全文获取类型
收费全文 | 3621篇 |
免费 | 175篇 |
国内免费 | 66篇 |
专业分类
化学 | 377篇 |
晶体学 | 4篇 |
力学 | 323篇 |
综合类 | 42篇 |
数学 | 1884篇 |
物理学 | 1232篇 |
出版年
2023年 | 22篇 |
2022年 | 15篇 |
2021年 | 55篇 |
2020年 | 73篇 |
2019年 | 69篇 |
2018年 | 45篇 |
2017年 | 88篇 |
2016年 | 88篇 |
2015年 | 54篇 |
2014年 | 118篇 |
2013年 | 261篇 |
2012年 | 120篇 |
2011年 | 167篇 |
2010年 | 119篇 |
2009年 | 170篇 |
2008年 | 152篇 |
2007年 | 185篇 |
2006年 | 178篇 |
2005年 | 157篇 |
2004年 | 135篇 |
2003年 | 145篇 |
2002年 | 165篇 |
2001年 | 126篇 |
2000年 | 127篇 |
1999年 | 132篇 |
1998年 | 105篇 |
1997年 | 57篇 |
1996年 | 61篇 |
1995年 | 79篇 |
1994年 | 50篇 |
1993年 | 47篇 |
1992年 | 30篇 |
1991年 | 28篇 |
1990年 | 19篇 |
1989年 | 29篇 |
1988年 | 27篇 |
1987年 | 16篇 |
1986年 | 21篇 |
1985年 | 72篇 |
1984年 | 78篇 |
1983年 | 56篇 |
1982年 | 40篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1978年 | 12篇 |
1977年 | 15篇 |
1976年 | 10篇 |
1975年 | 6篇 |
1974年 | 6篇 |
排序方式: 共有3862条查询结果,搜索用时 15 毫秒
991.
本文利用来自正态总体的常相关样本的一个统计量的分布,对广东省和辽宁省的一些企业1988年一1989年的利润额增值进行了分析. 相似文献
992.
H. E. Stanley S. V. Buldyrev A. L. Goldberger S. Havlin R. N. Mantegna C. K. Peng M. Simons 《Il Nuovo Cimento D》1994,16(9):1339-1356
Summary We present evidence supporting the idea that the DNA sequence in genes containingnoncoding regions is correlated, and that the correlation is remarkably long range-indeed, base pairsthousands of base pairs distant are correlated. We do not find such a long-range correlation in the coding regions of the gene; we utilize this fact
to build aCoding Sequence Finder algorithm, which uses statistical ideas to locate the coding regions of an unknown DNA sequence. We resolve the problem of the ?non-stationarity?
feature of the sequence of base pairs (that the relative concentration of purines and pyrimidines changes in different regions
of the mosaic-like chain) by describing a new algorithm calledDetrended Fluctuation Analysis (DFA). We address the claim of Voss that there is no difference in the statistical properties of coding and noncoding regions
of DNA by systematically applying the DFA algorithm, as well as standard FFT analysis, to every DNA sequence (33 301 coding
and 29 453 non-coding) in the entire GenBank database. We describe a simple model to account for the presence of long-range
power law correlations (and the systematic variation of the scaling exponent α with evolution) which is based upon a generalization
of the classic Lévy walk. Finally, we describe briefly some recent work showing that thenoncoding sequences have certain statistical features in common with natural languages. Specifically, we adapt to DNA the Zipf approach
to analyzing linguistic texts, and the Shannon approach to quantifying the ?redundancy? of a linguistic text in terms of a
measurable entropy function. We suggest that noncoding regions in eukaryotes may display a smaller entropy and larger redundancy
than coding regions for plants and invertebrates, further supporting the possibility that noncoding regions of DNA may carry
biological information.
Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. 相似文献
993.
Eugene Tsykin 《Applied Mathematical Modelling》1984,8(4):288-292
The common procedure of nonlinear approximation using linear multiple regressions developed in logarithmic space suffers from a number of inherent defects. Nonlinear approximation with choice of free parameters allows us to derive multiple regressions of varied structure, consisting of several members and an intercept. The calculations required for the proposed approximation are simple and can be implemented using standard statistical subprograms. More than 50 equations of the proposed type have been fitted for various purposes. The coefficient of determination is generally higher than 0.90. The application of the proposed method is demonstrated with two numerical examples and a number of equations derived for practical application in hydrology, geomorphology and water pollution studies. The proposed method can be used in any area of study where nonlinear multiple approximations are applicable. 相似文献
994.
