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   112652篇
  免费   11754篇
  国内免费   13510篇
系统科学   6637篇
丛书文集   2776篇
教育与普及   355篇
理论与方法论   412篇
现状及发展   727篇
研究方法   12篇
综合类   126996篇
自然研究   1篇
  2024年   282篇
  2023年   742篇
  2022年   1589篇
  2021年   2050篇
  2020年   2409篇
  2019年   4314篇
  2018年   4407篇
  2017年   4843篇
  2016年   5244篇
  2015年   6202篇
  2014年   7684篇
  2013年   7621篇
  2012年   7894篇
  2011年   8190篇
  2010年   8462篇
  2009年   8116篇
  2008年   8294篇
  2007年   8458篇
  2006年   7552篇
  2005年   6504篇
  2004年   5608篇
  2003年   4019篇
  2002年   4200篇
  2001年   3366篇
  2000年   2911篇
  1999年   1484篇
  1998年   589篇
  1997年   554篇
  1996年   362篇
  1995年   330篇
  1994年   374篇
  1993年   538篇
  1992年   507篇
  1991年   508篇
  1990年   354篇
  1989年   254篇
  1988年   319篇
  1987年   166篇
  1986年   207篇
  1985年   96篇
  1984年   45篇
  1983年   24篇
  1982年   17篇
  1981年   16篇
  1980年   16篇
  1977年   23篇
  1972年   66篇
  1968年   11篇
  1967年   9篇
  1955年   11篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
高速道路入口匝道控制方法综述   总被引:8,自引:2,他引:8  
高速道路入口匝道控制,在国外许多大城市已有广泛的应用和研究.其控制方法依据控制范围、复杂程度可归纳为静态控制、单点动态控制、动态协调控制三类.国内在此领域内尚未展开大规模的研究,目前也没有实际应用的案例。本文对几种基本的高速道路入口匝道控制方法及其优缺点进行总结和分析,并对相关研究的发展提出看法.  相似文献   
942.
本文介绍了AAC音频编码标准中滤波器组的原理和快速算法并且给出了软件实现的流程、测试方案以及优化策略。  相似文献   
943.
2.4GHz无线数据传输系统   总被引:1,自引:0,他引:1  
本文介绍了一种利用TI的C5409DSP和TLV320AIC26 Codec芯片以及Nordic公司的nRF2401 2.4GHz无线收发芯片实现的数字音频采集、播放和无线数据传输系统.具体讲解了TLV320AIC26与nRF2401芯片的特点和使用方法以及DSP与之的硬件和软件设计.同时,还介绍了整个无线数据传输系统的设计思路和简明的通信协议.  相似文献   
944.
With an F2 population from the cross of two cucumber inbred lines, S06 and S52, sequence-related amplified polymorphism (SRAP) was used to construct a genetic linkage map in cucumber (Cucumis sativus L.). Sixty-four SRAP primer combinations generated 108 polymorphic bands in the F2 population analysis. The average of polymorphic bands produced by one primer pair was 1.5, and the maximum was 5. Using Mapmaker 3.0, a linkage map was constructed, which consisted of 77 SRAP markers distributed in nine linkage groups (LOD≥3.0) and spanned 1114.2 cM with an average interval of 14.5 cM between markers. The gene for the first-flower-node trait, termed ffn, was mapped to linkage group Ⅸ, flanked by DC1EM5 and ME7EM2A at 10.3 cM and 12.1 cM distance, respectively.  相似文献   
945.
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   
946.
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   
947.
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   
948.
The aim of the work was to investigate the fate of injectant coal in blast furnaces and the origin of extractable materials in blast furnace carryover dusts. Two sets of samples including injectant coal and the corresponding carryover dusts from a full sized blast furnace and a pilot scale rig have been examined. The samples were extracted using 1-methyl-2-pyrrolidinone (NMP) solvent and the extracts studied by size exclusion chromatography (SEC). The blast furnace carryover dust extracts contained high molecular weight carbonaceous material, of apparent mass corresponding to 107?108 u, by polystyrene calibration. In contrast, the feed coke and char prepared in a wire mesh reactor under high temperature conditions did not give any extractable material. Meanwhile, controlled combustion experiments in a high-pressure wire mesh reactor suggest that the extent of combustion of injectant coal in the blast furnace tuyeres and raceways is limited by time of exposure and very low oxygen concentration. It is thus likely that the extractable, soot-like material in the blast furnace dust originated in tars is released by the injectant coal. Our results suggest that the unburned tars were thermally altered during the upward path within the furnace, giving rise to the formation of heavy molecular weight (soot-like) materials.  相似文献   
949.
实施西部大开发战略,加快中西部地区发展,是党中央面向新世纪做出的战略决策。位于内蒙古自治区西部的鄂托克旗,是一个典型的蒙古族聚居地区,自然资源丰富、生态环境脆弱,具有我国西部民族地区的典型特征。鄂托克旗所面临的生态环境问题在我国西部民族地区具有较强的代表性,其形成原因及解决方法对于我国西部大开发中的西部民族地区具有普遍意义。  相似文献   
950.
应用拉格朗日方程,得到了有阻尼弹簧测力机构在简谐激励作用下具有周期系数的非线性运动微分方程-- Duffing-Mathieu方程;根据非线性振动的多尺度解法,得到了系统满足3次超谐共振情况的一次近似解以及对应的定常解,并对其进行了数值计算,分析了激振力、谐调值、系统阻尼、弹簧刚度等对系统的影响,揭示了一些新的动力学现象.  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