全文获取类型
收费全文 | 88337篇 |
免费 | 7019篇 |
国内免费 | 6911篇 |
专业分类
化学 | 31570篇 |
晶体学 | 356篇 |
力学 | 7563篇 |
综合类 | 1425篇 |
数学 | 33585篇 |
物理学 | 27768篇 |
出版年
2023年 | 688篇 |
2022年 | 1297篇 |
2021年 | 1769篇 |
2020年 | 1813篇 |
2019年 | 2021篇 |
2018年 | 1618篇 |
2017年 | 1976篇 |
2016年 | 2476篇 |
2015年 | 2166篇 |
2014年 | 3227篇 |
2013年 | 5877篇 |
2012年 | 3547篇 |
2011年 | 4001篇 |
2010年 | 3527篇 |
2009年 | 4872篇 |
2008年 | 5471篇 |
2007年 | 5700篇 |
2006年 | 5191篇 |
2005年 | 4362篇 |
2004年 | 3936篇 |
2003年 | 4047篇 |
2002年 | 3693篇 |
2001年 | 3086篇 |
2000年 | 3032篇 |
1999年 | 2660篇 |
1998年 | 2544篇 |
1997年 | 2014篇 |
1996年 | 1795篇 |
1995年 | 1615篇 |
1994年 | 1489篇 |
1993年 | 1283篇 |
1992年 | 1243篇 |
1991年 | 942篇 |
1990年 | 790篇 |
1989年 | 712篇 |
1988年 | 677篇 |
1987年 | 524篇 |
1986年 | 452篇 |
1985年 | 573篇 |
1984年 | 535篇 |
1983年 | 246篇 |
1982年 | 394篇 |
1981年 | 452篇 |
1980年 | 344篇 |
1979年 | 332篇 |
1978年 | 258篇 |
1977年 | 243篇 |
1976年 | 189篇 |
1974年 | 120篇 |
1973年 | 121篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
利用K-泛函研究了修正的Baskakov型算子的Stechin-Marchaud型不等式,由此不等式,我们得到了关于ω2φλ的逆结果. 相似文献
942.
Roberto Bassani 《Meccanica》2006,41(4):375-389
S. Earnshaw, a mathematical physicist born in England two centuries ago, was the author of studies on the transmission of
sound and light. His theory on the transmission of light by “detached particles” has also influenced some studies on the stability
of bodies in potential fields, particularly studies on the levitation of permanent magnets, i.e. on passive magnetic levitation.
This theorem was so well known that even Maxwell mentioned it, and is today cited almost as an axiom in the ambit of passive
magnetization. However, what is less well known is that the theory actually only relates to magnetostatics. In this paper,
the mathematic model of the theory that refers to stability is outlined, along with some works that refer to it, in particular
those on passive magnetic levitation. It is then pointed out that spaces of low instability exist in magnetostatics, and that
stability may exist in the magneto-stationary and in magneto-dynamic ambit. 相似文献
943.
Li Changjun; Li Zheng; Evans David J.; Zhang Tie 《IMA Journal of Numerical Analysis》2003,23(4):581-592
Golub et al. (2001, BIT, 41, 7185) gave a generalizedsuccessive over-relaxation method for the augmented systems.In this paper, the connection between the SOR-like method andthe preconditioned conjugate gradient (PCG) method for the augmentedsystems is investigated. It is shown that the PCG method isat least as accurate (fast) as the SOR-like method. Numericalexamples demonstrate that the PCG method is much faster thanthe SOR-like method. 相似文献
944.
电磁继电器触点参数的测试 总被引:1,自引:0,他引:1
本文介绍了电磁继电器触点电参数的测试方法,包括接触电阻、绝缘电阻参数测试。 相似文献
945.
We study the system of nonlinear differential equations which expresses the constancy of the algebraic invariants of the Jacobian matrix for smooth vector fields in three-dimensional space. This system is equivalent to the equations of gas dynamics which describe the barochronous motions of a gas (the pressure and density depend only on the time). We present the results of computation of the admissible local Lie group and construction of the general solution of the system. We mention a few new problems that arise here. 相似文献
946.
