全文获取类型
收费全文 | 3753篇 |
免费 | 304篇 |
国内免费 | 399篇 |
专业分类
化学 | 948篇 |
晶体学 | 13篇 |
力学 | 1035篇 |
综合类 | 77篇 |
数学 | 1203篇 |
物理学 | 1180篇 |
出版年
2024年 | 8篇 |
2023年 | 32篇 |
2022年 | 68篇 |
2021年 | 133篇 |
2020年 | 116篇 |
2019年 | 93篇 |
2018年 | 94篇 |
2017年 | 157篇 |
2016年 | 240篇 |
2015年 | 157篇 |
2014年 | 199篇 |
2013年 | 240篇 |
2012年 | 186篇 |
2011年 | 213篇 |
2010年 | 192篇 |
2009年 | 230篇 |
2008年 | 226篇 |
2007年 | 207篇 |
2006年 | 198篇 |
2005年 | 182篇 |
2004年 | 177篇 |
2003年 | 144篇 |
2002年 | 123篇 |
2001年 | 106篇 |
2000年 | 87篇 |
1999年 | 86篇 |
1998年 | 80篇 |
1997年 | 69篇 |
1996年 | 57篇 |
1995年 | 59篇 |
1994年 | 53篇 |
1993年 | 28篇 |
1992年 | 24篇 |
1991年 | 23篇 |
1990年 | 24篇 |
1989年 | 14篇 |
1988年 | 21篇 |
1987年 | 18篇 |
1986年 | 13篇 |
1985年 | 15篇 |
1984年 | 12篇 |
1982年 | 11篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1974年 | 4篇 |
1973年 | 2篇 |
排序方式: 共有4456条查询结果,搜索用时 31 毫秒
171.
172.
Simple and robust digital holography for high-resolution imaging 总被引:2,自引:0,他引:2
Based on the point spread function of holographic system,the lateral resolution of digital holographic imaging system without any pre-magnification is studied.The expression of resolution limitation of holographic imaging system is thus presented.We investigate the possibilities to improve the lateral resolution.The simple experimental setup with an off-axis arrangement is built.By using a U.S.Air Force(USAF)test target as microscopic object,the recorded holograms are reconstructed digitally based on the principle of Fresnel diffraction.The lateral resolution of 2.76 μm without any pre-magnification is demonstrated experimentally,which matches the theoretical prediction well. 相似文献
173.
Tonatiuh Matos Darío Núñez F. Siddhartha Guzmán Erandy Ramírez 《General Relativity and Gravitation》2002,34(2):283-305
In an arbitrary axisymmetric stationary spacetime, we determine the expression for the tangential velocity of test objects following a circular stable geodesic motion in the equatorial plane, as function of the metric coefficients. Next, we impose the condition, observed in large samples of disks galaxies, that the magnitude of such tangential velocity be radii independent in the dark matter dominated region, obtaining a constraint equation among the metric coefficients, and thus arriving to an iff (iff means: if and only if.) condition: The tangential velocity of test particles is radii independent iff the metric coefficients satisfied the mentioned constraint equation. Furthermore, for the static case, the constraint equation can be easily integrated, leaving the spacetime at the equatorial plane essentially with only one independent metric coefficient. With the geometry thus fixed, we compute the Einstein tensor and equate it to an arbitrary stress energy tensor, in order to determine the type of energy-matter which could produce such a geometry. Within an approximation, we deduce a constraint equation among the components of the stress energy tensor. We test in that constraint equation several well known types of matter, which have been proposed as dark matter candidates and are able to point for possible right ones. Finally, we also present the spherically symmetric static case and apply the mentioned procedure to perfect fluid stress energy tensor, recovering the Newtonian result as well as the one obtained in the axisymmetric case. We also present arguments on the need to use GR to study types of matter different than the dust one. 相似文献
174.
以3-氨基-5-硝基2,1-苯并异噻唑(BIT) 为重氮组分,在适当的条件下,与单苯胺类及萘胺磺酸类衍生物偶联, 得到了从红到兰色的一系列分散染料. 这些染料均未见报导,并对其中部分染料进行了检测,证明各项性能优良,应用前景看好. 相似文献
175.
We describe a technique for generating a special class, called QPEC, of mathematical programs with equilibrium constraints, MPEC. A QPEC is a quadratic MPEC, that is an optimization problem whose objective function is quadratic, first-level constraints are linear, and second-level (equilibrium) constraints are given by a parametric affine variational inequality or one of its specialisations. The generator, written in MATLAB, allows the user to control different properties of the QPEC and its solution. Options include the proportion of degenerate constraints in both the first and second level, ill-conditioning, convexity of the objective, monotonicity and symmetry of the second-level problem, and so on. We believe these properties may substantially effect efficiency of existing methods for MPEC, and illustrate this numerically by applying several methods to generator test problems. Documentation and relevant codes can be found by visiting http://www.ms.unimelb.edu.au/danny/qpecgendoc.html. 相似文献
176.
177.
Several modifications of the upsetting test at room temperature are discussed under the aspect of suppressing friction and other possible sources of error.In the upsetting test according to Rastegaev, friction is reduced, and the testpiece remains cylindrical up to high strains, however the specimen's height is determined with a large error. Therefore it is proposed to measure the increase of the specimen's diameter instead of the reduction of height.Upsetting under sticking friction conditions can formally be treated like upsetting with lubrication for calculating the stress-strain curve. However, the applicability and the limits of this method have not yet been studied sufficiently.In the plane-strain upsetting test, friction is not suppressed completely, but the error of height measurement propagates only weakly into the calculated stress-strain curve. 相似文献
178.
For a simple multivariate regression model, nonparametric estimation of the (vector of) intercept following a preliminary test on the regression vector is considered. Along with the asymptotic distribution of these estimators, their asymptotic bias and dispersion matrices are studied and allied efficiency results are presented. 相似文献
179.
Michael D. Perlman 《Journal of multivariate analysis》1974,4(1):22-30
It is shown that for the MANOVA problem the power function of the test based on the trace of a multivariate beta matrix is monotonically increasing in each noncentrality parameter provided that the cutoff point is not too large. This result is also true for the problem of testing independence of two sets of variates. 相似文献
180.
本文就乌鲁木齐市不同民族,不同职业的居民对保险产品的认同水平运用χ2统计量[1]及关系矩阵进行齐一性假设检验,并对结果进行分析,得出了合乎情理的结论。根据居民职业的不同我们可以将文卫科、企管、工人、农牧、服务业、其他职业归为一类,而"公务员"、"金融"、"个体"各为一类。不同民族的居民对保险的认同水平同样差异显著:回族、维族与其它民族的差异性很小;回族与汉族有差异不大;而维族与汉族之间差异最大。 相似文献