a sampled-data controller composed of a sampler, the digital controller and a hold. The stability may be lost when the digital controller is implemented with a digital control processor due to the finite word length (FWL) effects. There are two main aspects involved in controller implementation. The first one is concerned with estimating the smallest word length that ensures stability. This is concerned with solving a stability robustness Problem. The other aspect is concerned with the optimal controller Structure Problem.
G. Li - One of the best experts on this subject based on the ideXlab platform.
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Polynomial operator based sparse controller Structures ith stability consideration
IEE Proceedings - Control Theory and Applications, 2005Co-Authors: G. LiAbstract:T o ne efficient controller Structures are derived based o a polynomial operator approach. The first one ca be considered as a improved versio of the recently proposed direct-form II transposed (DFIIt) Structure i the /spl rho/-operator, i hich the first-order /spl rho/-operators are replaced ith a set of second-order operators, hile the second one is the equivalent state-space realisation. A pole modulus sensitivity based stability measure is obtained and the corresponding expressio of the stability robustness for each Structure is derived. The optimal Structure Problem is solved by maximising the stability robustness under the parameter dynamical range constraints for fixed-point implementations. A design example is given, hich sho s that the ne ly developed Structures ca achieve much better stability performance tha those Structures i first-order /spl rho/-operators and furthermore, outperform the fully parametrised optimal realisatio i terms of both stability robustness and implementatio efficiency.
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Polynomial operator-based digital controller Structures of high stability performance and computation efficiency
ICARCV 2004 8th Control Automation Robotics and Vision Conference 2004., 2004Co-Authors: G. LiAbstract:In this paper, the optimal controller Structure Problem is investigated with finite word length (FWL) consideration. Based on the polynomial operator concept, a new sparse controller Structure is proposed. This Structure is efficient in terms of implementation and can be optimized to reduce FWL effects. A pole modulus sensitivity based stability measure is derived and the optimal controller Structures are defined as those that maximize the proposed measure. The Problem of finding optimal sparse Structures is solved using exhaustive searching with a practical consideration. A design example is given, which shows that the newly developed Structure can beat the fully parametrized optimal state-space realization in terms of both computation efficiency and stability performance.
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An improved stability measure for digital filter implementation
2003 IEEE International Conference on Acoustics Speech and Signal Processing 2003. Proceedings. (ICASSP '03)., 2003Co-Authors: G. LiAbstract:We consider the stability robustness Problem of a digital filter implemented with finite word length (FWL). Based on pole modulus sensitivities, a new stability robustness related measure is derived. This measure is less conservative than that derived with the classical pole sensitivity measure. It is shown that the normal realizations are a set of optimal realizations that maximize this proposed stability robustness measure. The stability performance of the generalized direct-form II transposed (DFIIt) Structure is analysed using this measure. The optimal generalized DFIIt Structure Problem is defined so as to identify those DFIIt Structures that maximize the proposed stability measure, which is solved using an exhaustive searching method. Numerical examples are given to show the design procedure.
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Analysis of sensitivity measures of finite-precision digital controller Structures with closed-loop stability bounds
IEE Proceedings - Control Theory and Applications, 1998Co-Authors: Robert S.h. Istepanian, Jie Wu, G. Li, J. ChuAbstract:The Problem of digital controller Structures and the effect of finite-word-length (FWL) implementation on the closed-loop stability of digital feedback control systems is addressed. A framework is presented to derive two lower stability bounds for a closed-loop system, which are controller Structure dependent, and then to solve the optimal FWL controller Structure Problem by maximising one of these lower bounds. This yields an improved method to design optimal finite-precision controller Structures with better numerical accuracy and closed-loop stability characteristics. Comparisons using numerical examples from two digital controller Structures demonstrate that the procedure proposed for the more efficient measure yields an improved finite controller realisation with combined lower bits and higher lower stability bounds
