全文获取类型
收费全文 | 23177篇 |
免费 | 4551篇 |
国内免费 | 2472篇 |
专业分类
化学 | 13143篇 |
晶体学 | 192篇 |
力学 | 1345篇 |
综合类 | 318篇 |
数学 | 5637篇 |
物理学 | 9565篇 |
出版年
2024年 | 38篇 |
2023年 | 262篇 |
2022年 | 433篇 |
2021年 | 706篇 |
2020年 | 813篇 |
2019年 | 734篇 |
2018年 | 655篇 |
2017年 | 785篇 |
2016年 | 1143篇 |
2015年 | 1071篇 |
2014年 | 1437篇 |
2013年 | 2113篇 |
2012年 | 1550篇 |
2011年 | 1617篇 |
2010年 | 1381篇 |
2009年 | 1531篇 |
2008年 | 1678篇 |
2007年 | 1670篇 |
2006年 | 1419篇 |
2005年 | 1327篇 |
2004年 | 1067篇 |
2003年 | 958篇 |
2002年 | 767篇 |
2001年 | 580篇 |
2000年 | 596篇 |
1999年 | 528篇 |
1998年 | 495篇 |
1997年 | 394篇 |
1996年 | 267篇 |
1995年 | 284篇 |
1994年 | 275篇 |
1993年 | 191篇 |
1992年 | 175篇 |
1991年 | 134篇 |
1990年 | 115篇 |
1989年 | 119篇 |
1988年 | 120篇 |
1987年 | 98篇 |
1986年 | 79篇 |
1985年 | 116篇 |
1984年 | 76篇 |
1983年 | 44篇 |
1982年 | 69篇 |
1981年 | 45篇 |
1980年 | 35篇 |
1979年 | 40篇 |
1978年 | 44篇 |
1977年 | 30篇 |
1976年 | 25篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
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
102.
本从第一性原理出发,计算了充磁线圈产生的磁场,脉冲充磁的超导圆盘中的感应电流密度和俘获场分布.以超导体中的电流运动方程为基础,通过磁通动力学方程E=Ec(J/Jc)^n和物质方程B=μ0H表示超导圆盘的超导特性.计算表明第一个脉冲充磁电流的峰值和磁通蠕动指数对于超导圆盘中的感应电流分布非常重要.同时研究了充磁电流的宽度,波形,第二个充磁电流的峰值和充磁线圈的形状对于俘获场的影响.计算表明不断减小脉冲充磁电流峰值的反复充磁可以保持超导圆盘中的感应电流密度的平台在一确定水平. 相似文献
103.
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
104.
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
105.
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
106.
Yoshikazu Takada 《Annals of the Institute of Statistical Mathematics》1998,50(2):325-335
This paper gives a condition which implies the nonexistence of parametric statistical procedures with bounded risk or error performance characteristics. Many examples for which such a condition is satisfied are considered. 相似文献
107.
108.
109.
本研究应用聚类分析的方法,对人离休髁突×片进行数字图像技术处理所获得的影像密度数值做了详细分析,取得人髁突不同区的骨密度分布及不同年龄阶段髁突密度变化的情况。该方法避免了人为因素的影响,其结果较客观地反映了髁突的骨质密度;为研究病理状态时髁突骨密度的改变提供参考。 相似文献
110.
In this paper, we provide numerical means to compute the quasi-stationary (QS) distributions inM/GI/1/K queues with state-dependent arrivals andGI/M/1/K queues with state-dependent services. These queues are described as finite quasi-birth-death processes by approximating the general distributions in terms of phase-type distributions. Then, we reduce the problem of obtaining the QS distribution to determining the Perron-Frobenius eigenvalue of some Hessenberg matrix. Based on these arguments, we develop a numerical algorithm to compute the QS distributions. The doubly-limiting conditional distribution is also obtained by following this approach. Since the results obtained are free of phase-type representations, they are applicable for general distributions. Finally, numerical examples are given to demonstrate the power of our method. 相似文献