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   9882篇
  免费   728篇
  国内免费   4篇
  10614篇
  2023年   60篇
  2022年   114篇
  2021年   206篇
  2020年   175篇
  2019年   216篇
  2018年   278篇
  2017年   227篇
  2016年   332篇
  2015年   563篇
  2014年   561篇
  2013年   691篇
  2012年   842篇
  2011年   783篇
  2010年   521篇
  2009年   470篇
  2008年   576篇
  2007年   544篇
  2006年   528篇
  2005年   482篇
  2004年   448篇
  2003年   366篇
  2002年   346篇
  2001年   88篇
  2000年   74篇
  1999年   93篇
  1998年   122篇
  1997年   91篇
  1996年   68篇
  1995年   62篇
  1994年   65篇
  1993年   63篇
  1992年   45篇
  1991年   42篇
  1990年   44篇
  1989年   39篇
  1988年   33篇
  1987年   27篇
  1986年   25篇
  1985年   29篇
  1984年   40篇
  1983年   22篇
  1982年   28篇
  1981年   26篇
  1980年   14篇
  1979年   13篇
  1978年   13篇
  1976年   13篇
  1975年   17篇
  1974年   10篇
  1973年   12篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
This paper introduces an indicator system to evaluate sustainability in established coastal tourism destinations, applying the recommendations and definitions of the World Tourism Organization (WTO). We also develop a new synthetic indicator to simplify the measurement of sustainability and facilitate the comparative analysis of destination ranking. This measurement was obtained by applying a procedure to reduce the number of subjective decisions made by the analyst, using a novel two-stage aggregation methodology based on principal component analysis and on the distance to a reference point. The synthetic indicator obtained was applied to Spanish coastal destinations, and the results serve as a guideline for tourism planning. The conclusions of this research can be extrapolated to the study of other tourism destinations.  相似文献   
72.
Many terpenoids are biosynthesized after a cascade of cyclizations and rearrangements of carbocations mediated by terpenoid synthases, as exemplified in th  相似文献   
73.
74.
75.
The Mediterranean Basin is a global biodiversity hotspot, hosting a number of native species belonging to families that are found almost exclusively in tropical climates. Yet, whether or not these taxa were able to survive in the Mediterranean region during the Quaternary climatic oscillations remains unknown. Focusing on the European free-tailed bat (Tadarida teniotis), we aimed to (a) identify potential ancient populations and glacial refugia; (b) determine the post-glacial colonization routes across the Mediterranean; and (c) evaluate current population structure and demography. Mitochondrial and nuclear markers were used to understand T. teniotis evolutionary and demographic history. We show that T. teniotis is likely restricted to the Western Palearctic, with mitochondrial phylogeny suggesting a split between an Anatolian/Middle East clade and a European clade. Nuclear data pointed to three genetic populations, one of which is an isolated and highly differentiated group in the Canary Islands, another distributed across Iberia, Morocco, and France, and a third stretching from Italy to the east, with admixture following a pattern of isolation by distance. Evolutionary and demographic reconstruction supports a pre-Last Glacial Maximum (LGM) colonization of Italy and the Anatolian/Middle East, while the remaining populations were colonized from Italy after the Younger Dryas. We also found support for demographic expansion following the Iberian colonization. The results show that during the LGM T. teniotis persisted in Mediterranean refugia and has subsequently expanded to its current circum-Mediterranean range. Our findings raise questions regarding the physiological and ecological traits that enabled species with tropical affinities to survive in colder climates.  相似文献   
76.
The International Journal of Life Cycle Assessment - Given the increasing importance of political decision-making to reduce emission targets, the main purpose of the current paper is to identify...  相似文献   
77.
Despite early reperfusion, patients with ST segment elevation myocardial infarction (STEMI) may present large myocardial necrosis and significant impairment of ventricular function. The present study aimed to evaluate the role of subtypes of B lymphocytes and related cytokines in the infarcted mass and left ventricular ejection fraction obtained by cardiac magnetic resonance imaging performed after 30 days of STEMI. This prospective study included 120 subjects with STEMI submitted to pharmacoinvasive strategy. Blood samples were collected in subjects in the first (D1) and 30th (D30) days post STEMI. The amount of CD11b+ B1 lymphocytes (cells/ml) at D1 were related to the infarcted mass (rho = 0.43; P=0.033), measured by cardiac MRI at D30. These B1 cells were associated with CD4+ T lymphocytes at D1 and D30, while B2 classic lymphocytes at day 30 were related to left ventricular ejection fraction (LVEF). Higher titers of circulating IL-4 and IL-10 were observed at D30 versus D1 (P=0.013 and P<0.001, respectively). Titers of IL-6 at D1 were associated with infarcted mass (rho = 0.41, P<0.001) and inversely related to LVEF (rho = −0.38, P<0.001). After multiple linear regression analysis, high-sensitivity troponin T and IL-6 collected at day 1 were independent predictors of infarcted mass and, at day 30, only HDL-C. Regarding LVEF, high-sensitivity troponin T and high-sensitivity C-reactive protein were independent predictors at day 1, and B2 classic lymphocytes, at day 30. In subjects with STEMI, despite early reperfusion, the amount of infarcted mass and ventricular performance were related to inflammatory responses triggered by circulating B lymphocytes.  相似文献   
78.
International Journal of Primatology - Some nonhuman primates use tools to modify the behavior of other animals in social contexts. For example, during their proceptive periods, female bearded...  相似文献   
79.
80.
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