首页 | 官方网站   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   41534篇
  免费   3978篇
  国内免费   2711篇
农业科学   4060篇
艺术   9篇
地球科学   2713篇
航空航天   311篇
生物科学   3259篇
文化教育   1514篇
经济   2782篇
环境安全   1176篇
自然科学   3373篇
社会科学   4067篇
历史地理   67篇
工业技术   10999篇
语言文字   26篇
文学   1篇
数理化   3180篇
医药卫生   9344篇
军事   76篇
哲学宗教   349篇
政治法律   217篇
交通运输   700篇
  2024年   64篇
  2023年   760篇
  2022年   1161篇
  2021年   1512篇
  2020年   1791篇
  2019年   1527篇
  2018年   1477篇
  2017年   1935篇
  2016年   1771篇
  2015年   1689篇
  2014年   2369篇
  2013年   3964篇
  2012年   2902篇
  2011年   2683篇
  2010年   2100篇
  2009年   2234篇
  2008年   2157篇
  2007年   2239篇
  2006年   1894篇
  2005年   1522篇
  2004年   1379篇
  2003年   1125篇
  2002年   989篇
  2001年   871篇
  2000年   680篇
  1999年   682篇
  1998年   593篇
  1997年   476篇
  1996年   433篇
  1995年   403篇
  1994年   343篇
  1993年   346篇
  1992年   373篇
  1991年   308篇
  1990年   247篇
  1989年   213篇
  1988年   188篇
  1987年   123篇
  1986年   104篇
  1985年   107篇
  1984年   89篇
  1983年   59篇
  1982年   60篇
  1981年   39篇
  1980年   58篇
  1979年   31篇
  1978年   26篇
  1977年   30篇
  1976年   19篇
  1973年   21篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
2.
Islands, which provide multiple ecosystem services, are subject to increasing urbanization pressure due to the ongoing marine development, especially in developing countries. Insights into the island urbanization mechanism and its ecological consequences are essential to sustainable development. In the present paper, the satellite images, nighttime lights, and topographic data were integrated to characterize the spatially explicit urbanization process and mechanism during 1995–2011 in the Zhoushan Island, East China. Furthermore, the corresponding spatially explicit changes in ecosystem services, including net primary productivity (NPP), carbon sequestration and oxygen production (CSOP), nutrient cycling, crop production, and habitat quality, were quantified based on the Carnegie–Ames–Stanford Approach (CASA) and Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) models. The results showed that the Zhoushan Island had experienced a rapid urbanization over the years, with significant urban encroachment on the farmland and tidal flat. Moreover, the urban land expansion was positively correlated with that of the nighttime lights and negatively correlated with the elevation, slope, and the distance to shoreline. These indicated that the urban expansion was resulted from the enhancement of socioeconomic activities, and concentrated in the near-shore areas with low altitude and gentle slope. The urban encroachment on other land use types resulted in a decrease of 3.4 Gg C a−1 NPP, 8.7 Gg a−1 CSOP, 13.2 Gg a−1 nutrient cycling, and 12.3 t a−1 crop production, respectively. In addition, the habitat quality in 11% area of this island degraded substantially. Therefore, to achieve sustainable development of islands, it is urgent to implement more stringent policies, such as island spatial regulation, environmental impact assessment, intensive land use, and urban greening, etc.  相似文献   
3.
Abstract. In this study, plant functional types are understood as groups of plants with similar biological traits displaying significant optima or maxima on a gradient plane of resource supply and disturbance intensity. The biological traits refer to expansion, vegetative regeneration, generative reproduction, dispersal and seed bank longevity. 129 vegetation samples were taken in an agricultural landscape in southwestern Germany, covering a wide range of terrestrial vegetation types – but with the exception of forests and wetlands. For each site, also soil data were recorded. Mean daily soil moisture was estimated with a simple model. Soil moisture, balanced nitrogen supply and available phosphorus were combined into a factor ‘resource supply’. In addition, disturbance intensity was estimated for each site. This factor was based on (1) frequency of disturbance, (2) disturbance depth below or above the soil surface, and (3) proportion of the area affected by a discrete disturbance event. 30 plant groups with similar biological characteristics resulted from a cluster analysis, based on a compilation of 19 biological traits for a regional species pool. Logistic regression on a gradient plane of disturbance intensity and resource supply yielded response curves for 28 groups. The dependent variable was defined as the probability of encountering all members of a group in a sample. 17 groups display a significant response curve on the gradient plane. Plants with a potential for long- range dispersal are concentrated on sites with low or high disturbance intensities (e.g. fallow land, fields, lawns). On sites with medium disturbance intensity (e.g. meadows) and low to medium resource supply, small-range dispersal predominates. There are no distinct trends concerning seed bank longevity. The potential for vertical and lateral expansion increases with decreasing disturbance intensity. Only at medium disturbance intensities does vertical expansion correlate positively with resource supply. Rapid detachment of daughter individuals occurs more often on productive sites than on less productive sites. Diversity of groups with similar biological traits is highest on sites with medium disturbance intensities.  相似文献   
4.
