全文获取类型
收费全文 | 261220篇 |
免费 | 1946篇 |
国内免费 | 538篇 |
专业分类
化学 | 134493篇 |
晶体学 | 4243篇 |
力学 | 13046篇 |
综合类 | 11篇 |
数学 | 28944篇 |
物理学 | 82967篇 |
出版年
2021年 | 2553篇 |
2020年 | 2676篇 |
2019年 | 3274篇 |
2018年 | 4711篇 |
2017年 | 4750篇 |
2016年 | 6264篇 |
2015年 | 3091篇 |
2014年 | 5519篇 |
2013年 | 11058篇 |
2012年 | 8985篇 |
2011年 | 10488篇 |
2010年 | 8347篇 |
2009年 | 8550篇 |
2008年 | 9868篇 |
2007年 | 9721篇 |
2006年 | 8765篇 |
2005年 | 7714篇 |
2004年 | 7311篇 |
2003年 | 6712篇 |
2002年 | 6707篇 |
2001年 | 7273篇 |
2000年 | 5392篇 |
1999年 | 4118篇 |
1998年 | 3612篇 |
1997年 | 3576篇 |
1996年 | 3261篇 |
1995年 | 2863篇 |
1994年 | 2941篇 |
1993年 | 2870篇 |
1992年 | 3182篇 |
1991年 | 3350篇 |
1990年 | 3189篇 |
1989年 | 3320篇 |
1988年 | 3148篇 |
1987年 | 3105篇 |
1986年 | 2973篇 |
1985年 | 3690篇 |
1984年 | 3857篇 |
1983年 | 3223篇 |
1982年 | 3243篇 |
1981年 | 3117篇 |
1980年 | 2900篇 |
1979年 | 3289篇 |
1978年 | 3401篇 |
1977年 | 3580篇 |
1976年 | 3483篇 |
1975年 | 3234篇 |
1974年 | 3216篇 |
1973年 | 3302篇 |
1972年 | 2510篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
131.
In the present paper, we find a class of linear homogeneous differential equations of order n + 1 (n > 1) whose fundamental system of solutions is constructed from the fundamental system of solutions of a second-order differential
equation. The spectral properties of differential operators generated by these differential expressions are investigated.
In particular, sufficient conditions are obtained for the coefficients of a second-order differential equation under which
the case of maximal deficiency indices is realized.
Dedicated to the memory of B. M. Levitan 相似文献
132.
E. N. Sobol’ O. I. Baum A. V. Bol’shunov V. I. Sipliviy N. Yu. Ignat’eva O. L. Zakharkina V. V. Lunin A. I. Omel’chenko V. A. Kamenskiy A. V. Myakov 《Laser Physics》2006,16(5):735-740
A new approach to alterations in eye refraction upon nondestructive laser action on the sclera and cornea is studied. It is demonstrated in in vivo experiments on rabbit eyes that sequential laser irradiation of the sclera and cornea yields a significant alteration in the eye refraction. The collagen structure of the sclera and cornea is studied after the nondestructive laser action with noninvasive polarization-sensitive optical coherence tomography. It is demonstrated that collagen fibers that provide for the cornea tension and applanation partially survive in the zone of the local denaturation of sclera. An irradiation mode that corresponds to an increase in the cornea’s plasticity and does not cause visible structural changes is chosen. The simplest theoretical model for alterations in the eye refraction upon the nonablative laser action on sclera is analyzed. The alteration in the cornea curvature upon stretching resulting from the local sclera coagulation and the corresponding decrease in its volume is calculated. The model makes it possible to approximately estimate the laser irradiation modes that provide the desired alterations in eye refraction. 相似文献
133.
