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Shmuel Fishman

Publications and source records attributed to Shmuel Fishman.

12 recordsLinked to original sources

Scaling and universality of the complexity of analog computation.

We apply a probabilistic approach to study the computational complexity of analog computers which solve linear programming problems. We numerically analyze various ensembles of linear programming problems and obtain, for each of these ensembles, the probability distribution functions of certain quantities which measure the computational complexity, known as the convergence rate, the barrier and the computation time. We find that in the limit of very large problems these probability distributions are universal scaling functions. In other words, the probability distribution function for each of these three quantities becomes, in the limit of large problem size, a function of a single scaling variable, which is a certain composition of the quantity in question and the size of the system. Moreover, various ensembles studied seem to lead essentially to the same scaling functions, which depend only on the variance of the ensemble. These results extend analytical and numerical results obtained recently for the Gaussian ensemble, and support the conjecture that these scaling functions are universal.

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Time-independent approximations for periodically driven systems with friction.

The classical dynamics of a particle that is driven by a rapidly oscillating potential (with frequency omega) is studied. The motion is separated into a slow part and a fast part that oscillates around the slow part. The motion of the slow part is found to be described by a time-independent equation that is derived as an expansion in orders of omega(-1) (in this paper terms to the order omega(-3) are calculated explicitly). This time-independent equation is used to calculate the attracting fixed points and their basins of attraction. The results are found to be in excellent agreement with numerical solutions of the original time-dependent problem.

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Changes in the slope of the first major deflection of the ECG complex during acute coronary occlusion.

To evaluate the effect of acute coronary occlusion (ACO) during percutaneous coronary intervention (PCI) on the slope of the first major deflection of the QRS complex (the initial QRS slope). Standard ECG signals of 18 patients (89 leads), undergoing PCI were recorded prior to and during ACO. The initial QRS slope was calculated in the baseline state and during ACO. Changes in the standard ECG were detected in 36 of 89 leads (40%). The initial QRS slope during ACO was significantly different from baseline in 74 of 89 leads (83%). The specificity of the change in the slope during ACO was low (29%).

Angioplasty, Balloon, Coronary↗

Dynamics of an ion chain in a harmonic potential.

Cold ions in anisotropic harmonic potentials can form ion chains of various sizes. Here, the density of ions is not uniform, and thus the eigenmodes are not phononic-like waves. We study chains of N>>1 ions and evaluate analytically the long-wavelength modes and the density of states in the short-wavelength limit. These results reproduce with good approximation the dynamics of chains consisting of dozens of ions. Moreover, they allow one to determine the critical transverse frequency required for the stability of the linear structure, which is found to be in agreement with results obtained by different theoretical methods [Phys. Rev. Lett. 71, 2753 (1993)]] and by numerical simulations [Phys. Rev. Lett. 70, 818 (1993)]]. We introduce and explore the thermodynamic limit for the ion chain. The thermodynamic functions are found to exhibit deviations from extensivity.

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Eigenmodes and thermodynamics of a Coulomb chain in a harmonic potential.

The density of ions trapped in a harmonic potential in one dimension is not uniform. Consequently the eigenmodes are not phononlike waves. We calculate the long-wavelength modes in the continuum limit, and evaluate the density of states in the short-wavelength limit for chains of N>>1 ions. Remarkably, the results that are found analytically in the thermodynamic limit provide a good estimate of the spectrum of excitations of small chains down to few tens of ions. The spectra are used to compute the thermodynamic functions of the chain. Deviations from the extensivity of the thermodynamic quantities are found. An analytic expression for the critical transverse frequency determining the stability of a linear chain is derived.

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Decoherence as a probe of coherent quantum dynamics.

The effect of decoherence, induced by spontaneous emission, on the dynamics of cold atoms periodically kicked by an optical lattice is experimentally and theoretically studied. Ideally, the mean energy growth is essentially unaffected by weak decoherence, but the resonant momentum distributions are fundamentally altered. It is shown that experiments are inevitably sensitive to certain nontrivial features of these distributions, in a way that explains the puzzle of the observed enhancement of resonances by decoherence [Phys. Rev. Lett. 87, 074102 (2001)]. This clarifies both the nature of the coherent evolution, and the way in which decoherence disrupts it.

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Classical scaling theory of quantum resonances.

