全文获取类型
收费全文 | 9956篇 |
免费 | 1259篇 |
国内免费 | 885篇 |
专业分类
化学 | 2798篇 |
晶体学 | 34篇 |
力学 | 3258篇 |
综合类 | 237篇 |
数学 | 2894篇 |
物理学 | 2879篇 |
出版年
2024年 | 24篇 |
2023年 | 88篇 |
2022年 | 180篇 |
2021年 | 317篇 |
2020年 | 359篇 |
2019年 | 254篇 |
2018年 | 297篇 |
2017年 | 403篇 |
2016年 | 529篇 |
2015年 | 405篇 |
2014年 | 443篇 |
2013年 | 756篇 |
2012年 | 522篇 |
2011年 | 546篇 |
2010年 | 460篇 |
2009年 | 504篇 |
2008年 | 522篇 |
2007年 | 510篇 |
2006年 | 548篇 |
2005年 | 490篇 |
2004年 | 459篇 |
2003年 | 433篇 |
2002年 | 342篇 |
2001年 | 314篇 |
2000年 | 309篇 |
1999年 | 276篇 |
1998年 | 238篇 |
1997年 | 207篇 |
1996年 | 199篇 |
1995年 | 180篇 |
1994年 | 153篇 |
1993年 | 112篇 |
1992年 | 101篇 |
1991年 | 87篇 |
1990年 | 90篇 |
1989年 | 50篇 |
1988年 | 73篇 |
1987年 | 50篇 |
1986年 | 35篇 |
1985年 | 44篇 |
1984年 | 39篇 |
1983年 | 10篇 |
1982年 | 38篇 |
1981年 | 15篇 |
1980年 | 10篇 |
1979年 | 13篇 |
1978年 | 16篇 |
1977年 | 11篇 |
1976年 | 6篇 |
1957年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
Two groups of 10 speech-language pathology graduate students were each given 7 weeks of singing lessons to determine whether voice lessons could have an effect on their clinical and perceptual skills. Pre-, mid-, and posttests to measure various skills were designed and implemented. With use of paired sample statistical testing, statistically significant results were obtained. In addition, the subjective responses of the students show that the lessons were effective in improving pitch perception, breath control, and legato production or easy onset. This study supports efforts to integrate curricula in vocal performance and speech-language pathology. 相似文献
33.
We develop a production policy that controls work-in-process (WIP) levels and satisfies demand in a multistage manufacturing system with significant uncertainty in yield, rework, and demand. The problem addressed in this paper is more general than those in the literature in three aspects: (i) multiple products are processed at multiple workstations, and the capacity of each workstation is limited and shared by multiple operations; (ii) the behavior of a production policy is investigated over an infinite-time horizon, and thus the system stability can be evaluated; (iii) the representation of yield and rework uncertainty is generalized. Generalizing both the system structure and the nature of uncertainty requires a new mathematical development in the theory of infinite-horizon stochastic dynamic programming. The theoretical contributions of this paper are the existence proofs of the optimal stationary control for a stochastic dynamic programming problem and the finite covariances of WIP and production levels under the general expression of uncertainty. We develop a simple and explicit sufficient condition that guarantees the existence of both the optimal stationary control and the system stability. We describe how a production policy can be constructed for the manufacturing system based on the propositions derived. 相似文献
34.
Efficient Sequential Quadratic Programming Implementations for Equality-Constrained Discrete-Time Optimal Control 总被引:1,自引:0,他引:1
Efficient sequential quadratic programming (SQP) implementations are presented for equality-constrained, discrete-time, optimal control problems. The algorithm developed calculates the search direction for the equality-based variant of SQP and is applicable to problems with either fixed or free final time. Problem solutions are obtained by solving iteratively a series of constrained quadratic programs. The number of mathematical operations required for each iteration is proportional to the number of discrete times N. This is contrasted by conventional methods in which this number is proportional to N
3. The algorithm results in quadratic convergence of the iterates under the same conditions as those for SQP and simplifies to an existing dynamic programming approach when there are no constraints and the final time is fixed. A simple test problem and two application problems are presented. The application examples include a satellite dynamics problem and a set of brachistochrone problems involving viscous friction. 相似文献
35.
Asymptotic chi-squared test statistics for testing the equality of moment vectors are developed. The test statistics proposed are generalized Wald test statistics that specialize for different settings by inserting an appropriate asymptotic variance matrix of sample moments. Scaled test statistics are also considered for dealing with nonstandard conditions. The specialization will be carried out for testing the equality of multinomial populations, and the equality of variance and correlation matrices for both normal and nonnormal data. When testing the equality of correlation matrices, a scaled version of the normal theory chi-squared statistic is proven to be an asymptotically exact chi-squared statistic in the case of elliptical data. 相似文献
36.
Y. Yavin 《Journal of Optimization Theory and Applications》1993,78(1):77-91
Stochastic optimal control techniques are applied to compare the performance of identical medium-range air-to-air missiles which have different thrust-mass profiles. The measure of the performance is the probability of reaching a lock-on-point with a favorable range of guidance and flight parameters, during a fixed time interval [0,t
f
], given that, during the flight, the trajectories of the missile are subjected to a variety of constraints including dynamic pressure constraints. 相似文献
37.
Hidetoshi Shimodaira 《Annals of the Institute of Statistical Mathematics》1997,49(3):395-410
The asymptotic error probability of Linhart's model selection test isevaluated, and compared with the nominal significance level. We examine thecase where the expected discrepancies of the candidate models from the truemodel are asymptotically equal. The local alternatives method is employed inthe limiting operation of the asymptotic evaluation. Although the errorprobability under the null hypothesis is actually shown to be equal to orless than the level for most situations, intolerable violations of the errorcontrol are observed for nested models: It is often erroneously concludedthat the smaller model is significantly better than the larger model. Toprevent this violation, a modification of Linhart's test statistic isproposed. The effectiveness of the proposed test is confirmed throughtheoretical analysis and numerical simulations. 相似文献
38.
尚英姿 《武汉大学学报(理学版)》2007,53(1):37-40
为了解决蛋白质三维结构比对需要处理大量的旋转、平移变换,直接用动态规划将变得十分繁琐这一问题,在保留蛋白质空间结构属性特征的基础上,对蛋白质三维数据进行了预先的处理.通过计算蛋白质结构在旋转和平移下的几何不变量,将蛋白质的三维结构坐标变换为具有旋转、平移不变性的一维序列.进一步给出了“距离”以及“相似得分”的定义.在此基础上采用动态规划方法给出了新的蛋白质结构比对算法.对专家分类的蛋白质结构数据库进行测试,结果显示准确、快速. 相似文献
39.
Poisson回归模型广泛应用于分析计数型数据 ,Dean&Lawless(1989)和Dean(1992 )讨论了非重复测量得到的计数型数据的偏大离差存在性的检验问题 .本文分别利用随机系数模型和对数非线性模型讨论了基于重复测量得到的计数型数据的偏大离差的检验问题 ,得到了检验的score统计量 . 相似文献
40.
Dmitry I. Belov Ronald D. Armstrong 《Computational Optimization and Applications》2006,33(2-3):319-332
This paper introduces a novel approach for extracting the maximum number of non-overlapping test forms from a large collection
of overlapping test sections assembled from a given item bank. The approach involves solving maximum set packing problems
(MSPs). A branch-and-bound MSP algorithm is developed along with techniques adapted from constraint programming to estimate
lower and upper bounds on the optimal MSP solution. The algorithm is general and can be applied in other applications including
combinatorial auctions. The results of computer simulations and experiments with an operational item bank are presented.
An erratum to this article is available at . 相似文献