N. S. Simonović 《Few-Body Systems》2006,38(2-4):139-145
In agreement with the Kohn theorem the relative motion (rel) of three electrons in a two-dimensional parabolic trap separates
from the centre-of-mass (CM) motion. By introducing new coordinates the Hamiltonian for relative motion in the approximation
of non-interacting electrons can be taken to the normal form. The eigenstates of the normalized Hamiltonian are products of
the Fock-Darwin states for normal modes. The energy levels for relative motion are obtained by diagonalizing the exact Hamiltonian
in the eigenbasis for the non-interacting case. In this basis the interaction matrix elements can be obtained in the analytical
form. Since the rank of the Hamiltonian matrix is significantly reduced, the calculations are faster and more accurate than
those for the full (CM + rel) motion. This advantage is especially important for the calculations of excited states and the
analysis of energy spectra. 相似文献
134.
O. I. Berngardt 《Radiophysics and Quantum Electronics》2006,49(6):415-431
We propose a model of coherent-echo signals during the monostatic sounding of the ionosphere. The model is based on the previously
obtained radar equation for separate samples of the scattered signal. The dielectric-permittivity perturbation described by
a discrete set of spatial harmonics modulated in space and time is used as the scattering irregularities. The model was tested
using Irkutsk incoherent-scatter radar data obtained during the coherent-echo observations in July 15 and 16, 2000. The test
shows that the model is suitable for describing the observed characteristics of separate sample spectra of the coherent-echo
signals.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 6, pp. 459–477, June 2006. 相似文献
135.
136.
A stability theorem is proved for mappings with bounded distortion over John domains in the Heisenberg group furnished with the Carnot–Caratheodory metric. 相似文献
137.
We investigate the linear stability of the Bickley jet in the framework of the beta-plane approximation. Because singular inviscid neutral modes exist in the retrograde case , it is necessary to add viscosity to interpret them. One of these modes was found in closed form by Howard and Drazin [1] . However, its critical point is at the center of the jet and it was therefore not possible for these authors to ascertain the relationship of this mode to the stability problem or to discuss how to continue the eigenfunction across the singularity.
The viscous critical layer problem associated with this singularity is considerably more difficult than the usual one (which leads to integrals of the Airy function) because and, consequently, a second-order turning point is involved. Our analysis shows that the Howard–Drazin mode is degenerate in the domain where it is valid as a limit of the viscous problem (wavenumber α2 ≤ 9/2 ), that is, it corresponds to both an odd and an even mode. This conclusion is confirmed by direct numerical solution of the Orr–Sommerfeld equation which shows, in addition, that viscosity is destabilizing along portions of the stability boundary. For a retrograde jet, instability is found to occur beyond the inviscid critical value of β, that is, in the region where the flow would be stable according to the Rayleigh–Kuo condition. 相似文献
The viscous critical layer problem associated with this singularity is considerably more difficult than the usual one (which leads to integrals of the Airy function) because and, consequently, a second-order turning point is involved. Our analysis shows that the Howard–Drazin mode is degenerate in the domain where it is valid as a limit of the viscous problem (wavenumber α
138.
The spin splitting caused by the terms linear in wavevector in the effective Hamiltonian containing can give rise to the new magneto-oscillation phenomena in two-dimensional systems. It is shown that the joint action of the spin-dependent contributions due to the heterostructure asymmetry and to the lack of inversion center in the bulk material suppresses beats that arise in the magneto-oscillation phenomena in the presence of the terms of only one of these types. 相似文献
139.
Optics and Spectroscopy - Specific features of the light-beam transverse structure transformation in the process of four-wave coupling in the Fabry-Perot interferometer with a resonant nonlinearity... 相似文献
140.
A Dirac picture perturbation theory is developed for the time evolution operator in classical dynamics in the spirit of the Schwinger–Feynman–Dyson perturbation expansion and detailed rules are derived for computations. Complexification formalisms are given for the time evolution operator suitable for phase space analyses, and then extended to a two-dimensional setting for a study of the geometrical Berry phase as an example. Finally a direct integration of Hamilton's equations is shown to lead naturally to a path integral expression, as a resolution of the identity, as applied to arbitrary functions of generalized coordinates and momenta. 相似文献