《Journal of separation science》2003,26(5):402-408
When nitrated polynuclear aromatic hydrocarbons (NPAH) were recognised as long‐term toxic compounds and were also identified in several anthropogenic emissions and among the by‐products of photochemical smog they became objects of investigation in ambient air in both polluted and clean environments. In Italy the first investigations focussing on atmospheric NPAH were performed in the early eighties and encompassed both the canyon streets and dwelling areas of Rome. Because NPAH were indicated as markers of vehicle emission, new investigations were performed in other cities and also in rural and (nominally) unpolluted locations. In general, the first measurements of NPAH were episodic and could not be assumed to be strictly representative of the sites investigated. In contrast, nowadays consistent data bases on NPAH have been built up and are discussed in the technical literature, so that both neat concentrations and the yearly timed variation of the most important NPAH congeners have been measured in Italian air. Although concentrations of NPAH in air are usually much lower than those of PAH, owing to their very high potency in inducing mutations and tumours NPAH are regarded as having an important health impact on populations living in Italian urban areas. In this regard the genotoxicity of atmospheric aerosols is partly associated with nitrofluoranthenes and nitropyrenes whereas nitronaphthalenes and methylnitronaphthalenes seem to contribute to that of the gaseous organic fraction. Several civic and regional administrations are introducing control of atmospheric NPAH among ways of improving air quality in cities. 相似文献
947.
A. Tseronis D. Tsilimantos N. Uzunoglu 《International Journal of Infrared and Millimeter Waves》2005,26(2):279-295
Radiation phenomena observed in a wedge shape ended dielectric slab waveguide are analyzed using mode matching technique. The case of transverse electric polarization (TE) being parallel to dielectric slab waveguide is assumed. In order to describe the fields in the wedge region, a stack of dielectric plates is assumed and in each layer the fields are expanded in terms of the mixed spectrum of guided and radiated modes. A similar expansion is used in the constant thickness slab waveguide while in free space medium a continuous-radiation mode expansion is used. Then a mode matching approach is applied, incorporating the orthogonality properties of mixed spectrum modes, in order to compute the wave fields inside the dielectric slab waveguide and wedge medium. Mode matching is achieved by discretizing the continuous radiation mode spectrum leading into a numerically stable solution provided a sufficient large number of points are used to convert integrals into finite summations. Numerical computations are carried out for various wedge geometries and shapes including linear and exponential profiles. 相似文献
948.
ZhengBin Zhang Chunying Liu Peifeng Li Zhenzhen Wu Cai Lin Huawei Huang Lei Xing Liansheng Liu 《Frontiers of Chemistry in China》2006,1(1):86-97
Many food algae and red tide algae were cultivated in the f/2 medium, and the nitric oxide (NO) concentration of the medium
and the cell density were determined. The experiments on algae were conducted when different concentrations of NO were added
into the medium using two methods. The results show that low concentrations of NO were self-produced by marine algae during
the exponential growth period and were about nmol/L level. But at the end of the period, i.e., 2 or 3 days before the cell
density reaches the maximum, an NO peak appeared, with the NO concentration reaching 10 nmol/L. The NO threshold concentration
exists according to the influence of exogenous NO on the marine phytoplankton growth. One type is the threshold concentration
that can promote algae growth, and its value is between 10 and 1 nmol level, or even lower. The other type can inhibit the
phytoplankton growth, which is about μmol level or higher. The results indicate that red tide algae are far more sensitive
to NO than are food algae. The fundamental experimental outcome above may provide a new clue for red tide chemical forecast
by inspecting the NO change. 相似文献
949.
950.
Fabio Schoen 《Journal of Global Optimization》1994,4(1):17-35
In this paper a new algorithm is proposed for global optimization problems. The main idea is that of modifying a standard clustering approach by sequentially sampling the objective function while adaptively deciding an appropriate sample size. Theoretical as well as computational results are presented. 相似文献