全文获取类型
收费全文 | 20257篇 |
免费 | 2101篇 |
国内免费 | 1182篇 |
专业分类
化学 | 3019篇 |
晶体学 | 82篇 |
力学 | 2092篇 |
综合类 | 320篇 |
数学 | 10127篇 |
物理学 | 7900篇 |
出版年
2024年 | 25篇 |
2023年 | 141篇 |
2022年 | 225篇 |
2021年 | 420篇 |
2020年 | 492篇 |
2019年 | 496篇 |
2018年 | 466篇 |
2017年 | 587篇 |
2016年 | 679篇 |
2015年 | 626篇 |
2014年 | 1039篇 |
2013年 | 1686篇 |
2012年 | 946篇 |
2011年 | 1075篇 |
2010年 | 893篇 |
2009年 | 1211篇 |
2008年 | 1235篇 |
2007年 | 1369篇 |
2006年 | 1196篇 |
2005年 | 990篇 |
2004年 | 889篇 |
2003年 | 870篇 |
2002年 | 805篇 |
2001年 | 644篇 |
2000年 | 685篇 |
1999年 | 603篇 |
1998年 | 513篇 |
1997年 | 475篇 |
1996年 | 355篇 |
1995年 | 262篇 |
1994年 | 224篇 |
1993年 | 181篇 |
1992年 | 186篇 |
1991年 | 123篇 |
1990年 | 96篇 |
1989年 | 83篇 |
1988年 | 79篇 |
1987年 | 84篇 |
1986年 | 64篇 |
1985年 | 71篇 |
1984年 | 71篇 |
1983年 | 33篇 |
1982年 | 56篇 |
1981年 | 58篇 |
1980年 | 47篇 |
1979年 | 57篇 |
1978年 | 20篇 |
1977年 | 32篇 |
1976年 | 27篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
由于可见光成像系统的聚焦范围有限,因而在成像过程中,除聚焦良好的物体能生成清晰的图像外,该物体前后一定距离外的所有物体都将呈现不同程度的模糊.为了获得场景内所有物体均清晰的图像,在分析了多聚焦图像成像机理的基础上,提出了一种基于小波包变换的融合方法.它是将成像系统先聚焦在一部分对象上,得到其清晰的图像;然后再将其聚焦在另一部分对象上,得到另一清晰的图像;最后把这两幅实验图像加以融合,从而获得场景内所有物体均清晰的图像.实验结果表明,基于小波包变换的融合方法能够将信号的频带进行多层次划分,对高频成分也能进一步地分解,可有效综合多聚焦图像. 相似文献
73.
74.
Differential game formulations provide an adequate basis for a guidance law synthesis against highly maneuvering targets. This paper deals with a guidance law based on a linear-quadratic differential game formulation. This guidance law has many attractive properties: it is continuous, linear with respect to the state variables, and its gain coefficients can be precalculated offline. Nevertheless, due to the lack of hard control constraints in the formulation, the magnitude of the control can exceed the admissible level imposed by the nature of the problem. In this paper, the upper bound of the interceptor control is obtained depending on the system parameters and the penalty coefficients of the game performance index. It is shown that the interceptor can guarantee an arbitrarily small miss distance without exceeding the control constraints if it has sufficient maneuverability and if the penalty coefficients are chosen properly. By manipulating the penalty coefficients, it is possible to reduce significantly the maneuverability requirements compared to the case of zero interceptor penalty coefficient. 相似文献
75.
Defining speed of diffusion as the amount of time it takes to get from one penetration level to a higher one, we introduce a dynamic model in which we study the link between pricing policy, speed of diffusion, and number of competitors in the market. Our analysis shows that, in the case of strategic (oligopolistic) competition, the speed of diffusion has an important influence on the optimal pricing policy. In particular, we find that higher speeds of diffusion create an incentive to strategically interacting firms to lower their prices. 相似文献
76.
In this paper, a unified approach for analysing finite dimensional approximations to a class of partial differential equations boundary value problems (second‐kind Fredholm differential equations) is introduced. The approach is shown to be general despite of its extremely simple form. In particular, it is expected to be useful in the convergence analysis of finite element methods for solving PDE problems. Three specific examples are presented to illustrate the broad applicability of the approach. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
77.
In this paper, we prove convergence rates for spherical spline Hermite interpolation on the sphere Sd−1 via an error estimate given in a technical report by Luo and Levesley. The functionals in the Hermite interpolation are either point evaluations of pseudodifferential operators or rotational differential operators, the desirable feature of these operators being that they map polynomials to polynomials. Convergence rates for certain derivatives are given in terms of maximum point separation. 相似文献
78.
K. Kubilius 《Acta Appl Math》2003,78(1-3):233-242
We consider the integral equation driven by a standard Brownian motion and by a fractional Brownian motion. Sufficient conditions under which the equation has a weak solution are obtained. 相似文献
79.
Sufficient conditions are established for the asymptotic behavior of solutionsof nonlinear delay differential equations x′(t)+sum from i=1 to m(pi(t)x(t-т_i))=F(t,x_t),t≥0where 0<т_1<т_2<…<т_m≤r,pi∈C([0,∞)),i=1,2,…,m,F∈C([0,∞)×C_0,R).C_0=C([-r,0],R)equipped with the sup norm ||·|| forsome r>0. A new result is established, some known results are improved. 相似文献
80.