全文获取类型
收费全文 | 19643篇 |
免费 | 2516篇 |
国内免费 | 867篇 |
专业分类
化学 | 2420篇 |
晶体学 | 172篇 |
力学 | 3340篇 |
综合类 | 300篇 |
数学 | 11438篇 |
物理学 | 5356篇 |
出版年
2024年 | 37篇 |
2023年 | 184篇 |
2022年 | 388篇 |
2021年 | 501篇 |
2020年 | 455篇 |
2019年 | 438篇 |
2018年 | 437篇 |
2017年 | 626篇 |
2016年 | 632篇 |
2015年 | 452篇 |
2014年 | 863篇 |
2013年 | 1242篇 |
2012年 | 1108篇 |
2011年 | 957篇 |
2010年 | 904篇 |
2009年 | 1069篇 |
2008年 | 1116篇 |
2007年 | 1268篇 |
2006年 | 1090篇 |
2005年 | 1047篇 |
2004年 | 866篇 |
2003年 | 890篇 |
2002年 | 790篇 |
2001年 | 699篇 |
2000年 | 674篇 |
1999年 | 579篇 |
1998年 | 509篇 |
1997年 | 479篇 |
1996年 | 326篇 |
1995年 | 326篇 |
1994年 | 258篇 |
1993年 | 252篇 |
1992年 | 201篇 |
1991年 | 174篇 |
1990年 | 149篇 |
1989年 | 98篇 |
1988年 | 99篇 |
1987年 | 86篇 |
1986年 | 76篇 |
1985年 | 114篇 |
1984年 | 100篇 |
1983年 | 39篇 |
1982年 | 55篇 |
1981年 | 64篇 |
1980年 | 40篇 |
1979年 | 64篇 |
1978年 | 59篇 |
1977年 | 59篇 |
1976年 | 37篇 |
1974年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
The general stability of single-layer cylindrical grids is studied in linear and nonlinear formulations. Dependence of the general buckling load on the geometry and stiffness parameters of a grid is established in an analytical form. Such grids are numerically analyzed for stability. It is established that the general buckling load is much less than the local buckling load. Typical general buckling modes are found. It is shown that such grids are weakly sensitive to imperfections 相似文献
62.
本对于全局优化问题提出一个改进的进化规划算法,该算法以概率p接收基于电磁理论求出合力方向作为随机搜索方向,以概率1-p接收按正态分布产生的随机搜索方向。改进算法不仅克服了传统进化规划算法随机搜索的盲目性,而且保留了传统进化规划算法全局搜索性。本算法应用于几个典型例题,数值结果表明本算法是可行的,有效的。 相似文献
63.
从决策有限理性角度,引入行为金融理论于机构投资风险优化系统,对多心理账户条件下机构投资的风险优化设计进行了研究。以Friedman和Savage之谜为释例,对机构投资风险优化中的诸多非标准金融异像进行了解释。以Shefrin和Statman行为证券组合理论为核心,建立了多心理账户条件下机构投资的风险优化模型,为机构投资的风险优化实践提供了一种量化分析工具。 相似文献
64.
本文提供了预条件不精确牛顿型方法结合非单调技术解光滑的非线性方程组.在合理的条件下证明了算法的整体收敛性.进一步,基于预条件收敛的性质,获得了算法的局部收敛速率,并指出如何选择势序列保证预条件不精确牛顿型的算法局部超线性收敛速率. 相似文献
65.
66.
The Sample Average Approximation Method Applied to Stochastic Routing Problems: A Computational Study 总被引:1,自引:0,他引:1
Bram Verweij Shabbir Ahmed Anton J. Kleywegt George Nemhauser Alexander Shapiro 《Computational Optimization and Applications》2003,24(2-3):289-333
The sample average approximation (SAA) method is an approach for solving stochastic optimization problems by using Monte Carlo simulation. In this technique the expected objective function of the stochastic problem is approximated by a sample average estimate derived from a random sample. The resulting sample average approximating problem is then solved by deterministic optimization techniques. The process is repeated with different samples to obtain candidate solutions along with statistical estimates of their optimality gaps.We present a detailed computational study of the application of the SAA method to solve three classes of stochastic routing problems. These stochastic problems involve an extremely large number of scenarios and first-stage integer variables. For each of the three problem classes, we use decomposition and branch-and-cut to solve the approximating problem within the SAA scheme. Our computational results indicate that the proposed method is successful in solving problems with up to 21694 scenarios to within an estimated 1.0% of optimality. Furthermore, a surprising observation is that the number of optimality cuts required to solve the approximating problem to optimality does not significantly increase with the size of the sample. Therefore, the observed computation times needed to find optimal solutions to the approximating problems grow only linearly with the sample size. As a result, we are able to find provably near-optimal solutions to these difficult stochastic programs using only a moderate amount of computation time. 相似文献
67.
近年来,决定椭圆型方程系数反问题在地磁、地球物理、冶金和生物等实际问题上有着广泛的应用.本文讨论了二维的决定椭圆型方程系数反问题的数值求解方法.由误差平方和最小原则,这个反问题可化为一个变分问题,并进一步离散化为一个最优化问题,其目标函数依赖于要决定的方程系数.本文着重考察非线性共轭梯度法在此最优化问题数值计算中的表现,并与拟牛顿法作为对比.为了提高算法的效率我们适当选择加快收敛速度的预处理矩阵.同时还考察了线搜索方法的不同对优化算法的影响.数值实验的结果表明,非线性共轭梯度法在这类大规模优化问题中相对于拟牛顿法更有效. 相似文献
68.
A new approach to the robust handling of non‐linear constraints for GAs (genetic algorithms) optimization is proposed. A specific feature of the approach consists of the change in the conventional search strategy by employing search paths which pass through both feasible and infeasible points (contrary to the traditional approach where only feasible points may be included in a path). The method (driven by full Navier–Stokes computations) was applied to the problem of multiobjective optimization of aerodynamic shapes subject to various geometrical and aerodynamic constraints. The results demonstrated that the method retains high robustness of conventional GAs while keeping CFD computational volume to an acceptable level, which allowed the algorithm to be used in a demanding engineering environment. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
69.
70.
Yu. Satov B. Sharkov H. Haseroth Yu. Smakovski K. Makarov S. Kondrashev V. Roerich A. Stepanov H. Kugler R. Scrivens O. Camut A. Shumshurov A. Balabaev A. Charushin 《Journal of Russian Laser Research》2004,25(3):205-216
Experimental and numerical calculation results devoted to development of an optical system for an ion source based on a repetition rate CO2 laser are described. The laser chain consists of a master oscillator, gas absorber cells, and a four-pass amplifier. The optical system provides smooth laser pulses with variable duration and high spatial quality that ensures efficiency for plasma heating and ion generation. The parameters of the plasma ion component measured in the CERN laboratory are applied for a lead target illumination. 相似文献