Les Frair 《Applied Mathematical Modelling》1984,8(4):271-281
A mathematical model of the annoyance created at an airport by aircraft operations is developed. The model incorporates population distribution considerations around an airport and the annoyance caused by aircraft noise. The objective function of this model corresponds to seeking to minimize total population annoyance created by all aircraft operations in a 24-hour period. Several factors are included in this model as constraint relationships. Aircraft operations by type and time period are upper bounded. Demand for flight services is incorporated by including lower bounds on the number of operations by type of aircraft, runway used and time period. Also upper bounds on the number of operations for each runway are included. The mathematical model as formulated is recognized as corresponding to a nonlinear integer mathematical programming problem.The solution technique selected makes use of a successive linear approximation optimization algorithm. An especially attractive feature of this solution algorithm is that it is capable of obtaining solutions to large problems. For example, it would be feasible to attempt the solution of problems involving several thousand variables and over 500 linear constraints. This suggested solution algorithm was implemented on a computer and computational results obtained for example problems. 相似文献
995.
Summary We have considered a model of a lattice gas defined on a periodic tangled chain to study the DNA denaturation by a modified
transfer matrix method. By using an iterative algorithm we have obtained numerically different kinds of melting curves for
different configurations of the tangled chain and different types of interactions. In some special cases of configurations
and interactions we have found the same melting curves, which we have obtained before studying some simple lattice gas models,
using different techniques. This more generalized model and the new results could be useful for the experimental investigations. 相似文献
996.
997.
W. C. M. Kallenberg 《Annals of the Institute of Statistical Mathematics》1993,45(2):341-351
With a given Edgeworth expansion sequences of i.i.d. r.v.'s are associated such that the Edgeworth expansion for the standardized sum of these r.v.'s agrees with the given Edgeworth expansion. This facilitates interpretation and manipulation of Edgeworth expansions. The theory is applied to the power of linear rank statistics and to the combination of such statistics based on subsamples. Complicated expressions for the power become more transparent. As a consequence of the sum-structure it is seen why splitting the sample causes no loss of first order efficiency and only a small loss of second order efficiency. 相似文献
998.
高斯场下形体影响因子的动态特征以及微扰方程的修正 总被引:3,自引:0,他引:3
为解决微粒形体变化对微粒测量结果的影响,在半椭圆模型理论模型的基础上,对高斯分布散射场下任意形体微粒散射的影响因子进行了讨论。建立了影响因子与形体特征参量和微粒粒度之间的数学关系,并求得了其影响因子数值解的分布。基于对影响因子数值解分布动态特征的讨论,拟合得到了高斯分布散射场下任意形体微粒影响因子函数解的解析式,由此提出了对任意形体微粒特征分布概率函数微扰方程的修正。研究结果表明,该方法对于大粒度微粒形体变化对测量结果影响的修正效果尤佳。 相似文献
999.
正Fibonacci数的标准分解式中因子5的指数 总被引:1,自引:0,他引:1
袁明豪 《数学的实践与认识》2007,37(7):166-170
根据Fibonacci数列的定义,利用初等数论的知识和数学归纳法,讨论了正Fibonacci数Fn的标准分解式中因子5的指数与下标n的关系,得到下列结论:正Fibonacci数Fn的标准分解式中因子5的指数,与下标n的标准分解式中因子5的指数一致. 相似文献
1000.
"图解"数学物理方法的教学实践 总被引:5,自引:0,他引:5
《数学物理方法》虽然是数学课,但是它是介绍在物理中应用的数学.物理课程多媒体教学的经验启发我们,合理地选择《数学物理方法》课程中有物理背景的内容,恰当地使用可视化以展现数学公式的物理图像,就能提高教学效果,对一些随时间演变的方程,还可以用动画来表现.我们用两种方法实现可视化,一是用解析解作图,另一个是通过数值求解方程,再将数值计算结果作图.本文介绍我们在《数学物理方法》课开展可视化教学的实践. 相似文献