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   483篇
  免费   3篇
  国内免费   3篇
系统科学   10篇
教育与普及   1篇
理论与方法论   5篇
现状及发展   196篇
研究方法   90篇
综合类   177篇
自然研究   10篇
  2018年   14篇
  2017年   9篇
  2016年   13篇
  2015年   11篇
  2014年   9篇
  2013年   14篇
  2012年   45篇
  2011年   54篇
  2010年   22篇
  2009年   9篇
  2008年   32篇
  2007年   22篇
  2006年   33篇
  2005年   24篇
  2004年   30篇
  2003年   22篇
  2002年   23篇
  1999年   3篇
  1997年   2篇
  1995年   2篇
  1991年   1篇
  1987年   1篇
  1985年   2篇
  1983年   1篇
  1982年   2篇
  1981年   6篇
  1980年   6篇
  1979年   3篇
  1978年   2篇
  1977年   8篇
  1976年   4篇
  1975年   5篇
  1974年   2篇
  1973年   8篇
  1972年   6篇
  1971年   6篇
  1970年   4篇
  1969年   2篇
  1968年   2篇
  1966年   3篇
  1965年   2篇
  1964年   3篇
  1962年   1篇
  1960年   1篇
  1959年   1篇
  1957年   1篇
  1956年   1篇
  1955年   5篇
  1954年   1篇
  1947年   1篇
排序方式: 共有489条查询结果,搜索用时 203 毫秒
31.
We use survey data on five bilateral exchange rates to provide empirical evidence of the fact that professional forecasters of foreign exchange rates behave irrationally, in the specific sense that they respond inaccurately to available information in the market when forming their predictions. In particular, we find systematic biases in the forecasts resulting in the overreaction of analysts to past information contained in the exchange rate dynamics: forecasters change their prediction more than it would be rational on the basis of past realized changes. In addition, forecasters are heterogeneous in their irrationality: low performers in previous periods show a more pronounced overreaction effect. This can be read as an indication of perpetration of past errors and continued inability to learn from the past. In the second part of the paper, we exploit the novel structure of our dataset, which consists of survey data extracted from the Bloomberg platform and readily available to anyone. This feature allows us to consider their own and others' past forecasts as part of the information set that analysts use in making their predictions. By using past forecasts as proxies for relevant macroeconomic variables, we find evidence that analysts fail to correctly process not only the information contained in the spot rate past dynamics but also the information in this broader set. We see this as confirmation of the existence of inefficiency and heterogeneity between low and high performers also when full information is available. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   
32.
The taxonomy of the family Raspailiidae has always been controversial. The family was first included in the order Poecilosclerida. It was then allocated to the order Axinellida and later moved back to Poecilosclerida. Currently with the development of molecular tools it has been assigned to the order Axinellida. In this contribution we describe 10 species from the Mexican Pacific Ocean. Seven of them are new to science: Raspailia (Parasyringella) rubra sp. nov., Raspailia (Raspaxilla) hymani (Dickinson 1945 Dickinson, MG. 1945. Sponges on the Gulf of California. Allan Hancock Pac Exp, 11(1): 1251.  [Google Scholar]), Raspailia (Raspaxilla) hyle (de Laubenfels 1930 de Laubenfels, MW. 1930. The sponges of California. (Abstracts of dissertations for the degree of doctor of philosophy). Stanford Univ Bull, 5(98): 2429.  [Google Scholar]), Aulospongus cerebella (Dickinson 1945 Dickinson, MG. 1945. Sponges on the Gulf of California. Allan Hancock Pac Exp, 11(1): 1251.  [Google Scholar]), Aulospongus californianus sp. nov., Aulospongus aurantiacus sp. nov., Eurypon patriciae sp. nov., Eurypon tylospinosum sp. nov., Eurypon diversicolor sp. nov. and Eurypon brunus sp. nov. We discuss the genus Eurypon and include a table for all the species described worldwide with some comments about this genus. http://www.zoobank.org/urn:lsid:zoobank.org:pub:D462084B-EE9C-4C61-884A-C9DB70003B4A  相似文献   
33.
