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   6868篇
  免费   1143篇
  国内免费   416篇
化学   2034篇
晶体学   35篇
力学   509篇
综合类   235篇
数学   2385篇
物理学   3229篇
  2024年   29篇
  2023年   122篇
  2022年   477篇
  2021年   369篇
  2020年   229篇
  2019年   188篇
  2018年   146篇
  2017年   274篇
  2016年   330篇
  2015年   257篇
  2014年   456篇
  2013年   551篇
  2012年   443篇
  2011年   405篇
  2010年   382篇
  2009年   435篇
  2008年   402篇
  2007年   428篇
  2006年   343篇
  2005年   256篇
  2004年   218篇
  2003年   221篇
  2002年   214篇
  2001年   166篇
  2000年   175篇
  1999年   147篇
  1998年   121篇
  1997年   104篇
  1996年   106篇
  1995年   62篇
  1994年   76篇
  1993年   65篇
  1992年   34篇
  1991年   30篇
  1990年   30篇
  1989年   18篇
  1988年   17篇
  1987年   17篇
  1986年   20篇
  1985年   16篇
  1984年   13篇
  1982年   8篇
  1981年   7篇
  1979年   4篇
  1977年   5篇
  1976年   1篇
  1975年   1篇
  1972年   1篇
  1967年   1篇
  1959年   4篇
排序方式: 共有8427条查询结果,搜索用时 15 毫秒
81.
In part I of this work (the present article) the equilibrium state of temporary polymer networks is treated in the framework of thermodynamics and statistical mechanics. The network is described as an open system. Thereby we use a modified spring-bead model in which the beads represent junctions that decay and reform thus adding a viscous component to the assumed elastic behaviour of the permanent network. The relevant statistical equation — analogous to Liouville's equation — is solved. The grand-canonical probability density function and two of three equations of state are derived. Explicit formulae are given for several relevant probabilities. For instance the probabilityw (z)dz that a network chain connecting two junctions has a contour length betweenz andz +dz is given by the Wien type formulaw(z) =A z 3 exp {–B z} whereA andB do not depend onz.  相似文献   
82.
It is known that rank-two bimolecular mass-action systems do not admit limit cycles. With a view to understanding which small mass-action systems admit oscillation, in this paper we study rank-two networks with bimolecular source complexes but allow target complexes with higher molecularities. As our goal is to find oscillatory networks of minimal size, we focus on networks with three reactions, the minimum number that is required for oscillation. However, some of our intermediate results are valid in greater generality. One key finding is that an isolated periodic orbit cannot occur in a three-reaction, trimolecular, mass-action system with bimolecular sources. In fact, we characterize all networks in this class that admit a periodic orbit; in every case, all nearby orbits are periodic too. Apart from the well-known Lotka and Ivanova reactions, we identify another network in this class that admits a center. This new network exhibits a vertical Andronov–Hopf bifurcation. Furthermore, we characterize all two-species, three-reaction, bimolecular-sourced networks that admit an Andronov–Hopf bifurcation with mass-action kinetics. These include two families of networks that admit a supercritical Andronov–Hopf bifurcation and hence a stable limit cycle. These networks necessarily have a target complex with a molecularity of at least four, and it turns out that there are exactly four such networks that are tetramolecular.  相似文献   
83.
G-actin prepared from pork liver and purified to give a product of 95–98% purity was polymerized to F-actin in a rotational dynamic viscometer. The rheological properties were investigated during the course of polymerisation and after polymerisation. G andG measured at frequencies around 0.1 Hz increased strongly during the first 3 hours of the reaction and then slowly approached a constant value in the range of 10–50 mPa. The rise during polymerisation was more than two orders of magnitude. When a solution of F-actin was subjected to steady shear at a high shear rate for a short period of time and subsequently dynamic measurements were performed, then a considerably smallerG (about 70% smaller) was observed immediately after cessation of the steady shear.G then increased with time and approached its initial value.When measuring the viscosity of F-actin solutions as a function of shear rate, a strong shear thinning effect was seen which did not vanish even at shear rates as low as 0.001 s–1. Even at this low shear rates, a pronounced yield maximum was observed before reaching the steady state. Oscillatory experiments showed a remarkably weak frequency dependence ofG. The results imply that F-actin solutions are largely structured forming a weak temporary network which can be easily destroyed by application of high shear rates. It seems most likely that the destruction occurs by a reversible, shear induced depolymerisation process.  相似文献   
84.
