全文获取类型
收费全文 | 13697篇 |
免费 | 1539篇 |
国内免费 | 952篇 |
专业分类
化学 | 2288篇 |
晶体学 | 30篇 |
力学 | 831篇 |
综合类 | 250篇 |
数学 | 6606篇 |
物理学 | 2720篇 |
无线电 | 3463篇 |
出版年
2024年 | 25篇 |
2023年 | 145篇 |
2022年 | 182篇 |
2021年 | 277篇 |
2020年 | 341篇 |
2019年 | 360篇 |
2018年 | 340篇 |
2017年 | 420篇 |
2016年 | 539篇 |
2015年 | 398篇 |
2014年 | 725篇 |
2013年 | 1124篇 |
2012年 | 704篇 |
2011年 | 845篇 |
2010年 | 779篇 |
2009年 | 823篇 |
2008年 | 893篇 |
2007年 | 935篇 |
2006年 | 787篇 |
2005年 | 728篇 |
2004年 | 630篇 |
2003年 | 610篇 |
2002年 | 557篇 |
2001年 | 442篇 |
2000年 | 402篇 |
1999年 | 348篇 |
1998年 | 305篇 |
1997年 | 239篇 |
1996年 | 209篇 |
1995年 | 187篇 |
1994年 | 131篇 |
1993年 | 128篇 |
1992年 | 111篇 |
1991年 | 101篇 |
1990年 | 58篇 |
1989年 | 58篇 |
1988年 | 41篇 |
1987年 | 26篇 |
1986年 | 34篇 |
1985年 | 31篇 |
1984年 | 28篇 |
1983年 | 15篇 |
1982年 | 19篇 |
1981年 | 18篇 |
1980年 | 16篇 |
1979年 | 18篇 |
1978年 | 13篇 |
1977年 | 14篇 |
1976年 | 8篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 630 毫秒
1.
2.
G. Peruginelli 《代数通讯》2018,46(11):4724-4738
We classify the maximal subrings of the ring of n×n matrices over a finite field, and show that these subrings may be divided into three types. We also describe all of the maximal subrings of a finite semisimple ring, and categorize them into two classes. As an application of these results, we calculate the covering number of a finite semisimple ring. 相似文献
3.
《Journal of Saudi Chemical Society》2020,24(10):754-764
A kinetic study of the reactions of potentially bioactive 2-amino-4-arylthiazoles with highly reactive 4,6-dinitrobenzofuroxan (DNBF) is reported herein in acetonitrile solution. The complexation reaction was followed by recording the UV–vis spectra with time at λmax = 482 nm. Electronic effects of substituents influencing the rate of reaction have been studied using structure-reactivity relationships. It is shown that the Hammett plot relative to the reaction of DNBF with 2-amino-4-(4-chlorophenyl)thiazole exhibit positive deviation from the log k1 versus σ correlation, while it showed excellent linear correlation in terms of Yukawa–Tsuno equation. It has be noticed that the nonlinear Hammett plot observed for 2-amino-4-(4-chlorophenyl) thiazole is not attributed to a change in rate-determining step but is due to nature of electronic effect of substituent caused by the resonance of stabilization of substrates. The second-order rate constant (k1) relating to the bond C–C and C-N forming step of the complexation processes of DNBF with 4-substituted-aminothiazoles and 2-amino-5-methyl-4-phenylthiazole, respectively, is fit into the linear relationship log k = sN (N + E), thereby permitting the assessment of the nucleophilicity parameter (N) of the 2-amino-4-arylthiazoles of the range (4.90 < N < 6.85). 2-amino-4-arylthiazoles is subsequently ranked by positioning its reactivity on the general nucleophilicity scale developed recently by Mayr and coworkers (2003) leading an interesting and a direct comparison over a large domain of π-, σ -, and n-nucleophiles. The global electrophilicity/nucleophilicity reactivity indexes of the 2-amino-4-arylthiazoles have been investigated by means of a density functional theory (DFT) method. . 相似文献
4.
5.
对复杂高阶系统进行模型简化,为研究与设计系统提供了方便的条件。对系统模型的降阶是控制系统设计与仿真工作者的一个重要研究课题,对高阶复杂系统的分析、设计、仿真等具有重要的理论意义和工程实用价值。针对目前比较成熟的基于系统矩阵的有序实Schur分解方法,提出了采用时矩输出拟合来改善降阶系统输出响应逼近程度的改进方法,从而使简化模型具有输出误差更小、计算简便等优点,通过给出的实例仿真证实了这一点。 相似文献
6.
7.
8.
基于直升机目标旋转部件的调制模型,本文针对防空雷达提出了分析直升机信号特征的一种新方法——WVD法,并对直升机目标旋转部件调制回波信号在不同噪声情况下加以仿真分析,结果表明:用WVD分析雷达回波中直升机信号特征是一种非常有效的方法。 相似文献
9.
As we approach 100 nm technology the interconnect issues are becoming one of the main concerns in the testing of gigahertz system-on-chips. Voltage distortion (noise) and delay violations (skew) contribute to the signal integrity loss and ultimately functional error, performance degradation and reliability problems. In this paper, we first define a model for integrity faults on the high-speed interconnects. Then, we present a BIST-based test methodology that includes two special cells to detect and measure noise and skew occurring on the interconnects of the gigahertz system-on-chips. Using an inexpensive test architecture the integrity information accumulated by these special cells can be scanned out for final test and reliability analysis. 相似文献
10.
In this paper, we study the existence of the uniformly minimum risk equivariant (UMRE) estimators of parameters in a class
of normal linear models, which include the normal variance components model, the growth curve model, the extended growth curve
model, and the seemingly unrelated regression equations model, and so on. The necessary and sufficient conditions are given
for the existence of UMRE estimators of the estimable linear functions of regression coefficients, the covariance matrixV and (trV)α, where α > 0 is known, in the models under an affine group of transformations for quadratic losses and matrix losses, respectively.
Under the (extended) growth curve model and the seemingly unrelated regression equations model, the conclusions given in literature
for estimating regression coefficients can be derived by applying the general results in this paper, and the sufficient conditions
for non-existence of UMRE estimators ofV and tr(V) are expanded to be necessary and sufficient conditions. In addition, the necessary and sufficient conditions that there
exist UMRE estimators of parameters in the variance components model are obtained for the first time. 相似文献