全文获取类型
收费全文 | 78680篇 |
免费 | 5285篇 |
国内免费 | 4787篇 |
专业分类
化学 | 44664篇 |
晶体学 | 422篇 |
力学 | 6540篇 |
综合类 | 1319篇 |
数学 | 17662篇 |
物理学 | 18145篇 |
出版年
2024年 | 101篇 |
2023年 | 628篇 |
2022年 | 1077篇 |
2021年 | 2565篇 |
2020年 | 1939篇 |
2019年 | 1936篇 |
2018年 | 1596篇 |
2017年 | 1948篇 |
2016年 | 2544篇 |
2015年 | 2157篇 |
2014年 | 2922篇 |
2013年 | 5224篇 |
2012年 | 3968篇 |
2011年 | 3824篇 |
2010年 | 3408篇 |
2009年 | 4358篇 |
2008年 | 4441篇 |
2007年 | 4853篇 |
2006年 | 3952篇 |
2005年 | 3179篇 |
2004年 | 3028篇 |
2003年 | 2778篇 |
2002年 | 4860篇 |
2001年 | 2315篇 |
2000年 | 1896篇 |
1999年 | 1628篇 |
1998年 | 1536篇 |
1997年 | 1294篇 |
1996年 | 1311篇 |
1995年 | 1183篇 |
1994年 | 1074篇 |
1993年 | 973篇 |
1992年 | 945篇 |
1991年 | 710篇 |
1990年 | 627篇 |
1989年 | 446篇 |
1988年 | 496篇 |
1987年 | 346篇 |
1986年 | 332篇 |
1985年 | 463篇 |
1984年 | 356篇 |
1983年 | 203篇 |
1982年 | 394篇 |
1981年 | 518篇 |
1980年 | 467篇 |
1979年 | 522篇 |
1978年 | 402篇 |
1977年 | 300篇 |
1976年 | 265篇 |
1973年 | 157篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Summary In this paper a Gauss-Jordan algorithm with column interchanges is presented and analysed. We show that, in contrast with Gaussian elimination, the Gauss-Jordan algorithm has essentially differing properties when using column interchanges instead of row interchanges for improving the numerical stability. For solutions obtained by Gauss-Jordan with column interchanges, a more satisfactory bound for the residual norm can be given. The analysis gives theoretical evidence that the algorithm yields numerical solutions as good as those obtained by Gaussian elimination and that, in most practical situations, the residuals are equally small. This is confirmed by numerical experiments. Moreover, timing experiments on a Cyber 205 vector computer show that the algorithm presented has good vectorisation properties. 相似文献
102.
This paper reports that the m-plane GaN layer is grown on (200)-plane LiAlO2 substrate by metal-organic chemical wpour deposition (MOCVD) method. Tetragonal-shaped crystallites appear at the smooth surface. Raman measurement illuminates the compressive stress in the layer which is released with increasing the layer's thickness. The high transmittance (80%), sharp band edge and excitonic absorption peak show that the GaN layer has good optical quality. The donor acceptor pair emission peak located at -3.41 eV with full-width at half maximum of 120 meV and no yellow peaks in the photoluminescence spectra partially show that no Li incorporated into GaN layer from the LiAlO2 substrate. 相似文献
103.
104.
105.
The high-temperature cubic phase of non-stoichiometric strontium ferrite SrFeOx (2.5≤x≤3.0) has been studied by in situ neutron powder diffraction in air over the temperature range 300-1273 K. The composition of SrFeOx changes within the range 2.56≤x≤2.81 from 1273 to 673 K, respectively.Rietveld refinements of the diffraction patterns show that the high-temperature cubic phase of SrFeOx is consistent with a face-centred Fm3c structure. This structure leads to agreement with previous density measurements. This cell allows the high-temperature structure of SrFeOx to be described in terms of a solid solution of the composition end members. Cubic SrFeOx at high temperature is found to closely obey Vegard's law. The density of cubic SrFeOx is also found to exhibit a linear relationship with composition. 相似文献
106.
107.
Copper bromide was used as a catalyst for the addition of pyrroles to enones. When both the reactants were used in equimolar amounts, mono and dialkylated products were obtained. However, the use of excess enone furnished only dialkylated products. Thus, copper bromide was shown to be an efficient catalyst for the dialkylation of pyrroles. 相似文献
108.
ON HYPERBOLIC TIME DISCOUNTING IN EXHAUSTIBLE RESOURCE MODELS: AN APPLICATION TO WORLD OIL RESOURCES
JOHN ROWSE 《Natural Resource Modeling》2006,19(2):243-277
ABSTRACT. Recent research on discounting in long term economic models involves hyperbolic discounting, in which the marginal discount rate shrinks as time passes. To investigate hyperbolic discounting and exhaustible resource allocation, this work develops a discrete‐time world oil model and model solution procedure, then uses the model to examine the consequences of adopting conventional (constant annual) discounting when hyperbolic discounting is appropriate, of adopting one hyperbolic discount rate path when a different hyperbolic path is appropriate, and of adopting hyperbolic discounting when conventional discounting is appropriate. Five conventional and two hyperbolic discount rate paths are considered. One hyperbolic path is that used by Nordhaus and Boyer [2000]; the other is that recommended by Weitzman [2001]. The generality of the findings is also assessed. 相似文献
109.
Dalila B. M. M. Fontes Eleni Hadjiconstantinou Nicos Christofides 《Journal of Global Optimization》2006,34(1):97-125
In this paper we obtain Lower Bounds (LBs) to concave cost network flow problems. The LBs are derived from state space relaxations
of a dynamic programming formulation, which involve the use of non-injective mapping functions guaranteing a reduction on
the cardinality of the state space. The general state space relaxation procedure is extended to address problems involving
transitions that go across several stages, as is the case of network flow problems. Applications for these LBs include: estimation
of the quality of heuristic solutions; local search methods that use information of the LB solution structure to find initial
solutions to restart the search (Fontes et al., 2003, Networks, 41, 221–228); and branch-and-bound (BB) methods having as
a bounding procedure a modified version of the LB algorithm developed here, (see Fontes et al., 2005a). These LBs are iteratively
improved by penalizing, in a Lagrangian fashion, customers not exactly satisfied or by performing state space modifications.
Both the penalties and the state space are updated by using the subgradient method. Additional constraints are developed to
improve further the LBs by reducing the searchable space. The computational results provided show that very good bounds can
be obtained for concave cost network flow problems, particularly for fixed-charge problems. 相似文献
110.
F. Balibrea 《Topology and its Applications》2006,153(12):2092-2095
Recently, Forti, Paganoni and Smítal constructed an example of a triangular map of the unite square, F(x,y)=(f(x),g(x,y)), possessing periodic orbits of all periods and such that no infinite ω-limit set of F contains a periodic point. In this note we show that the above quoted map F has a homoclinic orbit. As a consequence, we answer in the negative the problem presented by A.N. Sharkovsky in the eighties whether, for a triangular map of the square, existence of a homoclinic orbit implies the existence of an infinite ω-limit set containing a periodic point. It is well known that, for a continuous map of the interval, the answer is positive. 相似文献