全文获取类型
收费全文 | 117743篇 |
免费 | 11511篇 |
国内免费 | 6011篇 |
专业分类
化学 | 25869篇 |
晶体学 | 468篇 |
力学 | 6773篇 |
综合类 | 1241篇 |
数学 | 22207篇 |
物理学 | 30705篇 |
无线电 | 48002篇 |
出版年
2024年 | 220篇 |
2023年 | 1170篇 |
2022年 | 2108篇 |
2021年 | 2942篇 |
2020年 | 3034篇 |
2019年 | 2353篇 |
2018年 | 2243篇 |
2017年 | 3270篇 |
2016年 | 4058篇 |
2015年 | 3887篇 |
2014年 | 6584篇 |
2013年 | 7598篇 |
2012年 | 6960篇 |
2011年 | 7072篇 |
2010年 | 5372篇 |
2009年 | 6564篇 |
2008年 | 7445篇 |
2007年 | 8014篇 |
2006年 | 6898篇 |
2005年 | 5804篇 |
2004年 | 5081篇 |
2003年 | 5089篇 |
2002年 | 4869篇 |
2001年 | 3767篇 |
2000年 | 3384篇 |
1999年 | 2871篇 |
1998年 | 2621篇 |
1997年 | 1797篇 |
1996年 | 1677篇 |
1995年 | 1568篇 |
1994年 | 1398篇 |
1993年 | 1124篇 |
1992年 | 1068篇 |
1991年 | 794篇 |
1990年 | 667篇 |
1989年 | 477篇 |
1988年 | 455篇 |
1987年 | 384篇 |
1986年 | 333篇 |
1985年 | 327篇 |
1984年 | 322篇 |
1983年 | 198篇 |
1982年 | 242篇 |
1981年 | 224篇 |
1980年 | 160篇 |
1979年 | 180篇 |
1978年 | 144篇 |
1977年 | 132篇 |
1976年 | 85篇 |
1973年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 504 毫秒
941.
Parking Capacity and Pricing in Park'n Ride Trips: A Continuous Equilibrium Network Design Problem 总被引:2,自引:0,他引:2
In this paper we consider the problem of designing parking facilities for park'n ride trips. We present a new continuous equilibrium network design problem to decide the capacity and fare of these parking lots at a tactical level. We assume that the parking facilities have already been located and other topological decisions have already been taken.The modeling approach proposed is mathematical programming with equilibrium constraints. In the outer optimization problem, a central Authority evaluates the performance of the transport network for each network design decision. In the inner problem a multimodal traffic assignment with combined modes, formulated as a variational inequality problem, generates the share demand for modes of transportation, and for parking facilities as a function of the design variables of the parking lots. The objective is to make optimal parking investment and pricing decisions in order to minimize the total travel cost in a subnetwork of the multimodal transportation system.We present a new development in model formulation based on the use of generalized parking link cost as a design variable.The bilevel model is solved by a simulated annealing algorithm applied to the continuous and non-negative design decision variables. Numerical tests are reported in order to illustrate the use of the model, and the ability of the approach to solve applications of moderate size. 相似文献
942.
LaurieWallace 《世界电信》2002,15(3):51-53
为了构筑第三代无线网络,人们需要对现有基础设施进行彻底改造,包括网络的发射基站。配置在无线基站中的新型数字控制功率放大器能够改善频谱利用率和功率效率,从而提高系统容量。PMC-Sierra公司的数字控制功率放大器采用了数字校正信号处理器(DCSP)以及自适应控制处理器和补偿估值器(ACPCE),该放大器可改善无线基础设施的可靠性,同时降低系统成本、规模及复杂性。 相似文献
943.
横向放大率法测定光具组的基点 总被引:1,自引:1,他引:0
介绍了用横向放大率法确定两薄透镜组成的光具组基点的原理和方法,该方法采用线阵光电耦合器件(CCD)测量物经光学系统成像的横向放大率,进一步提高了测量精度. 相似文献
944.
Arne Barinka Stephan Dahlke Wolfgang Dahmen 《Advances in Computational Mathematics》2006,24(1-4):5-34
Recently adaptive wavelet methods have been developed which can be shown to exhibit an asymptotically optimal accuracy/work
balance for a wide class of variational problems including classical elliptic boundary value problems, boundary integral equations
as well as certain classes of noncoercive problems such as saddle point problems. A core ingredient of these schemes is the
approximate application of the involved operators in standard wavelet representation. Optimal computational complexity could
be shown under the assumption that the entries in properly compressed standard representations are known or computable in
average at unit cost. In this paper we propose concrete computational strategies and show under which circumstances this assumption
is justified in the context of elliptic boundary value problems.
Dedicated to Charles A. Micchelli on the occasion of his 60th birthday
Mathematics subject classifications (2000) 41A25, 41A46, 65F99, 65N12, 65N55.
This work has been supported in part by the Deutsche Forschungsgemeinschaft SFB 401, the first and third author are supported
in part by the European Community's Human Potential Programme under contract HPRN-CT-202-00286 (BREAKING COMPLEXITY). The
second author acknowledges the financial support provided through the European Union's Human Potential Programme, under contract
HPRN-CT-2002-00285 (HASSIP) and through DFG grant DA 360/4–1. 相似文献
945.
R. S. Kaler 《Fiber and Integrated Optics》2006,25(1):41-57
In this article, the comparison of large signal theory and small signal theory has been done with dispersive propagation of optical signal with IMDD (Intensity Modulation Direct Detection) systems for semiconductor lasers with higher-order dispersion terms. The expressions for an exact large signal theory and small signal theory including higher-order dispersion terms for propagation of an optical wave with sinusoidal amplitude and frequency modulation in a dispersive fiber have been derived. It is observed that small signal theory is more sensitive compared to large signal theory in terms of intensity modulation/direct detection systems. Also, it is reported that for large signal analysis the higher-order effects of dispersion can be ignored, whereas for small signal theory, the higher-order effects can be ignored for lower modulation frequencies only. The variation in the transfer function for various values of modulation indices are greater for small signal analysis than for large signal analysis. Also, as the intensity modulation index is increased, there is a decrease in the value of transfer function. The large signal model approximates the small signal model for lower values of the intensity modulation index. 相似文献
946.
947.
Nash Equilibria,Variational Inequalities,and Dynamical Systems 总被引:2,自引:0,他引:2
Cavazzuti E. Pappalardo M. Passacantando M. 《Journal of Optimization Theory and Applications》2002,114(3):491-506
In this paper, we introduce some relationships between Nash equilibria, variational equilibria, and dynamical equilibria for noncooperative games. 相似文献
948.
随予行 《大气与环境光学学报》2002,(6)
在简要回顾飞机操纵系统的发展历史的基础上,详细介绍了光传操纵系统的发展状况和应用前景。针对航空领域光纤传输的特点,对光传操纵系统中光电器件的选择及关键技术等进行了分析。 相似文献
949.
介绍了图像增强技术中,空间域处理方法中的邻域平均法的原理、算法,及该原理在windows平台上以C++ Builder为开发工具的具体程序实现。 相似文献
950.
基于机器视觉的离散傅里叶变换目标识别方法 总被引:1,自引:0,他引:1
提出了一种基于机器视觉与离散傅里叶变换的目标特征识别方法。利用计算机图像技术采集和处理图像信号;利用离散的傅里叶变换对图像数据提取特征,能够更好的辨别数据细节,从而可通过图像的比对来实现目标的识别。该方法在对实际的静止图像进行处理与计算后,能够很好的对图像的细节变化进行识别。 相似文献