全文获取类型
收费全文 | 13402篇 |
免费 | 1234篇 |
国内免费 | 882篇 |
专业分类
化学 | 1917篇 |
晶体学 | 27篇 |
力学 | 774篇 |
综合类 | 131篇 |
数学 | 8813篇 |
物理学 | 1884篇 |
无线电 | 1972篇 |
出版年
2024年 | 39篇 |
2023年 | 141篇 |
2022年 | 184篇 |
2021年 | 237篇 |
2020年 | 367篇 |
2019年 | 393篇 |
2018年 | 349篇 |
2017年 | 396篇 |
2016年 | 461篇 |
2015年 | 386篇 |
2014年 | 669篇 |
2013年 | 1186篇 |
2012年 | 690篇 |
2011年 | 751篇 |
2010年 | 642篇 |
2009年 | 842篇 |
2008年 | 800篇 |
2007年 | 875篇 |
2006年 | 757篇 |
2005年 | 631篇 |
2004年 | 581篇 |
2003年 | 552篇 |
2002年 | 507篇 |
2001年 | 405篇 |
2000年 | 409篇 |
1999年 | 377篇 |
1998年 | 307篇 |
1997年 | 302篇 |
1996年 | 193篇 |
1995年 | 145篇 |
1994年 | 125篇 |
1993年 | 79篇 |
1992年 | 104篇 |
1991年 | 50篇 |
1990年 | 58篇 |
1989年 | 47篇 |
1988年 | 37篇 |
1987年 | 27篇 |
1986年 | 37篇 |
1985年 | 51篇 |
1984年 | 61篇 |
1983年 | 27篇 |
1982年 | 43篇 |
1981年 | 41篇 |
1980年 | 37篇 |
1979年 | 37篇 |
1978年 | 18篇 |
1977年 | 27篇 |
1976年 | 23篇 |
1975年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
Frank J.S. Wang 《Stochastic Processes and their Applications》1977,5(2):173-193
Consider a population consisting of one type of individual living in a fixed region with area A. In [8], we constructed a stochastic population model in which the death rate is affected by the age of the individual and the birth rate is affected by the population density PA(t), i.e., the population size divided by the area A of the given region. In [8], we proposed a continuous deterministic model which in general is a nonlinear Volterra type integral equation and proved that under appropriate conditions the sequence PA(t) would converge to the solution P(t) of our integral equation in the sense that .In this paper, we obtain a “central limit theorem” for the random element √A(PA(t)?P(t)). We prove that under appropriate conditions √A(PA(t)?P(t)) will converge to a Gaussian process. (See Theorem 3.4 for the explicit formula of this Gaussian process.) 相似文献
22.
This research presents the results of an experimental study on the determination of pyrolysis behaviour and kinetics of six
crude oils by differential scanning calorimetry (DSC) and thermogravimetry (TG/DTG). Crude oil pyrolysis indicated two main
temperature ranges where loss of mass was observed. The first region between ambient to 400°C was distillation. The second
region between 400 and 600°C was visbreaking and thermal cracking. Arrhenius-type kinetic model is used to determine the kinetic
parameters of crude oils studied. It was observed that as crude oils gets heavier (°API decreases) cracking activation energy
increases. Activation energy of cracking also show a general trend with asphaltene content.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
23.
Four polyphenylacetylene samples, synthesized using different C6H5OH/Mo molar ratios, were investigated thermoanalytically by modulated temperature differential scanning calorimetry (MTDSC) in order to clarify a non-reversible exothermic event observed between 473 and 523 K on normal DSC. A stepwise, non-reversible change in heat capacity suggests the presence of internal reactions within the sample, that are followed by decomposition with loss of volatile products. 相似文献
24.
《Electroanalysis》2003,15(4):254-262
The new electrochemical double pulse technique, known as additive differential normal pulse voltammetry (ADNPV) when there is no restriction on the duration of both pulses, and additive differential pulse voltammetry (ADPV) when t2?t1, has been applied to a pseudo‐first‐order catalytic mechanism. The expressions obtained here are applicable to planar and spherical electrodes, of any radius. This is of great interest since the size of the electrode plays an important role in the preponderating of diffusive and kinetics processes. The signal obtained with this technique presents the same morphological characteristics as the triple pulse technique, double differential pulse voltammetry (DDPV) and is more advantageous than DDPV and than the double pulse one, differential pulse voltammetry (DPV). 相似文献
25.
Shuping Chen 《Applied Mathematics and Optimization》1992,26(1):95-110
Necessary and sufficient conditions are established in this paper for the existence of positive- and/or negative-definite solutions to the algebraic Riccati equation with indefinite coefficient. An iterative procedure is also given for computing such a solution.Project supported by the National Science Foundation of China and by the special program of the State Education Commission of China under grant 9033507. 相似文献
26.
Yoshisuke Ueda 《Journal of statistical physics》1979,20(2):181-196
This paper deals with turbulent or chaotic phenomena which occur in the system governed by Duffing's equation, a special type of two-dimensional periodic system. By using analog and digital computers, experiments are carried out with special reference to the change of attractors and of average power spectra of the random processes under the variation of the system parameters. On the basis of the experimental results, an outline of the random process is made clear. The results obtained in this paper will be applied to various physical problems and will also serve as material for the development of a proper mathematics of this phenomenon. 相似文献
27.
With the advent of single-cell RNA-sequencing (scRNA-seq), it is possible to measure the expression dynamics of genes at the single-cell level. Through scRNA-seq, a huge amount of expression data for several thousand(s) of genes over million(s) of cells are generated in a single experiment. Differential expression analysis is the primary downstream analysis of such data to identify gene markers for cell type detection and also provide inputs to other secondary analyses. Many statistical approaches for differential expression analysis have been reported in the literature. Therefore, we critically discuss the underlying statistical principles of the approaches and distinctly divide them into six major classes, i.e., generalized linear, generalized additive, Hurdle, mixture models, two-class parametric, and non-parametric approaches. We also succinctly discuss the limitations that are specific to each class of approaches, and how they are addressed by other subsequent classes of approach. A number of challenges are identified in this study that must be addressed to develop the next class of innovative approaches. Furthermore, we also emphasize the methodological challenges involved in differential expression analysis of scRNA-seq data that researchers must address to draw maximum benefit from this recent single-cell technology. This study will serve as a guide to genome researchers and experimental biologists to objectively select options for their analysis. 相似文献
28.
29.
A closed form expression for the bit error rate (BER) performance of frequency domaindifferential demodulation(FDDD) for orthogonal frequency division multiplexing system in flat fadingchannel is derived.The performance is evaluated by computer simulation and compared with the timedomain differential demodulation(TDDD).The results indicate that the performance of FDDD is betterthan that of TDDD,and the lower band of BER in the former is lower than that of the latter. 相似文献
30.
本文讨论了应用于傅里叶变换的微分性质的隐含假设,并叙述了信号不满足隐含假设时的修正方法。本文还给出了作为信号生成系统的微分方程描述,它使信号谱成为其生成系统的频率传递函数,指出了微分方程法的隐含假设。最后还给出了信号违反此假设时进行修正的方法。 相似文献