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   11650篇
  免费   844篇
  国内免费   386篇
系统科学   1296篇
丛书文集   203篇
教育与普及   132篇
理论与方法论   292篇
现状及发展   890篇
研究方法   17篇
综合类   10049篇
自然研究   1篇
  2024年   43篇
  2023年   89篇
  2022年   157篇
  2021年   167篇
  2020年   127篇
  2019年   72篇
  2018年   817篇
  2017年   849篇
  2016年   560篇
  2015年   280篇
  2014年   332篇
  2013年   315篇
  2012年   556篇
  2011年   1282篇
  2010年   1115篇
  2009年   770篇
  2008年   857篇
  2007年   1039篇
  2006年   239篇
  2005年   259篇
  2004年   276篇
  2003年   436篇
  2002年   468篇
  2001年   322篇
  2000年   221篇
  1999年   216篇
  1998年   120篇
  1997年   144篇
  1996年   136篇
  1995年   103篇
  1994年   104篇
  1993年   82篇
  1992年   80篇
  1991年   65篇
  1990年   58篇
  1989年   43篇
  1988年   39篇
  1987年   18篇
  1986年   11篇
  1985年   3篇
  1984年   4篇
  1983年   1篇
  1967年   1篇
  1955年   4篇
排序方式: 共有10000条查询结果,搜索用时 932 毫秒
71.
In this paper, we introduce a new class of generalized convex function, namely, a-pseudounivex function, by combining the concepts of pseudo-univex and α-invex functions. Further, we establish some relationships between vector variational-like inequality problems and vector optimization problems under the assumptions of α-pseudo-univex functions. Results obtained in this paper present a refinement and improvement of previously known results.  相似文献   
72.
从几个方面论述了TCP/IP体系结构怎样通过层间协作来保证通信的可靠性,以此说明任何一个网络体系结构必须具备完整性,并给出了网络的几种寻址方法.  相似文献   
73.
The geomagnetic fields, which play important roles in the ionospheric dynamo, can greatly affect the global distribution of ionospheric electric fields, currents and other ionospheric electrodynamics phenomena. In the study of ionospheric electrodynamics phenomena, such as the longitudinal variations of ionospheric electric fields, the non-dipolar component of the geomagnetic fields must be taken into account. In this paper, we deduce a theoretical electric field model for ionospheric dynamo at mid- and low-latitude which adopt a modified magnetic apex coordinates system. In the new electric field model, the geomagnetic fields can be calculated from either the IGRF model or the dipole field model, and the neutral winds and conductivities are calculated based on empirical models. Then the dynamo equation for the electric potential is finally solved in terms of the line-by-line iteration method, and the ionospheric electric fields and currents are derived from the calculated potential. Our model can reproduce the main features of the ionospheric electrodynamics processes, so it will be a useful tool for the investigation of the upper atmosphere and ionosphere.  相似文献   
74.
本文用小角x射线射法法对硒化蓖麻酸多相脂质体的抗癌作用从结构和功能上进行了研究。  相似文献   
75.
本文提出了滚齿时基面内切削厚度的计算方法.该方法可作为深入研究滚齿切削过程的理论基础.  相似文献   
76.
本文对现行泛函分析教材中的一个不动点的存在唯一性定理举出了一个反例,指出其唯一性不成立。  相似文献   
77.
We discuss the foundations of constructive mathematics, including recursive mathematics and intuitionism, in relation to classical mathematics. There are connections with the foundations of physics, due to the way in which the different branches of mathematics reflect reality. Many different axioms and their interrelationship are discussed. We show that there is a fundamental problem in BISH (Bishop’s school of constructive mathematics) with regard to its current definition of ‘continuous function’. This problem is closely related to the definition in BISH of ‘locally compact’. Possible approaches to this problem are discussed. Topology seems to be a key to understanding many issues. We offer several new simplifying axioms, which can form bridges between the various branches of constructive mathematics and classical mathematics (‘reuniting the antipodes’). We give a simplification of basic intuitionistic theory, especially with regard to so-called ‘bar induction’. We then plead for a limited number of axiomatic systems, which differentiate between the various branches of mathematics. Finally, in the appendix we offer BISH an elegant topological definition of ‘locally compact’, which unlike the current definition is equivalent to the usual classical and/or intuitionistic definition in classical and intuitionistic mathematics, respectively.  相似文献   
78.
现代科学计算和工程等很多问题中都是用微分方程的形式进行描述,因而研究微分方程具有非常重要的实际意义。本文主要介绍如何使用MATLAB求解常微分方程初值问题。  相似文献   
79.
简要评估了“926”植物生长调节剂的经济效益和生态环境。分析了开发应用的前景。  相似文献   
80.
The Indian antHarpegnathos saltator may be unique among insects in using its jumping capacity not only as an escape mechanism but also as a normal means of locomotion, and for catching its prey in flight. High-speed cinematography used to analyse the various phases of the jump suggests thatHarpegnathos employs a novel jumping mechanism to mediate these behaviours: namely the synchronous activation of its middle and hindlegs. Electrophysiological recordings from muscles or nerves in pairs of middle and hindlegs show remarkably synchronous activity during fictive jumping, supporting the synchronous activation hypothesis.Harpegnathos is not the only ant to jump, and a cladistic analysis suggests that jumping behaviour evolved independently three times during ant evolutionary history.  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