全文获取类型
收费全文 | 176388篇 |
免费 | 575篇 |
国内免费 | 133篇 |
专业分类
化学 | 73549篇 |
晶体学 | 2564篇 |
力学 | 10422篇 |
综合类 | 1篇 |
数学 | 32454篇 |
物理学 | 58106篇 |
出版年
2021年 | 870篇 |
2020年 | 967篇 |
2019年 | 1111篇 |
2018年 | 10331篇 |
2017年 | 11206篇 |
2016年 | 6183篇 |
2015年 | 1854篇 |
2014年 | 1849篇 |
2013年 | 3401篇 |
2012年 | 7269篇 |
2011年 | 15928篇 |
2010年 | 9754篇 |
2009年 | 9892篇 |
2008年 | 13736篇 |
2007年 | 17312篇 |
2006年 | 2971篇 |
2005年 | 9168篇 |
2004年 | 5686篇 |
2003年 | 5622篇 |
2002年 | 3609篇 |
2001年 | 2402篇 |
2000年 | 1961篇 |
1999年 | 1294篇 |
1998年 | 1175篇 |
1997年 | 1057篇 |
1996年 | 1079篇 |
1995年 | 939篇 |
1994年 | 907篇 |
1993年 | 897篇 |
1992年 | 938篇 |
1991年 | 1014篇 |
1990年 | 956篇 |
1989年 | 991篇 |
1988年 | 913篇 |
1987年 | 861篇 |
1986年 | 822篇 |
1985年 | 961篇 |
1984年 | 1074篇 |
1983年 | 919篇 |
1982年 | 918篇 |
1981年 | 833篇 |
1980年 | 782篇 |
1979年 | 867篇 |
1978年 | 997篇 |
1977年 | 936篇 |
1976年 | 868篇 |
1975年 | 815篇 |
1974年 | 830篇 |
1973年 | 847篇 |
1972年 | 591篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
121.
S. P. Mukherjee A. K. Chattopadhyay 《The Journal of the Operational Research Society》1989,40(5):489-494
Consider an organization with several categories of staff, each having a specified number of members. The age of entry into service and the age of retirement are both specified. Withdrawals occur at time-independent rates. Under this set-up, the authors have computed the maximum ages at which promotions should take place, assuming that the staff members continuing at various ages are random. Some illustrative calculations have been carried out. 相似文献
122.
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
123.
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
124.
In the field of industrial vision, the 3D inspection of highly reflective metallic objects is still a delicate task. The specular
reflections prevent the use of a 3D laser scanner, whereas the phase shifting-based 3D systems are more adapted to inspect
surfaces with low curvature. This paper deals with a new automated 3D inspection system based on polarization analysis. Studying
the state of polarization of the reflected light is very useful for obtaining information on the normals of the surface. An
extension of the shape from polarization method from dielectric to metallic surfaces is demonstrated. Then, an improved relaxation
algorithm is provided in order to reconstruct the shape from the normal field given by the polarization analysis. Finally,
applications to shape defect detection are discussed and the efficiency of the system in discriminating defects on highly
reflective metallic objects made by stamping and polishing is presented.
The text was submitted by the authors in English. 相似文献
125.
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
126.
Th. Seyller K.V. Emtsev F. Speck A. Tadich J.D. Riley O. Rader A.M. Shikin 《Surface science》2006,600(18):3906-3911
Photoelectron spectroscopy, low-energy electron diffraction, and scanning probe microscopy were used to investigate the electronic and structural properties of graphite layers grown by solid state graphitization of SiC(0 0 0 1) surfaces. The process leads to well-ordered graphite layers which are rotated against the substrate lattice by 30°. On on-axis 6H-SiC(0 0 0 1) substrates we observe graphitic layers with up to several 100 nm wide terraces. ARUPS spectra of the graphite layers grown on on-axis 6H-SiC(0 0 0 1) surfaces are indicative of a well-developed band structure. For the graphite/n-type 6H-SiC(0 0 0 1) layer system we observe a Schottky barrier height of ?B,n = 0.3 ± 0.1 eV. ARUPS spectra of graphite layers grown on 8° off-axis oriented 4H-SiC(0 0 0 1) show unique replicas which are explained by a carpet-like growth mode combined with a step bunching of the substrate. 相似文献
127.
Dual fractional cutting plane algorithms, in which cutting planes are used to iteratively tighten a linear relaxation of an integer program,
are well-known and form the basis of the highly successful branch-and-cut method. It is rather less well-known that various primal cutting plane algorithms were developed in the 1960s, for example by Young. In a primal algorithm, the main role of the cutting
planes is to enable a feasible solution to the original problem to be improved. Research on these algorithms has been almost
non-existent.
In this paper we argue for a re-examination of these primal methods. We describe a new primal algorithm for pure 0-1 problems based on strong valid inequalities and give some encouraging computational results. Possible extensions to the case of general
mixed-integer programs are also discussed. 相似文献
128.
129.
Annegret K. Wagler 《Mathematical Methods of Operations Research》2002,56(1):127-149
An edge e of a perfect graph G is critical if G−e is imperfect. We would like to decide whether G−e is still “almost perfect” or already “very imperfect”. Via relaxations of the stable set polytope of a graph, we define two
superclasses of perfect graphs: rank-perfect and weakly rank-perfect graphs. Membership in those two classes indicates how
far an imperfect graph is away from being perfect. We study the cases, when a critical edge is removed from the line graph
of a bipartite graph or from the complement of such a graph. 相似文献
130.
Possible Loss and Recovery of Gibbsianness¶During the Stochastic Evolution of Gibbs Measures 总被引:1,自引:1,他引:0
A.C.D. van Enter R. Fernández F. den Hollander F. Redig 《Communications in Mathematical Physics》2002,226(1):101-130
We consider Ising-spin systems starting from an initial Gibbs measure ν and evolving under a spin-flip dynamics towards a
reversible Gibbs measure μ≠ν. Both ν and μ are assumed to have a translation-invariant finite-range interaction. We study
the Gibbsian character of the measure νS(t) at time t and show the following:
(1) For all ν and μ, νS(t) is Gibbs for small t.
(2) If both ν and μ have a high or infinite temperature, then νS(t) is Gibbs for all t > 0.
(3) If ν has a low non-zero temperature and a zero magnetic field and μ has a high or infinite temperature, then νS(t) is Gibbs for small t and non-Gibbs for large t.
(4) If ν has a low non-zero temperature and a non-zero magnetic field and μ has a high or infinite temperature, then νS(t) is Gibbs for small t, non-Gibbs for intermediate t, and Gibbs for large t.
The regime where μ has a low or zero temperature and t is not small remains open. This regime presumably allows for many different scenarios.
Received: 26 April 2001 / Accepted: 10 October 2001 相似文献