This article discusses the intersection of science and culture in the marketplace and explores the ways in which radium quack and medicinal products were packaged and labelled in the early twentieth century US. Although there is an interesting growing body of literature by art historians on package design, historians of science and medicine have paid little to no attention to the ways scientific and medical objects that were turned into commodities were packaged and commercialized. Thinking about packages not as mere containers but as multifunctional tools adds to historical accounts of science as a sociocultural enterprise and reminds us that science has always been part of consumer culture. This paper suggests that far from being receptacles that preserve their content and facilitate their transportation, bottles and boxes that contained radium products functioned as commercial and epistemic devices. It was the 1906 Pure Food and Drug Act that enforced such functions. Packages worked as commercial devices in the sense that they were used to boost sales. In addition, ‘epistemic’ points to the fact that the package is an artefact that ascribes meaning to and shapes its content while at the same time working as a device for distinguishing between patent and orthodox medicines.  相似文献   
34.
Schizophrenia is an etiologically heterogeneous psychiatric disease, which exists in familial and nonfamilial (sporadic) forms. Here, we examine the possibility that rare de novo copy number (CN) mutations with relatively high penetrance contribute to the genetic component of schizophrenia. We carried out a whole-genome scan and implemented a number of steps for finding and confirming CN mutations. Confirmed de novo mutations were significantly associated with schizophrenia (P = 0.00078) and were collectively approximately 8 times more frequent in sporadic (but not familial) cases with schizophrenia than in unaffected controls. In comparison, rare inherited CN mutations were only modestly enriched in sporadic cases. Our results suggest that rare de novo germline mutations contribute to schizophrenia vulnerability in sporadic cases and that rare genetic lesions at many different loci can account, at least in part, for the genetic heterogeneity of this disease.  相似文献   
35.
36.
Cornelia de Lange syndrome is a multisystem developmental disorder characterized by facial dysmorphisms, upper limb abnormalities, growth delay and cognitive retardation. Mutations in the NIPBL gene, a component of the cohesin complex, account for approximately half of the affected individuals. We report here that mutations in SMC1L1 (also known as SMC1), which encodes a different subunit of the cohesin complex, are responsible for CdLS in three male members of an affected family and in one sporadic case.  相似文献   
37.
In higher vertebrates, sulfatases belong to a conserved family of enzymes that are involved in the regulation of cell metabolism and in developmental cell signaling. They cleave the sulfate from sulfate esters contained in hormones, proteins, and complex macromolecules. A highly conserved cysteine in their active site is post-translationally converted into formylglycine by the formylglycine-generating enzyme encoded by SUMF1 (sulfatase modifying factor 1). This post-translational modification activates all sulfatases. Sulfatases are extensively glycosylated proteins and some of them follow trafficking pathways through cells, being secreted and taken up by distant cells. Many proteoglycans, glycoproteins, and glycolipids contain sulfated carbohydrates, which are sulfatase substrates. Indeed, sulfatases operate as decoding factors for a large amount of biological information contained in the structures of the sulfated sugar chains that are covalently linked to proteins and lipids. Modifications to these sulfate groups have pivotal roles in modulating specific signaling pathways and cell metabolism in mammals.  相似文献   
38.
Recognized as a “disease modifier”, physical activity (PA) is increasingly viewed as a more holistic, cost-saving method for prevention, treatment and management of human disease conditions. The traditional view that PA engages the monoaminergic and endorphinergic systems has been challenged by the discovery of the endocannabinoid system (ECS), composed of endogenous lipids, their target receptors, and metabolic enzymes. Indeed, direct and indirect evidence suggests that the ECS might mediate some of the PA-triggered effects throughout the body. Moreover, it is now emerging that PA itself is able to modulate ECS in different ways. Against this background, in the present review we shall discuss evidence of the cross-talk between PA and the ECS, ranging from brain to peripheral districts and highlighting how ECS must be tightly regulated during PA, in order to maintain its beneficial effects on cognition, mood, and nociception, while avoiding impaired energy metabolism, oxidative stress, and inflammatory processes.  相似文献   
39.
40.
Riassunto Nell'ambito del problema del polimorfismo cromosomico diRattus rattus è stato studiato il cariotipo di varie popolazioni europee (Italia, Svizzera, Francia ed Inghilterra) riferibili alle sottospeciefrugivorus, rattus edalexandrinus. Il numero diploide osservato è sempre stato2n=38 e non è emersa nessuna differenza morfologica tra i cariogrammi delle varie popolazioni.

This research was supported by the Gruppo di Studio per le popolazioni insulari del C.N.R..  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