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   8527篇
  免费   395篇
  国内免费   984篇
林业   418篇
农学   831篇
基础科学   1476篇
  1789篇
综合类   3396篇
农作物   448篇
水产渔业   145篇
畜牧兽医   377篇
园艺   192篇
植物保护   834篇
  2024年   80篇
  2023年   181篇
  2022年   300篇
  2021年   319篇
  2020年   348篇
  2019年   398篇
  2018年   226篇
  2017年   326篇
  2016年   478篇
  2015年   386篇
  2014年   484篇
  2013年   561篇
  2012年   710篇
  2011年   616篇
  2010年   486篇
  2009年   509篇
  2008年   392篇
  2007年   467篇
  2006年   402篇
  2005年   326篇
  2004年   253篇
  2003年   253篇
  2002年   169篇
  2001年   172篇
  2000年   141篇
  1999年   108篇
  1998年   110篇
  1997年   107篇
  1996年   85篇
  1995年   90篇
  1994年   74篇
  1993年   73篇
  1992年   64篇
  1991年   50篇
  1990年   47篇
  1989年   30篇
  1988年   23篇
  1987年   16篇
  1986年   8篇
  1985年   6篇
  1984年   11篇
  1983年   3篇
  1982年   1篇
  1981年   2篇
  1980年   2篇
  1979年   3篇
  1978年   4篇
  1977年   3篇
  1976年   1篇
  1955年   2篇
排序方式: 共有9906条查询结果,搜索用时 31 毫秒
61.
In this paper, by use of the pseudo-gradient vector field, the three critical points theorem in [2] is extended to the C1 function in the Ba-nach space, and therefore the three critical, points theorem in [5] is strengthened and the existence of the saddle point of the C2 function is discussed.  相似文献   
62.
Mathematical model of power frequency electric field is established based on Charge Simulation Method(CSM) to analyze power frequency electric field caused by EHV lines with overhead ground wires and its effect, which are entering the megalopolis. Calculative method on the electric field under the transmission lines is analyzed and validated. According to the criterion of environmental evaluation, the distributing characteristic of electric field is summarized, in case of various arrangement manners of lines. At last, some advice and methods of eliminating electric field under the transmission lines entering the megalopolis is proposed with regard to protecting environment.  相似文献   
63.
64.
Based on [l].this paper presents a new spline-BEM for calculating 3-D electrostatic fields,in which bi-cubic B-splines are used as basis functions. The corner problem in BEM and the extreme conditions for splines are circumvented. The numerical results presented show that the global approximation .convergence and stability of the method are satisfactory.  相似文献   
65.
Based on the infroduction of the present process of quenched internal gear, a new technolgy of machining quenched internal gear-EDM generating technology is presented with its principle and machining processes. .  相似文献   
66.
This paper has applied the var cost to the theory of spot price.An augmented Lagrange method is employed to develop the exact model of spot price of active power and reactive power,and the physical means of Lagrange multipliers are explained.The numerical examples are given to demonstrate the validity of the proposed model.  相似文献   
67.
Some conceptions of stable quaternion matrices and some judging criteria are given.The Robrach theorem,Taussky theorem,Ostrowski Schneider theorem and Gerschgorin theorem are generalized to quaternion field.The conclusion about two quaternion matrices,i.e.,inferior positive matrices are unstable,interior negative matrices are stable and some other results are pointed out.  相似文献   
68.
The mechanism of television interference is discussed from High voltage transmission line. Then the computational model is established, and scattering fields are calculated with the finite-difference time-domain(FDTD) method, and the results are visualized. The scattering effects of High voltage transmission lines on horizontal polarization waves (corresponding to TM mode) are greater than on vertical polarization waves (corresponding to TE mode).  相似文献   
69.
70.
A nonlinear finite element analysis of 4 RC deep flexural simply supported beams with the program VT2 is presented. Crack development and failure modes , longitudinal reinforcement strain distributions and load -deflection curves were compared as a result in order to evaluate the validity of the disturbed stress field model to analyse such deep simply supported beam.  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