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   5356篇
  免费   196篇
  国内免费   630篇
系统科学   264篇
丛书文集   153篇
教育与普及   100篇
理论与方法论   39篇
现状及发展   5篇
综合类   5621篇
  2024年   12篇
  2023年   35篇
  2022年   41篇
  2021年   36篇
  2020年   40篇
  2019年   25篇
  2018年   21篇
  2017年   38篇
  2016年   59篇
  2015年   148篇
  2014年   309篇
  2013年   231篇
  2012年   314篇
  2011年   399篇
  2010年   352篇
  2009年   403篇
  2008年   539篇
  2007年   691篇
  2006年   467篇
  2005年   374篇
  2004年   229篇
  2003年   203篇
  2002年   196篇
  2001年   311篇
  2000年   176篇
  1999年   72篇
  1998年   68篇
  1997年   59篇
  1996年   48篇
  1995年   31篇
  1994年   30篇
  1993年   33篇
  1992年   29篇
  1991年   37篇
  1990年   39篇
  1989年   44篇
  1988年   14篇
  1987年   7篇
  1986年   4篇
  1985年   14篇
  1984年   1篇
  1963年   1篇
  1957年   1篇
  1943年   1篇
排序方式: 共有6182条查询结果,搜索用时 359 毫秒
81.
海南昌江县石灰岩地区轮叶戟群落特征研究   总被引:10,自引:0,他引:10  
根据样方调查,分析了海南岛昌江县石灰岩地区轮叶戟Lasiococca comberi var.pseudoverticillata群落的种类组成、外貌和结构特征,并就分析结果与其它地区进行了相关性分析和讨论.结果表明,在1 800 m+2样地中,有维管植物164种,隶属于64科129属,其中以热带性分布属占明显优势,所占比例为94.87%.群落外貌终年常绿,生活型以高位芽为主,占76.07%,乔木多为单叶,叶型以中型叶为主,占70.73%.群落成层现象不明显;通过对优势种群的年龄结构分析显示,该群落处于增长状态,按照Raunkiaer的频度等级定律,A级频度值为71.95,其中有79种植物只出现于一个样方中.通过相关性分析显示,该地植物在属的区系成分上受气候的影响比基质的大,该群落所属的植被类型处于热带常绿季雨林向热带雨林演替的过渡阶段,并需用自然恢复的方式对植被进行恢复和保护.  相似文献   
82.
民族文化旅游受到民族地区高度的重视,但是一些民族地区在旅游开发中,因缺乏民族文化的内涵和特色,不能很好地满足旅游者对民族文化的了解和体验;还有的民族地区,单纯地强调旅游业带来的经济效益,忽视外来文化对民族文化的冲击,导致民族文化的失真或失落.本文从旅游与文化关系的理论入手,结合甘南藏族自治州藏民族文化旅游发展的实际,提出进一步挖掘民族文化的旅游价值,开发具有民族特色的旅游产品;用科学的方法和手段,多层次、多角度地展示民族文化的魅力;防止民族文化的失真和失落,使民族文化得以传承的观点.  相似文献   
83.
Three-dimensional magnetic resonance spectroscopic imaging (3DMRSI) is helpful to identify prostate cancer (PC)from benign prostate hyperplasia (BPH) and to show the distribution of tumor infiltration. Combined with the (Cho + Cre)/Cit ratio, the z-score model can effectively discriminate prostate cancer from stromal benign prostate hyperplasia (sBPH) and detect small malignant lesions (SML).  相似文献   
84.
