首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   49064篇
  免费   3641篇
  国内免费   2556篇
系统科学   3102篇
丛书文集   1596篇
教育与普及   1489篇
理论与方法论   433篇
现状及发展   1118篇
研究方法   133篇
综合类   47383篇
自然研究   7篇
  2025年   10篇
  2024年   547篇
  2023年   629篇
  2022年   828篇
  2021年   992篇
  2020年   616篇
  2019年   281篇
  2018年   1090篇
  2017年   1220篇
  2016年   945篇
  2015年   1163篇
  2014年   1618篇
  2013年   1599篇
  2012年   2053篇
  2011年   2855篇
  2010年   2737篇
  2009年   2598篇
  2008年   2870篇
  2007年   3127篇
  2006年   2036篇
  2005年   2076篇
  2004年   1533篇
  2003年   1494篇
  2002年   1493篇
  2001年   1550篇
  2000年   1455篇
  1999年   2220篇
  1998年   1748篇
  1997年   1885篇
  1996年   1716篇
  1995年   1438篇
  1994年   1282篇
  1993年   1133篇
  1992年   944篇
  1991年   866篇
  1990年   763篇
  1989年   664篇
  1988年   540篇
  1987年   337篇
  1986年   175篇
  1985年   63篇
  1984年   12篇
  1983年   5篇
  1982年   5篇
  1981年   7篇
  1980年   7篇
  1973年   8篇
  1971年   5篇
  1967年   3篇
  1957年   3篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
With the rapid development of the era of big data and the Internet of Things technology, the demand for wearable electronics is also increasing.Compared with tr...  相似文献   
12.
Fullerene-based electron-transporting layers (ETLs) significantly influence the defect passivation and device performance of inverted perovskite solar cells (PS...  相似文献   
13.
共轭帽分子分割法研究蛋白质的电荷密度   总被引:6,自引:6,他引:0  
共轭帽分子分割(MFCC)法将蛋白质处理为一系列加了适当的共轭帽的单个的氨基酸片断,各片段的物理量可以用量化软件算出,整个蛋白质的物理量由各片段的物理量组合而得到.本文将加MFCC法应用于生物大分子的研究,得到了现在抗肿瘤研究中具有重要意义的p53蛋白的电荷密度和静电势.  相似文献   
14.
Maternal stress during pregnancy is prevailing worldwide,which exposes fetuses to intrauterine hyper glucocorticoids (GC),programming offspring to obesity and m...  相似文献   
15.
浅评我国生态环境监测现状   总被引:5,自引:0,他引:5  
生态环境监测是环境监测中一个全新的概念,是环境生态建设的技术保证和支持体系.本文阐释了生态环境监测的基本内涵,并结合我国生态环境监测现状,提出了不断完善生态环境监测的一些建议.  相似文献   
16.
This paper is concerned with model averaging estimation for conditional volatility models. Given a set of candidate models with different functional forms, we propose a model averaging estimator and forecast for conditional volatility, and construct the corresponding weight-choosing criterion. Under some regulatory conditions, we show that the weight selected by the criterion asymptotically minimizes the true Kullback–Leibler divergence, which is the distributional approximation error, as well as the Itakura–Saito distance, which is the distance between the true and estimated or forecast conditional volatility. Monte Carlo experiments support our newly proposed method. As for the empirical applications of our method, we investigate a total of nine major stock market indices and make a 1-day-ahead volatility forecast for each data set. Empirical results show that the model averaging forecast achieves the highest accuracy in terms of all types of loss functions in most cases, which captures the movement of the unknown true conditional volatility.  相似文献   
17.
Online auctions have become increasingly popular in recent years. There is a growing body of research on this topic, whereas modeling online auction price curves constitutes one of the most interesting problems. Most research treats price curves as deterministic functions, which ignores the random effects of external and internal factors. To account for the randomness, a more realistic model using stochastic differential equations is proposed in this paper. The online auction price is modeled by a stochastic differential equation in which the deterministic part is equivalent to the second‐order differential equation model proposed in Wang et al. (Journal of the American Statistical Association, 2008, 103, 1100–1118). The model also includes a component representing the measurement errors. Explicit expressions for the likelihood function are also obtained, from which statistical inference can be conducted. Forecast accuracy of the proposed model is compared with the ODE (ordinary differential equation) approach. Simulation results show that the proposed model performs better.  相似文献   
18.
Finding a mate is a crucial step in sexual reproduction. The mutualism between figs and their pollinators is exploited by many figwasps that develop within their galls and attain sexual maturity simultaneously. Male pollinators carry the specific burden of detecting and opening mating holes in galls containing conspecific females before mating with them. The mating behaviourand antennal sensilla of male pollinators associated with Ficus semicordata was investigated to understand mate recognition. Male Ceratosolen gravelyi located female-containing galls from a distance, but only attempted to chew a mating hole after antennal contact with the gall surface was made. They showed similar responses to females and body extracts but failed to respond to washed female bodies. This behaviour indicates that unidentified chemicals present on the body surfaces of females are sufficient to elicit olfactory attraction and tactile confirmation. Multiporous plate sensilla are candidates for olfactory reception from a distance while basiconic sensilla may be involved in contact chemoreceptor. All of the sensilla are highly localised, on the distal part of the terminal flagellomere, suggesting a response to selection for strong directionality in the complex odour environment of the fig in which messages produced by hundreds of females may confuse precise mate localisation.  相似文献   
19.
DNA methylation is the major epigenetic modification and it is involved in the negative regulation of gene expression. Its alteration can lead to neoplastic transformation. Several biomolecular approaches are nowadays used to study this modification on DNA, but also on RNA molecules, which are known to play a role in different biological processes. RNA methylation is one of the most common RNA modifications and 5-methylcytosine presence has recently been suggested in mRNA. However, an analysis of nucleic acid methylation at electron microscope is still lacking. Therefore, we visualized DNA methylation status and RNA methylation sites in the interphase nucleus of HeLa cells and rat hepatocytes by ultrastructural immunocytochemistry and cytochemical staining. This approach represents an efficient alternative to study nucleic acid methylation. In particular, this ultrastructural method makes the visualization of this epigenetic modification on a single RNA molecule possible, thus overcoming the technical limitations for a (pre-)mRNA methylation analysis.  相似文献   
20.
Interferon-alpha (IFN-α) is a potent anti-viral cytokine, critical to the host immune response against viruses. IFN-α is first produced upon viral detection by pathogen recognition receptors. Following its expression, IFN-α embarks upon a complex downstream signalling cascade called the JAK/STAT pathway. This signalling pathway results in the expression of hundreds of effector genes known as interferon stimulated genes (ISGs). These genes are the basis for an elaborate effector mechanism and ultimately, the clearance of viral infection. ISGs mark an elegant mechanism of anti-viral host defence that warrants renewed research focus in our global efforts to treat existing and emerging viruses. By understanding the mechanistic role of individual ISGs we anticipate the discovery of a new “treasure trove” of anti-viral mediators that may pave the way for more effective, targeted and less toxic anti-viral therapies. Therefore, with the aim of highlighting the value of the innate type 1 IFN response in our battle against viral infection, this review outlines both historic and recent advances in understanding the IFN-α JAK/STAT pathway, with a focus on new research discoveries relating to specific ISGs and their potential role in curing existing and future emergent viral infections.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号