The quantum resonances occurring with delta-kicked particles are studied with the help of a fictitious classical limit, establishing a direct correspondence between the nearly resonant quantum motion and the classical resonances of a related system. A scaling law which characterizes the structure of the resonant peaks is derived and numerically demonstrated.

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Time independent description of rapidly oscillating potentials.

The classical and quantum dynamics in a high frequency field are found to be described by an effective time independent Hamiltonian. It is calculated in a systematic expansion in the inverse of the frequency (omega) to order omega(-4). The work is an extension of the classical result for the Kapitza pendulum, which was calculated in the past to order omega(-2). The analysis makes use of an implementation of the method of separation of time scales and of a quantum gauge transformation in the framework of Floquet theory. The effective time independent Hamiltonian enables one to explore the dynamics in the presence of rapidly oscillating fields, in the framework of theories that were developed for systems with time independent Hamiltonians. The results are relevant, in particular, for exploring the dynamics of cold atoms.

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Stable quantum resonances in atom optics.

A theory for stabilization of quantum resonances by a mechanism similar to one leading to classical resonances in nonlinear systems is presented. It explains recent surprising experimental results, obtained for cold cesium atoms when driven in the presence of gravity, and leads to further predictions. The theory makes use of invariance properties of the system allowing for separation into independent kicked rotor problems. The analysis relies on a fictitious classical limit where the small parameter is not Planck's constant, but rather the detuning from the frequency that is resonant in the absence of gravity.

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Localized perturbations of integrable quantum billiards.

The statistics of energy levels of a rectangular billiard that is perturbed by a strong localized potential are studied analytically and numerically, when this perturbation is at the center or at a typical position. Different results are found for these two types of position. If the scatterer is at the center, the symmetry leads to additional contributions, some of which are related to the angular dependence of the potential. The limit of the delta-like scatterer is obtained explicitly. The form factor, which is the Fourier transform of the energy-energy correlation function, is calculated analytically, in the framework of the semiclassical geometrical theory of diffraction, and numerically. Contributions of classical orbits that are nondiagonal are calculated and are found to be essential.

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Evaluation of the phase-plane ECG as a technique for detecting acute coronary occlusion.

OBJECTIVE: To evaluate the phase plane (PP) ECG as a method for detecting acute coronary occlusion (ACO). BACKGROUND: Balloon inflation in a coronary artery during PTCA produces acute myocardial ischemia. The sensitivity of the standard ECG for detecting ACO is approximately 50%, depending on the number of leads recorded. METHODS: The standard ECG signals of 18 patients (91 leads), undergoing PTCA were sampled and converted to digital data, prior to, and during acute coronary occlusion. PPs were constructed by projecting the ECG signals and their first derivatives onto a two-dimensional plane. Standard ECG signals and PPs, prior to ACO, were compared to their respective recordings and PPs during ACO. RESULTS: Using the standard ECG analysis, the acute occlusion was detected in 39 of 91 leads (43%), and in 15 of 18 patients (83%), whereas using the PP analysis it was detected in 82 of 91 leads (90%), and in all 18 patients (100%) (P<0.001, for leads). The median number of leads per patient demonstrating standard ECG changes was 2.0, whereas for the PP analysis it was 5.5 (P<0.001). The specificity of the PP method was 83.5%. CONCLUSIONS: The sensitivity of the PP method for detecting ACO during PTCA was superior to that of the standard ECG analysis. A smaller lead system is required to detect changes of ACO, during PTCA, when the PP method is used. The PP method is simple, low-priced, and may serve to detect acute myocardial ischemia in a number of clinical settings.

Acute Disease↗

The correlation dimension of rat hearts in an experimentally controlled environment.

The electric response of several isolated rat hearts in a controlled environment was studied experimentally. The correlation dimension D(2) was estimated and was found to be between 4 and 6.5 when the response was nearly periodic. The variation of D(2) with the concentration of calcium was studied and a general trend of its increase with increasing concentration was found. Two types of ventricular fibrillation (VF) were observed, one that corresponds to a stochastic signal where D(2) is unbounded and the other to a low dimensional dynamical system with 3.5</=D(2)</=4. It was found also that for the heart the correlation dimension is only an approximate concept. A new method for the estimation of D(2), that is suitable for the case when it is only approximate was introduced. A surrogate data method suitable for the analysis of nearly periodic series was implemented. (c) 2000 American Institute of Physics.

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