The combined adjustment project of the nationwide astronomical geodetic networks and 2000' national GPS control network involve 48519 terrestrial observational stations and 2666 GPS observational stations. In the terrestrial network, 25107 stations are height fixed and 23412 stations are in three dimensions. 440793 terrestrial observations and 183668 unknowns are involved in the adjustment. The three-dimensional observational models for the adjustment have been established. The crustal deformation influences and corrections are researched. Fast methods of solving partitioned adjustment for super large-scale geodetic network have been studied. The standard deviations of three-dimensional coordinates for all stations are evaluated. As a result of the combined adjustment, a unified national geodetic network with nearly 50000 stations is established, the high accurate three-dimensional geocentric coordinates are obtained. The standard deviation for geoid is averagely 0.2 m, for vertical deflection is about 1.5″, for the horizontal position is about 0.12 m, for the geodetic height is about 0.14 m, and for the three-dimensional position is averagely 0.3 m.  相似文献   
85.
The development of functional magnetic resonance imaging (fMRI) technology has made it possible to carry out functional brain imaging experiments in small animals. Usually, group data is required to form the assessment of population, which can not only increase the sensitivity of the overall experiment, but also allow the generalization of the conclusion to the whole population. In order to average the signals of functional brain images from different subjects, it is necessary to put all the mapping images into the same standard space (template image). However, up to now, most animal brain templates remain unavailable and it must be done by ourselves. In this study, a template image based on the brains of eight male Wistar rats is obtained, and it is successfully used in our present Alzheimer disease (AD)-like rat model studies as template for spatially normalizing images to the same stereotaxical space. The fMRI results processed with statistical parametric mapping (SPM99) software are in agreement with the results from immunohistochemical experiment, which proves that this method is universally applicable to the pathologic models of other small animals and to human brain lesion studies.  相似文献   
86.
Modal logics are good candidates for a formal theory of agents. The efficiency of reasoning method in modal logics is very important, because it determines whether or not the reasoning method can be widely used in systems based on agent. In this paper, we modify the extension rule theorem proving method we presented before, and then apply it to P-logic that is translated from modal logic by functional transformation. At last, we give the proof of its soundness and completeness.  相似文献   
87.
This paper focuses on the visual servo control of an uncalibrated robotic arm with an eye-in-hand camera. Without a prior knowledge of the kinematics of the robotic arm or camera calibration, the proposed hybrid Jacobian controller can track a moving object using visual feedback and joint-space velocity feedback. The proposed hybrid control method is a combination of the uncalibrated visual servoing and approximate Jacobian feedback control. First, the Jacobian matrix from joint-space to image-space is estimated by recursive least-squares (RLS) algorithm, and then the approximate Jacobian feedback controller is designed by using visual feedback and joint-space velocity feedback. The performances of the proposed control methods are illustrated by computer simulations.  相似文献   
88.
Analyses of the mating-type ratio of protoplast monokaryons to spores in 28 strains of Lentinula edodes were performed through statistical methods. The results showed that the segregation distortion phenomenon commonly exists in both types of Lentinula edodes, 17 out of the 28 samples did not display the expected 1∶1 ratio of protoplast monokaryons; and among them 8 fruiting strains distorted 1∶1∶1∶1 segregation of spores. The degree of distortion in all tested wild strains was significantly lower than that of cultured strains. In fruiting test of positive-negative parent, predominant spores were those with the mating type identical to dikaryotic parent, indicating that the predominance of nuclei mainly depends on the mating type of dikaryotic parent. Data from di-mon mating test verified that monokaryons with acceptor nuclear type are present exclusively in a large amount after dedikaryotization, suggesting that the specificities among B factors and some other unknown factors in cytoplasm are probably responsible for the phenomenon of skewed segregation.  相似文献   
89.
The optimal model and simulating model of the Hanjiang cascade reservoirs operation with one day as its calculating period are established to coordinate the contradictions between power generating and shipping. The optimal model is solved by progressive optimality algorithm method adopting the theory of large system decomposing and coordinating. By analyzing the water allocation results obtained from six schemes, which are two cascades, four cascades and seven cascades joint operation considering shipping or not, the contradiction degree between power generating and shipping is investigated and some joint operation schemes are proposed.  相似文献   
90.
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