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991.
J. F. Voloch 《Bulletin of the Brazilian Mathematical Society》1990,21(1):91-94
Letf:CE be a non-constant rational map between curves over a finite field, whereE is elliptic. We estimate the number of rational points ofC whose image underf generate the group of rational points ofE. 相似文献
992.
Basford JR 《Journal of laser applications》1990,2(1):57-63
Low energy laser therapy has gained varying acceptance as a treatment for a broad range of soft tissue, musculoskeletal and neurological conditions. A controversial, but surprisingly large body of research with cell cultures suggests that laser irradiation can nondestructively alter cellular processes. Unfortunately, animal and human studies are often contradictory and difficult to evaluate due to differing study designs. As a result, the clinical effectiveness of low energy laser therapy remains debatable. Nevertheless, the findings are intriguing and deserve further investigation. 相似文献
993.
994.
We have measured the resistivity of the YBa2Cu3O7–Ag composite system with Ag volume fraction,p
Ag ranging from 0–100%. The percolation behavior and the normal-state resistivity are studied. We find a three-dimensional threshold for the Ag matrix occurring atp
Ag 20% and a threshold for the superconducting matrix occurring at 1–p
Ag30%. The electrical resistivity is interpreted in terms of a resistivity model for granular YBa2Cu3O7 proposed by Halbritter et al. [Z. Phys. B —Condensed Matter71, 411 (1988)]. Based on this model, we show that the resistivities between the YBa2Cu3O7 grains and flaws in the grains are greatly diminished in the samples, indicating significantly improved grain growth behavior of the YBa2Cu3O7 phase resulting from the addition of Ag. With the diminishing of the grain boundary resistivities, an intrinsic value for the temperature coefficient of resistivity, (1/gr)(d/dT), can be inferred, which is 3.3×10–3/K at 300 K. This value is very close to that observed in the Cu–O2 planes in single crystals and epitaxial films of YBa2Cu3O7. 相似文献
995.
J. Rantala J. Hartikainen J. Jaarinen 《Applied Physics A: Materials Science & Processing》1990,50(5):465-471
In this paper we describe the use of thermal microscopes in measuring the lengths of vertical cracks in silicon nitride. Results of theoretical calculations for temperature profiles in samples near the end of the cracks are demonstrated. These calculations showed that the effect of the crack decreases to a non-measurable level at distances of about 10 m from the end of the crack. Experimental measurements showed that thermally obtained crack lengths were at least 34% longer than those that were optically measured. The obtained crack length was independent of the heat modulation frequency used. 相似文献
996.
A. Esswein M. Griessl F. Hufnagel A. Lebrecht K. Schreier 《Applied Physics A: Materials Science & Processing》1990,51(5):382-386
The complex permittivities of some extra dense flint glasses (EDF glasses) have been studied. The dielectric features of the samples are dominated by their PbO content. Both refractive indices and dielectric losses exhibit a close relation to the concentration of Pb ions. The latter are located either at sites of the network atoms or filling the potential minima interstitially. They can be identified by their different relaxation mechanisms. The dispersion in the submillimeter wave and FIR area is characterized by a very broad distribution of comparably sharp resonant states. Thermal lens — or hysteresis effects can be excluded. 相似文献
997.
In this paper the existence and uniqueness of solutions of the following initial boundary value problem for non-linear symmetric hyperbolic equations of the first order are shown, where M = I + ? S , has the same from as the Kreiss' condition, but S must be sufficiently small ( I + is the unit matrix in the space generated by eigenvectors of the matrix ? A · n? , corresponding to positive eigenvalues) and n? is a unit outward vector normal to the boundary. The main result of the paper is obtaining an a priori estimate for non-linear equations. This estimate is obtained for sufficiently small time and norms of given data functions. The existence of solutions is proved by the method of successive approximations, which can be used because at each step such properties as symmetry of matrices and the numbers of positive and negative eigenvalues of the matrix ? A · n? are assured. This can be done because we restrict our attention to such systems of equations for which these properties are satisfied for solutions from some neighbourhood of initial data u 0. Therefore, using the fact that solutions in the class of continuous functions are sought, these properties can be satisfied for sufficiently small time. Moreover, some examples of initial boundary value problems for equations of hydrodynamics and magnetohydrodynamics are considered. 相似文献
998.
We consider the upper-branch neutral stability of flow in pipesof large aspect ratio, basically extending the work of F. T.Smith to the nonlinear regime. The inclusion of weak nonlinearityleads to an eigenproblem whose solution depends on the propertiesof three-dimensional nonlinear critical layers. Two specialcases are considered. The first is for very small amplitude perturbations, where R is a Reynolds numberbased on the height of the tube and which is assumed large.Then a fully analytical solution of the three-dimensional criticallayers is possible, from which the linear results of Smith maybe deduced. The second case studied is that of flow in a rectangularpipe, where a solution of the nonlinear critical layer problemcan be obtained. Further analysis of neutral modes in this lattercase suggests the possible existence, inter alia, of neutralmodes for finite aspect ratio tubes. These modes depend on thescaled amplitude and have O(1) wavespeeds. 相似文献
999.
The time evolution of the particle number density and spectrum of massive scalar particles, coupled conformally to a classical Friedman-Robertson-Walker gravitational field is followed numerically. Not only for a pure radiation cosmos, but also for one with an inflationary interlude, the density of particles produced remains constant in time. This results in a constant equivalent temperature scale up to the Compton time of the massive particles, and opens the possibility that during a sufficiently long inflation the energy density of the particles produced can exceed that of the radiation background. 相似文献
1000.
John R. Peterson Tamara J. Winter Hoang D. Do Robin D. Rogers 《Journal of chemical crystallography》1989,19(1):135-145
Structural and conformational information obtained from the crystal structure and solution1H nmr investigations of the title compound are compared. The 4-aryltetralone, C24H24O10, crystallizes as a chloroform solvate in the monoclinic space group, P21/n, witha=12.519(4),b=17.938(6),c=12.534(9)Å,=111.90(5)°, and Dcalc=1.51 g cm–3 forZ=4. The data for this compound were collected at –150°C. Least-squares refinement of 2796 observed [F
o5(F
o)] reflections led to the final agreement index ofR=0.062. A threefold static disorder was observed for one of the carboxyl groups. The second carboxyl group participates in an intramolecular hydrogen bond and is thus ordered. The1H nmr spectrum revealed the title compound to exist as a keto-enol tautomeric mixture in solution. Vicinal hydrogen coupling constant analysis proved reliable in ascertaining B-ring stereochemistry of 2,3-disubstituted-4-aryltetralones. 相似文献