全文获取类型
收费全文 | 44762篇 |
免费 | 6978篇 |
国内免费 | 4680篇 |
专业分类
化学 | 31161篇 |
晶体学 | 484篇 |
力学 | 2901篇 |
综合类 | 400篇 |
数学 | 4838篇 |
物理学 | 16636篇 |
出版年
2024年 | 170篇 |
2023年 | 905篇 |
2022年 | 1539篇 |
2021年 | 1544篇 |
2020年 | 1795篇 |
2019年 | 1786篇 |
2018年 | 1512篇 |
2017年 | 1388篇 |
2016年 | 2060篇 |
2015年 | 2115篇 |
2014年 | 2488篇 |
2013年 | 3227篇 |
2012年 | 4050篇 |
2011年 | 4034篇 |
2010年 | 2562篇 |
2009年 | 2636篇 |
2008年 | 2805篇 |
2007年 | 2535篇 |
2006年 | 2321篇 |
2005年 | 1921篇 |
2004年 | 1468篇 |
2003年 | 1181篇 |
2002年 | 1108篇 |
2001年 | 870篇 |
2000年 | 793篇 |
1999年 | 900篇 |
1998年 | 766篇 |
1997年 | 752篇 |
1996年 | 815篇 |
1995年 | 651篇 |
1994年 | 570篇 |
1993年 | 504篇 |
1992年 | 462篇 |
1991年 | 400篇 |
1990年 | 317篇 |
1989年 | 244篇 |
1988年 | 198篇 |
1987年 | 168篇 |
1986年 | 157篇 |
1985年 | 148篇 |
1984年 | 104篇 |
1983年 | 81篇 |
1982年 | 47篇 |
1981年 | 45篇 |
1980年 | 24篇 |
1979年 | 25篇 |
1978年 | 22篇 |
1977年 | 27篇 |
1975年 | 33篇 |
1974年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
In this paper, nonconvex multiobjective optimization problems are studied. New characterizations of a properly efficient solution in the sense of Geoffrion's are established in terms of the stability of one scalar optimization problem and the existence of an exact penalty function of a scalar constrained program, respectively. One of the characterizations is applied to derive necessary conditions for a properly efficient control-parameter pair of a nonconvex multiobjective discrete optimal control problem with linear constraints. 相似文献
92.
C.-S. Huang T. Li L. Wei Q.-S. Yan 《The European Physical Journal C - Particles and Fields》2002,23(1):195-199
It is shown that new sources of CP violation can be generated in models with more than one extra dimension. In the supersymmetric models on the space-time , where the radius moduli have auxiliary vacuum expectation values and the supersymmetry breaking is mediated by the Kaluza–Klein
states of the gauge supermultiplets, we analyze the gaugino masses and trilinear couplings for two scenarios and obtain the
result that there exist relative CP violating phases among the gaugino masses and trilinear couplings.
Received: 10 October 2001 / Published online: 20 December 2001 相似文献
93.
This paper proposes a new method that extends the efficient global optimization to address stochastic black-box systems. The
method is based on a kriging meta-model that provides a global prediction of the objective values and a measure of prediction
uncertainty at every point. The criterion for the infill sample selection is an augmented expected improvement function with
desirable properties for stochastic responses. The method is empirically compared with the revised simplex search, the simultaneous
perturbation stochastic approximation, and the DIRECT methods using six test problems from the literature. An application
case study on an inventory system is also documented. The results suggest that the proposed method has excellent consistency
and efficiency in finding global optimal solutions, and is particularly useful for expensive systems. 相似文献
94.
Y.-Q. Peng J.-H. Yang F.-P. Lu Q.-S. Yang H.-W. Xing X.-S. Li C.-A. Song 《Applied Physics A: Materials Science & Processing》2007,86(2):225-229
Based on the assumption of Gaussian energy distributions of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), analytical expressions of generalized Einstein relation in chemically doped organic semiconductor are developed, by approximation of Coulomb traps with a rectangle potential well. Numerical calculations show that traditional Einstein relations do not hold for chemically doped organic semiconductors. Similar to physical doping, the dependence of diffusion coefficient to mobility D/μ ratio on the carrier concentration has a maximum. An essential difference between chemical doping and physical doping is that, the D/μ ratio in chemically doped organic semiconductors depends not only on carrier concentration and doping concentration, but also on the applied electric field. PACS 71.20.Rv; 72.90.+y; 73.50.-h 相似文献
95.
The major goal of this paper is to examine the hypothesis that stock returns and return volatility are asymmetric, threshold nonlinear, functions of change in trading volume. A minor goal is to examine whether return spillover effects also display such asymmetry. Employing a double-threshold GARCH model with trading volume as a threshold variable, we find strong evidence supporting this hypothesis in five international market return series. Asymmetric causality tests lend further support to our trading volume threshold model and conclusions. Specifically, an increase in volume is positively associated, while decreasing volume is negatively associated, with the major price index in four of the five markets. The volatility of each series also displays an asymmetric reaction, four of the markets display higher volatility following increases in trading volume. Using posterior odds ratio, the proposed threshold model is strongly favored in three of the five markets, compared to a US news double threshold GARCH model and a symmetric GARCH model. We also find significant nonlinear asymmetric return spillover effects from the US market. 相似文献
96.
We study the four-directional traffic flow on a two-dimensional lattice. In the case of discrete densities, we assume equal number of vehicles in each lane. Except for the minimum density, the gridlock emerges swiftly. Two kinds of gridlock have been observed. The global gridlock dominates the system when the density is twice the minimum value. At higher densities, the system is pervaded by local gridlocks. We also analyze the time evolution of average speed. In the case of continuous densities, the vehicle numbers vary from lane to lane. The global gridlock is then destroyed by the fluctuations; while the local gridlock can still be observed. 相似文献
97.
Maria Mamalui Chien-Yueh Huang 《Physica E: Low-dimensional Systems and Nanostructures》2004,22(4):912-920
We present the theoretical study of the effect of external random field characterized by a Gaussian probability distribution function on the continuous phonon spectrum of one-dimensional (1D) chain, based on the Jacobian matrix method. The cumulative effect of the random field and simple isotopic defect is studied analytically and numerically. The Gaussian random field removes a square-root divergence appearing in the phonon spectrum of ideal 1D chain. The impurity phonon DOS shows strong dependence on the variance and the mean of the random field and exhibits very different behavior from the non-random case: the continuous spectrum is expanded and the δ-peak, describing discrete impurity vibrations in the non-random chain with the impurity, falls into a continuous zone. 相似文献
98.
By optimizing pump power ratio between 1st order backward pump and 2nd order forward pump on discrete Raman amplifier, we demonstrated over 2dB noise figure improvement without excessive non-linearity degradation. 相似文献
99.
100.
将心磁信号从干扰噪声中加以提取并有效地消除噪声干扰是心磁信号处理中尤为重要的环节 .从改进算法的角度出发,提出互补型自适应滤波器结构以实现心磁信号的消噪处理.该滤波器针对心磁这类非平稳信号进行设计,有效地解决了常规自适应滤波器应用于心磁信号处理时收敛速度和稳态误差的矛盾.通过仿真实验和心磁实验结果表明,该算法能有效地消除心磁信号的背景噪声和工频干扰噪声.同时该算法也可用于其他非平稳信号的消噪处理.
关键词:
自适应滤波
心磁图
最小均方误差 相似文献