It is commonly held that people believe themselves to be better than others, especially for outcomes under their control. However, such overconfidence is not universal. This paper presents evidence showing that people believe that they are below average on skill-based tasks that are difficult. A simple Bayesian explanation can account for these effects and for their robustness: On skill-based tasks, people generally have better information about themselves than about others, so their beliefs about others’ performances tend to be more regressive (thus less extreme) than their beliefs about their own performances. This explanation is tested in two experiments that examine these effects’ robustness to experience, feedback, and market forces. The discussion explores the implications for strategic planning in general and entrepreneurial entry in particular.  相似文献   
5.
免疫细胞浸润对癌症的诊断与预后有着重要意义。文中收集TCGA数据库已收录的非小细胞肺癌肿瘤与正常组织基因表达数据,利用CIBERSORT工具得到22种免疫细胞占比来评估免疫细胞浸润情况。以22种免疫细胞占比为特征,用机器学习方法构建了非小细胞肺癌肿瘤与正常组织的分类模型,其中随机森林方法构建的模型分类效果AUC=0.987、敏感性0.98及特异性0.84。并且用随机森林方法构建的肺腺癌和肺鳞癌肿瘤组织分类模型效果AUC=0.827、敏感性0.75及特异性0.77。用LASSO回归筛选22种免疫细胞特征,保留8种强相关特征组成的免疫细胞评分结合临床特征构建了非小细胞肺癌预后模型。经评估及验证,预后模型C-index=0.71并且3年和5年的校准曲线拟合良好,可以对预后风险度进行准确预测。本研究基于免疫细胞浸润所构建的分类模型与预后模型,旨在对非小细胞肺癌的诊断与预后研究提供新的策略。  相似文献   
6.
7.
8.
传统的最小二乘回归法关注于对当前数据集的准确估计, 容易导致模型的过拟合, 影响模型结论的可重复性。随着方法学领域的发展, 涌现出的新兴统计工具可以弥补传统方法的局限, 从过度关注回归系数值的解释转向提升研究结果的预测能力也愈加成为心理学领域重要的发展趋势。Lasso方法通过在模型估计中引入惩罚项的方式, 可以获得更高的预测准确度和模型概化能力, 同时也可以有效地处理过拟合和多重共线性问题, 有助于心理学理论的构建和完善。  相似文献   
9.
In North-western Germany woodland fragmentation has caused a decline in many forest plant species. Hedgerows partly offer a similar environment as forests and have been identified as potential habitats for forest plants in various studies from North America and Western Europe. The objective of this study was to examine whether this applies also to Central Europe and which variables affect the spatial distribution and abundance of forest plant species in hedgerows on a local scale. Three hedgerow networks north of the city of Bremen, Germany, were selected as study areas and divided into totally 515 hedgerow segments. In each segment we recorded all vascular plants and a large number of explanatory variables relating to structure, spatial configuration, environment and management. Averaged across species there was a predominant effect of environmental factors on the occurrence of forest species in the hedgerows, followed by spatial configuration and management. Hedgerow structure was found to be less important. In general, forest species were favored by low nutrient and light availability as well as high connectivity with other hedgerows or forest; they avoided hedgerows with a west-easterly orientation and an adjacent land use in the form of fields or grasslands. Forest species found and not found in hedgerows did not differ in their environmental preferences or life history traits. The number of threatened forest species in the hedgerows, however, was lower than expected with respect to their overall proportion to the total number of forest species in the region.  相似文献   
10.
基于低压氧弹燃烧法研究了充氧压力分别为0.6MPa和0.9MPa时两种无灰型柴油添加剂不同配比对助燃性能的影响。采用均匀实验设计和Matlab回归优化的方法,得到复配体系的最优配方,并对其进行了验证。结果表明,由Matlab回归分析得到的完全二次回归模型最理想,0.6MPa时由该模型得到的最优配方为:添加剂A和B的质量分数分别是0.300‰和0.700‰,并通过实验验证得到的发热量达到最大,变化率为8.697%。0.9MPa时,结合直观分析和回归分析,得到与实际相符的最优配方:添加剂A和B的质量分数分别是0.600‰和0.900‰,此时发热量皮化率为4.453%。  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-23

京公网安备 11010802026262号