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   59796篇
  免费   7871篇
  国内免费   5381篇
化学   11384篇
晶体学   987篇
力学   12084篇
综合类   700篇
数学   20591篇
物理学   15325篇
无线电   11977篇
  2024年   159篇
  2023年   681篇
  2022年   1107篇
  2021年   1296篇
  2020年   1716篇
  2019年   1454篇
  2018年   1474篇
  2017年   2089篇
  2016年   2407篇
  2015年   2021篇
  2014年   3251篇
  2013年   4428篇
  2012年   3775篇
  2011年   4121篇
  2010年   3461篇
  2009年   3759篇
  2008年   3710篇
  2007年   3872篇
  2006年   3491篇
  2005年   3241篇
  2004年   2772篇
  2003年   2503篇
  2002年   2232篇
  2001年   1898篇
  2000年   1756篇
  1999年   1604篇
  1998年   1435篇
  1997年   1185篇
  1996年   999篇
  1995年   846篇
  1994年   753篇
  1993年   583篇
  1992年   615篇
  1991年   472篇
  1990年   366篇
  1989年   286篇
  1988年   216篇
  1987年   158篇
  1986年   111篇
  1985年   153篇
  1984年   129篇
  1983年   78篇
  1982年   81篇
  1981年   67篇
  1980年   33篇
  1979年   51篇
  1978年   35篇
  1977年   44篇
  1976年   16篇
  1957年   12篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
The asymptotic correction technique of Paine, de Hoog and Anderssen can dramatically improve the accuracy of finite difference or finite element eigenvalues at negligible extra cost if closed form expressions are available for the errors in a simpler related problem. This paper gives closed form expressions for the errors in the eigenvalues of certain Sturm–Liouville problems obtained by various methods, thereby increasing the range of problems for which asymptotic correction can achieve maximum efficiency. It also investigates implementation of the method for more general problems.  相似文献   
72.
行激励级电路的设计是完全为行输出级服务的,它实质上是行输出级电路设计的一个重要而不可分割的组成部分。因此行激励级电路的设计,自然采用了从显象管偏转线圈和行输出管到激励变压器和行激励级的倒推设计法。  相似文献   
73.
A modified backward difference time discretization is presented for Galerkin approximations for nonlinear hyperbolic equation in two space variables. This procedure uses a local approximation of the coefficients based on patches of finite elements with these procedures, a multidimensional problem can be solved as a series of one‐dimensional problems. Optimal order H01 and L2 error estimates are derived. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2007  相似文献   
74.
A finite element solution of the Navier-Stokes equations for steady flow under the magnetic effect through a double-branched two-dimensional section of a three-dimensional model of the canine aorta is discussed. The numerical scheme involves transforming the physical co-ordinates to a curvilinear boundary-fitted co-ordinate system. The shear stress at the wall is calculated for a Reynolds number of 1000 with the branch-to-main aortic flow rate ratio as a parameter. The results are compared with earlier works involving experimental data and found to be in reasonable qualitative agreement. The steady flow, shear stress and branch flow under the effect of a magnetic field have been discussed in detail.  相似文献   
75.
本文研究了全部根构成一个等比数列的多项式的性质,并将所得的结果用到数值积分的Romberg方法^「1」,得到了相应遥改进。  相似文献   
76.
The use of photonic materials has been used in the theory of optical waves. The PBG (Photonic Band Gap) theory and material, was developed recently for optical frequencies and can be easily applied to millimeter waves, microwaves and planar antenna frequencies. The presence of photonic materials as substrate in antennas has some good characteristics such as, supression of light spontaneous emission and suppression of surface waves, allowing the application in planar antenna array. In this work an elaborate analysis using the full wave Transverse Transmission Line - TTL method, that provides efficient and concise results is applied to the planar antennas array with PBG substrate.  相似文献   
77.
We study asymptotic properties of solutions to an extension to arbitrary dimensions of the astrophysical model proposed by Chavanis et al. to explain phenomena of gravitational collapse in clouds of self-gravitating particles. In particular, we show that in the two-dimensional case the solutions can be continued to global ones, while in three space dimensions large data of negative energy blow up in a finite time. Relations between isothermal, Streater's energy-transport and the present models are also studied.  相似文献   
78.
The goal of this paper is to provide a tool, the Global Measure Formula, that will facilitate the study of the limit set of discrete geometrically finite groups of isometries of the rank one symmetric spaces. We consider the shadow of a ball from a fixed reference point onto the boundary, and prove a formula that describes the measure of the shadow in terms of the center of the shadowed ball, generalizing a result from real hyperbolic geometry.  相似文献   
79.
一种用于卫星电视接收的FZP天线   总被引:1,自引:1,他引:0  
本文用物理光学法对FZP反射器的聚焦场、聚焦效率及偏轴扫描特性等进行初步分析,并由此提出一种使用FZP天线实现多颗卫星信号同时接收的技术方案。原理性样机实际接收结果证实了其可行性  相似文献   
80.
Detailed pH-dependent steady state and picosecond time-resolved tryptophan fluorescence studies on thiocyanate and azide complexes of horseradish peroxidase have been carried out. The fluorescence decay of the single tryptophan in these species was fitted to a discrete three exponential model. Maximum entropy method analysis also gave three distinct regions of lifetime distributions. The fast subnanosecond lifetime component was found to have > 97% amplitude contribution while other two longer lifetime components have small contributions. Small contributions from the nanosecond lifetime components possibly arise from apoprotein impurity or some small amount of disordered heme conformer of the protein. pH dependence of the fast picosecond lifetime components was found to show a systematic behavior which has been interpreted in the light of obligatory conformation change associated with activation of the enzyme at low pH.  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