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971.
After studying Gaussian type quadrature formulae with mixed boundary conditions, we suggest a fast algorithm for computing their nodes and weights. It is shown that the latter are computed in the same manner as in the theory of the classical Gauss quadrature formulae. In fact, all nodes and weights are again computed as eigenvalues and eigenvectors of a real symmetric tridiagonal matrix. Hence, we can adapt existing procedures for generating such quadrature formulae. Comparative results with various methods now in use are given. In the second part of this paper, new algorithms for spectral approximations for second-order elliptic problems are derived. The key to the efficiency of our algorithms is to find an appropriate spectral approximation by using the most accurate quadrature formula, which takes the boundary conditions into account in such a way that the resulting discrete system has a diagonal mass matrix. Hence, our algorithms can be used to introduce explicit resolutions for the time-dependent problems. This is the so-called lumped mass method. The performance of the approach is illustrated with several numerical examples in one and two space dimensions.
972.
Consider the classical nonparametric regression problem yi = f(ti) + ii = 1,...,n where ti = i/n, and i are i.i.d. zero mean normal with variance 2. The aim is to estimate the true function f which is assumed to belong to the smoothness class described by the Besov space B
pq
q
. These are functions belonging to Lp with derivatives up to order s, in Lp sense. The parameter q controls a further finer degree of smoothness. In a Bayesian setting, a prior on B
pq
q
is chosen following Abramovich, Sapatinas and Silverman (1998). We show that the optimal Bayesian estimator of f is then also a.s. in B
pq
q
if the loss function is chosen to be the Besov norm of B
pq
q
. Because it is impossible to compute this optimal Bayesian estimator analytically, we propose a stochastic algorithm based on an approximation of the Bayesian risk and simulated annealing. Some simulations are presented to show that the algorithm performs well and that the new estimator is competitive when compared to the more standard posterior mean. 相似文献
973.
N. Z. Mamadalieva N. Sh. Ramazanov J.-P. Girault R. Lafont Z. Saatov 《Chemistry of Natural Compounds》2004,40(5):488-491
20-Hydroxyecdysone-22-benzoate was for the first time synthesized from 20-hydroxyecdysone.Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 401–403, September–October, 2004. 相似文献
974.
The effects of adsorbed H on the Mo1−xRex(110), x=0, 0.05, 0.15, and 0.25, surfaces have been investigated using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). For the x=0.15 alloy only, a c(2×2) LEED pattern is observed at a coverage Θ0.25 ML. A (2×2) pattern is observed for H coverages around Θ0.5 ML from surfaces with x=0, 0.05, and 0.15. Both c(2×2) and (2×2) patterns are attributed to reconstruction of the substrate. At higher coverages, a (1×1) pattern is observed. For the alloy surface with x=0.25, only a (1×1) pattern is obtained for all H coverages. Two H vibrations are observed in HREELS spectra for all Re concentrations, which shift to higher energies at intermediate coverages. Both peaks exhibit an isotopic shift, confirming their assignment to hydrogen. For Re concentrations of x=0.15 and higher, a third HREELS peak appears at 50 meV as H (D) coverage approaches saturation. This peak does not shift in energy with isotopic substitution, yet cannot be explained by contamination. The intrinsic width of the loss peaks depends on the Re concentration in the surface region and becomes broader with increasing x. This broadening can be attributed to surface inhomogeneity, but may also reflect increased delocalization of the adsorbed hydrogen atom. 相似文献
975.
S. Albeverio Yu. G. Kondratiev R. A. Minlos A. L. Rebenko 《Journal of statistical physics》1998,92(5-6):1153-1172
We construct the distribution of the infinite-dimensional Markov process associated with a finite-temperature Gibbs state for a quantum mechanical anharmonic crystal. The corresponding state is constructed via a cluster expansion technique for an arbitrary fixed temperature and, correspondingly, small enough masses of particles. 相似文献
976.
We want in this note to clarify some aspects of the Machian foundation of the concept of mass in classical mechanics; specifically, we show how the relations of transitivity for the mass-ratios, necessary for a well grounded definition of mass, can be derived from Machian postulates. 相似文献
977.
W. Goldacker M. Quilitz B. Obst H. Eckelmann 《Physica C: Superconductivity and its Applications》1998,310(1-4):182-186
Hysteresis losses and coupling losses, a main component of the AC losses in Bi(2223) tapes, can effectively be reduced by enhancing the resistivity of the matrix material between the filaments and applying a filament twist. Since through alloying the sheath, as using AgAu(8 wt.%), the resistivity can only be raised by a factor <10 (77 K), a new conductor configuration with a quite novel composite matrix having resistive SrCO3 barriers inside the Ag matrix between the filaments was developed. These new barriers, a cheap and commercial material, withstand the tape annealing, do not react with the superconductor, sinter dense and have a good bonding to Ag. Applying two different preparation techniques for 19 filament prototype tapes, critical current densities up to 20.7 kA cm−2 were achieved. We report on tape preparation, the effect on the phase texture and the superconducting properties of such barrier tapes. 相似文献
978.
L. Vlahos S. Trakadas A. Gouliamos G. Plataniotis C. Papavasiliou 《Magnetic resonance imaging》1993,11(8):1227-1229
A case of extramedullary hemopoiesis in an 18-yr-old girl suffering from beta-thalassemia is reported. Extramedullary hemopoietic masses were localized apart from the paraspinal region in the retrocrural areas as well as the presacral region associated with massive destruction of the sacrum. The importance of recognition of hemopoietic masses in these regions in conditions other than hereditary hemolytic anaemias is briefly discussed. 相似文献
979.
980.
Ann Cohen Brandwein Stefan Ralescu William E. Strawderman 《Annals of the Institute of Statistical Mathematics》1993,45(3):551-565
We consider estimation of a location vector for particular subclasses of spherically symmetric distributions in the presence of a known or unknown scale parameter. Specifically, for these spherically symmetric distributions we obtain slightly more general conditions and larger classes of estimators than Brandwein and Strawderman (1991,Ann. Statist.,19, 1639–1650) under which estimators of the formX +ag(X) dominateX for quadratic loss, concave functions of quadratic loss and general quadratic loss.Research supported by NSF grant DMS-88-22622 相似文献