提出并建立了一个基于马尔可夫调制的泊松过程MMPP(M arkovModu lated Poisson Process)和间歇泊松过程IPP(Interrupted Poisson Process)的系统模型,从新用户的到达和系统中用户发起的新业务两个方面着手,考察了在受系统门限值约束情况下的接入系统的新用户数量和系统内用户发  相似文献   
85.
港口柔性评价   总被引:1,自引:0,他引:1       下载免费PDF全文
港口柔性体现了港口对社会经济环境变化的反应能力和适应能力,科学的港口柔性评价是提高其柔性水平的基础.根据柔性的多维度特征和港口生产服务实际,从战略柔性、结构柔性、管理柔性、生产柔性和可持续柔性五个方面建立了港口柔性评价指标体系;选用模糊ANP方法建立评价模型,即根据各柔性指标之间的依存和反馈关系,用网络分析法(ANP)确定模糊指标权重.模糊ANP方法能够较准确地给出港口柔性的定量评价,为港口采取正确的发展策略、提高柔性水平提供参考.  相似文献   
86.
董继扬  张军英  陈忠 《物理学报》2007,56(9):5013-5020
将竞争机理引入网络的自动波产生与传播过程中,提出自动波竞争神经网络(ACNN)模型,并成功地应用于求解单源最短路问题,给出了基于ACNN的最短路求解算法. 与其他神经网络最短路算法相比,基于ACNN的最短路算法具有网络所需的神经元数目少、神经元及网络的结构简单、易于软硬件的实现、以及全并行方式计算等特点,可用于求解非对称赋权图的最短路径树问题,且其计算时间(迭代次数)仅正比于最短路径上的跃点数,与赋权图的复杂度、路径总长、边长的精度、通路总数等因素无关. 计算机仿真结果表明该算法的有效性和快速求解能力. 关键词: 单源最短路问题 自动波竞争神经网络 脉冲耦合神经网络  相似文献   
87.
本文介绍了多层神经网络的基本结构和主要概念,并对训练多层神经网络的Back-Propagation学习算法(即后向传递误差算法,简称后向算法)的原理和实施步骤作了详尽的分析和推导。在多层神经网络中运用这一算法,提出了平面波方位角估测的新方法。计算机模拟结果显示,这一方法是可行  相似文献   
88.
基于神经网络的三维宽场显微图像复原研究   总被引:8,自引:0,他引:8  
陈华  金伟其  张楠  石俊生  王霞 《光子学报》2006,35(3):473-476
提出一种利用BP神经网络进行三维宽场显微图像复原的非线性映射方法,将三维图像转化为二维图像进行处理,利用神经网络的学习能力,通过训练,建立含有散焦信息的二维模糊图像与二维清晰图像之间的映射关系,然后对切片堆叠进行逐幅复原,从而实现显微图像的三维复原.得到的复原图像在视觉上和定量分析上都获得了很好的效果.由于采用小规模神经网络,训练时间短,计算量小,使实时复原成为可能.  相似文献   
89.
The following article describes an advanced dense wavelength division multiplexing (DWDM) Optical Network developed by L-3 Photonics. The network, configured as an amplified optical bus, carries traffic simultaneously in both directions, using multiple wavelengths. As a result, data distribution is of the form peer-to-multi-peer, it is protocol independent, and it is scalable. The network leverages the rapid growth in commercial optical technologies, including wavelength division multiplexing (WDM), and when applied to military and commercial platforms such as aircraft, ships, unmanned and other vehicles, provides a cost-effective, low-weight, high-speed, and high noise-immune data distribution system.  相似文献   
90.
A simplified Olami-Feder-Christensen model on a random network has been studied. We propose a new toppling rule — when there is an unstable site toppling, the energy of the site is redistributed to its nearest neighbors randomly not averagely. The simulation results indicate that the model displays self-organized criticality when the system is conservative, and the avalanche size probability distribution of the system obeys finite size scaling. When the system is nonconservative, the model does not display scaling behavior. Simulation results of our model with different nearest neighbors q is also compared, which indicates that the spatial topology does not alter the critical behavior of the system.  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