全文获取类型
收费全文 | 24766篇 |
免费 | 3626篇 |
国内免费 | 1754篇 |
专业分类
化学 | 9017篇 |
晶体学 | 131篇 |
力学 | 2025篇 |
综合类 | 654篇 |
数学 | 8433篇 |
物理学 | 9886篇 |
出版年
2024年 | 38篇 |
2023年 | 283篇 |
2022年 | 884篇 |
2021年 | 932篇 |
2020年 | 837篇 |
2019年 | 636篇 |
2018年 | 599篇 |
2017年 | 906篇 |
2016年 | 1051篇 |
2015年 | 867篇 |
2014年 | 1427篇 |
2013年 | 1731篇 |
2012年 | 1453篇 |
2011年 | 1475篇 |
2010年 | 1279篇 |
2009年 | 1513篇 |
2008年 | 1556篇 |
2007年 | 1574篇 |
2006年 | 1366篇 |
2005年 | 1201篇 |
2004年 | 1017篇 |
2003年 | 959篇 |
2002年 | 880篇 |
2001年 | 666篇 |
2000年 | 684篇 |
1999年 | 590篇 |
1998年 | 580篇 |
1997年 | 447篇 |
1996年 | 393篇 |
1995年 | 335篇 |
1994年 | 328篇 |
1993年 | 247篇 |
1992年 | 204篇 |
1991年 | 149篇 |
1990年 | 128篇 |
1989年 | 118篇 |
1988年 | 98篇 |
1987年 | 89篇 |
1986年 | 81篇 |
1985年 | 114篇 |
1984年 | 72篇 |
1983年 | 40篇 |
1982年 | 61篇 |
1981年 | 33篇 |
1980年 | 28篇 |
1979年 | 46篇 |
1978年 | 38篇 |
1977年 | 29篇 |
1976年 | 25篇 |
1973年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
S. Aravinda N. Shamala Rituparna S. Roy P Balaram 《Journal of Chemical Sciences》2003,115(5-6):373-400
An overview of the use of non-protein amino acids in the design of conformationally well-defined peptides, based on work from
the author’s laboratory, is discussed. The crystal structures of several designed oligopeptides illustrate the useα-aminoisobutyric acid (Aib) in the construction of helices, D-amino acids in the design of helix termination segments andDPro-Xxx segments for nucleating ofβ-hairpin structures.β- andγ-amino acid residues have been used to expand the range of designed polypeptide structures.
Dedicated to Professor C N R Rao on his 70th birthday 相似文献
92.
The form of the probability density derived from the evolution in time of a previously truncated frequency distribution of animal Liveweights is of interest in animal husbandry. Truncated frequency distributions arise when the heavier animals are sold for slaughter and the lighter animals retained. The demands of modern quality assurance schemes require that, given information on animal growth, the farmer is able to estimate the number of animals that would meet the specifications at some time in the future after truncation. Assuming that animal growth can be described by a linear stochastic differential equation, we derive an explicit expression for the probability density of animal Liveweights at any time after the truncation of an initial Gaussian density. It is shown that this probability density converges rapidly to a Gaussian density, so that after about 20 days of typical growth rates for lambs, the resulting density is practically indistinguishable from Gaussian. 相似文献
93.
The distributions of limit cycles of cubic vector fields (P2, Q3) are considered in this paper, where P2 and Q3 are polynomials of x and y of order two and three, respectively. It is possibly seven different distributions of limit cycles given in [1]. We now prove that in which three kinds of distributions are impossible and other four kinds all can be realized by concrete vector fields of (P2,Q3). Some related results are also given. 相似文献
94.
工程施工中运输网络优化方案 总被引:5,自引:0,他引:5
施工工地运输方案的优化设计可以归结为 :按施工期要求设计运输线路或验证已有线路通行能力 ,计算总线路中影响提高流量的关键路段 ,取得最小费用最大流 .本文运用图论理论这一数学工具把实际问题抽象为有向网络 ,进而建立数学模型 .此方法理论上严密 ,解题步聚直观清晰 ,对水利、公路、水路、铁路等其它运输系统有普遍意义 . 相似文献
95.
两段级联掺铒光纤放大器的优化研究 总被引:7,自引:4,他引:3
基于Giles模型,研究了980 nm和1480 nm泵浦的两段级联掺铒光纤放大器(EDFA)的设计,得出了掺铒光纤(EDF)的最佳长度和光隔离器的最佳位置随泵浦功率和信号功率的变化关系.比较980 nm和1480 nm泵浦的两段级联EDFA,可以发现,前者的最佳EDF长度短,而光隔离器最佳位置距EDFA输入端远. 相似文献
96.
Space time-index plots for probing dynamical nonstationarity 总被引:3,自引:0,他引:3
We propose a simple method to efficiently probe dynamical nonstationarity in observed time series. In a space time-index plot, the density distributions as a function of normalized time-index are V-shaped due to nonstationarity. We show that this method is workable for short data sets and typical examples are illustrated. 相似文献
97.
HANXI'AN HUANGXILI 《高校应用数学学报(英文版)》1998,13(4):445-450
For two rational quadratic B-spline curves with same control vertexes, the cross ratio of four eollinear points are represented; which are any one of the vertexes, and the two points that the ray initialing from the vertex intersects with the corresponding segments of the twocurves, and the point the ray intersecting with the connecting line between the two neighboring vertexes. Different from rational quadratic Beeier curves, the value is generally related with the loeation of the ray, and the necessary and sufficient condition o5 the ratio being independent of the ray‘s loeation is showed. Alsn another cross ratio o5 the following four collinear points are suggested, i.e. one vertex, the points that the ray from the initlal vertex intersects respectivdy with the curve segmentt the line connecting the segments end points, and the line connecting the two neighboring vertexes. This cross ratio is concerned only whh the ray‘s location, butnot with the weights of the curve. Furthermore, the cross ratio is projective invariant under the projective transformation between the two segments. 相似文献
98.
神经网络法计算镉(Ⅱ),羟基及碳酸根三元体系的形态分布 总被引:2,自引:0,他引:2
采用前馈线笥网络BP算法,计算了Cd62+-OH^-CO^2-3三元体系的累积稳定常数。用Hopfield反馈网络研究了体系中络合物的形态分布。溶液中溶解的CO2对lgβ1的计算结果有重要影响,对lgβ2,lgβ3,lgβ4的结果影响不大。 相似文献
99.
In this paper, we present a unified treatment and analysis of a dynamic traffic network model with elastic demands formulated and studied as a projected dynamical system. We propose a travel route choice adjustment process that satisfies the projected dynamical system. Under certain conditions, stability and asymptotical stability of the equilibrium patterns are then derived. Finally, two discrete-time algorithms, the Euler method and the Heun method, are proposed for the computation of the solutions, and convergence results established. The convergence results depend crucially on stability analysis. The performance of the algorithms is then illustrated on several transportation networks. 相似文献
100.
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