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   140758篇
  免费   10850篇
  国内免费   11023篇
化学   47637篇
晶体学   1959篇
力学   9608篇
综合类   1477篇
数学   42322篇
物理学   59628篇
  2023年   710篇
  2022年   974篇
  2021年   1338篇
  2020年   1954篇
  2019年   2393篇
  2018年   2104篇
  2017年   2003篇
  2016年   1956篇
  2015年   1733篇
  2014年   3189篇
  2013年   5885篇
  2012年   3639篇
  2011年   4636篇
  2010年   4831篇
  2009年   10317篇
  2008年   11818篇
  2007年   10400篇
  2006年   9638篇
  2005年   7291篇
  2004年   6813篇
  2003年   6956篇
  2002年   7111篇
  2001年   5707篇
  2000年   5475篇
  1999年   4993篇
  1998年   4339篇
  1997年   3447篇
  1996年   3051篇
  1995年   3430篇
  1994年   3274篇
  1993年   2610篇
  1992年   2098篇
  1991年   1581篇
  1990年   1270篇
  1989年   1137篇
  1988年   1024篇
  1987年   786篇
  1986年   530篇
  1985年   1295篇
  1984年   994篇
  1983年   655篇
  1982年   921篇
  1981年   1064篇
  1980年   909篇
  1979年   787篇
  1978年   726篇
  1977年   643篇
  1976年   630篇
  1974年   416篇
  1973年   528篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
We consider the inverse problem of identifying a general source term, which is a function of both time variable and the spatial variable, in a parabolic PDE from the knowledge of boundary measurements of the solution on some portion of the lateral boundary. We transform this inverse problem into a problem of solving a compact linear operator equation. For the regularization of the operator equation with noisy data, we employ the standard Tikhonov regularization, and its finite dimensional realization is done using a discretization procedure involving the space $L^2(0,\tau;L^2(Ω))$. For illustrating the specification of an a priori source condition, we have explicitly obtained the range space of the adjoint of the operator involved in the operator equation.  相似文献   
2.
This paper proposes a method for solving optimisation problems involving piecewise quadratic functions. The method provides a solution in a finite number of iterations, and the computational complexity of the proposed method is locally polynomial of the problem dimension, i.e., if the initial point belongs to the sufficiently small neighbourhood of the solution set. Proposed method could be applied for solving large systems of linear inequalities.  相似文献   
3.
In the assessment of most complex socioeconomic phenomena with the use of multicriteria methods, continuous data are used, the source of which are most often public statistics. However, there are complex phenomena such as quality of life and quality of services in the assessment, for which questionnaire surveys and ordinal measurement scales are used. In this case, the use of classic multicriteria methods is very difficult, taking into account the way of presenting this type of data by official statistics, as well as their permissible transformations and arithmetic operations. Therefore, the main purpose of this study was the presentation of a novel framework which can be applied for assessing socioeconomic phenomena on the basis of survey data. It was assumed that the object assessments may contain positive or negative opinions and an element of uncertainty expressed in the form a “no”, “difficult to say”, or “no opinion” answers. For this reason, the intuitionistic fuzzy TOPSIS (IF-TOPSIS) method is proposed. To demonstrate the potential of this solution, the results of measuring the subjective quality of life of the inhabitants of 83 cities in EU countries, EFTA countries, the UK, the Western Balkans, and Turkey are presented. For most cities, a high level of subjective quality of life was observed using the proposed approach. The highest level of quality of life was observed in Zurich, whereas the lowest was observed in Palermo.  相似文献   
4.
In this paper, we consider a scenario where the base station (BS) collects time-sensitive data from multiple sensors through time-varying and error-prone channels. We characterize the data freshness at the terminal end through a class of monotone increasing functions related to Age of information (AoI). Our goal is to design an optimal policy to minimize the average age penalty of all sensors in infinite horizon under bandwidth and power constraint. By formulating the scheduling problem into a constrained Markov decision process (CMDP), we reveal the threshold structure for the optimal policy and approximate the optimal decision by solving a truncated linear programming (LP). Finally, a bandwidth-truncated policy is proposed to satisfy both power and bandwidth constraint. Through theoretical analysis and numerical simulations, we prove the proposed policy is asymptotic optimal in the large sensor regime.  相似文献   
5.
Compared with the traditional magnetic bubble, a skyrmion has a smaller size, and better stability and therefore is considered as a very promising candidate for future memory devices. When skyrmions are manipulated, erased and created, the density of skyrmions can be varied, however the relationship between the radii and the densities of skyrmions needs more exploration. In this paper, we study this problem both theoretically and by using the lattice simulation. The average radius of skyrmions as a function of material parameters, the strength of the external magnetic field and the density of skyrmions is obtained and verified. With this explicit function, the skyrmion radius can be easily predicted, which is helpful for the future study of skyrmion memory devices.  相似文献   
6.
遵循“学生主体、教师主导”的教育教学理念,借助信息化教学工具,合理有效地设计好每一个教学环节.采用演示、体验、探究、问题导向和分组讨论等教学手段和方法,让学生在课堂中自主发现问题、分析问题并解决问题,实现“学生做中学、教师做中教”,大大提升教学效果.  相似文献   
7.
8.
给出了求解多自由度动力学系统响应的M atlab程序,这些程序基于振型叠加法可用于求解由质量矩阵M和刚度矩阵K以及常见阻尼矩阵描述的线性离散系统的时域和频域解.对于无阻尼系统,用户可以选择数值解或符号解析解(以时间或频率表示),并利用复模态叠加法计算了阻尼系统的数值解.总结了模态叠加方法下动力学响应的求解,并在简短的M atlab程序中实现.以三自由度系统和悬臂梁模型为例说明了程序的应用.这些程序也可用于工程应用中,通过对商用有限元软件包产生的质量和刚度进行后处理,产生感兴趣的时域和频域响应.  相似文献   
9.
设G=(V,E)是一个连通图.称一个边集合S■E是一个k限制边割,如果G-S的每个连通分支至少有k个顶点.称G的所有k限制边割中所含边数最少的边割的基数为G的k限制边连通度,记为λ_k(G).定义ξ_k(G)=min{[X,■]:|X|=k,G[X]连通,■=V(G)\X}.称图G是极大k限制边连通的,如果λ_k(G)=ξ_k(G).本文给出了围长为g>6的极大3限制边连通二部图的充分条件.  相似文献   
10.
We compute the parameters of the linear codes that are associated with all projective embeddings of Grassmann varieties.  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