全文获取类型
收费全文 | 10836篇 |
免费 | 1388篇 |
国内免费 | 2190篇 |
专业分类
化学 | 6214篇 |
晶体学 | 144篇 |
力学 | 542篇 |
综合类 | 276篇 |
数学 | 3656篇 |
物理学 | 3582篇 |
出版年
2024年 | 12篇 |
2023年 | 167篇 |
2022年 | 279篇 |
2021年 | 415篇 |
2020年 | 323篇 |
2019年 | 283篇 |
2018年 | 226篇 |
2017年 | 370篇 |
2016年 | 437篇 |
2015年 | 363篇 |
2014年 | 626篇 |
2013年 | 850篇 |
2012年 | 609篇 |
2011年 | 619篇 |
2010年 | 558篇 |
2009年 | 648篇 |
2008年 | 667篇 |
2007年 | 734篇 |
2006年 | 572篇 |
2005年 | 486篇 |
2004年 | 447篇 |
2003年 | 462篇 |
2002年 | 484篇 |
2001年 | 426篇 |
2000年 | 340篇 |
1999年 | 293篇 |
1998年 | 280篇 |
1997年 | 225篇 |
1996年 | 205篇 |
1995年 | 135篇 |
1994年 | 168篇 |
1993年 | 174篇 |
1992年 | 153篇 |
1991年 | 121篇 |
1990年 | 131篇 |
1989年 | 147篇 |
1988年 | 134篇 |
1987年 | 155篇 |
1986年 | 153篇 |
1985年 | 141篇 |
1984年 | 133篇 |
1983年 | 83篇 |
1982年 | 32篇 |
1981年 | 28篇 |
1980年 | 16篇 |
1979年 | 20篇 |
1978年 | 14篇 |
1977年 | 17篇 |
1975年 | 10篇 |
1974年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
A transient molecular network model is built to describe the nonlinear viscoelasticity of polymers by considering the effect
of entanglement loss and regeneration on the relaxation of molecular strands. It is an extension of previous network theories.
The experimental data on three thermoplastic polymers (ABS, PVC and PA6) obtained under various loading conditions are used
to test the model. Agreement between the theoretical and experimental curves shows that the suggested model can describe successfully
the relaxation behavior of the thermoplastic polymers under different loading rates by using relatively few relaxation modes.
Thus the micromechanism responsible for strain-rate dependence of relaxation process and the origin of nonlinear viscoelasticity
may be disclosed.
The project supported by the National Natural Science Foundation of China and Doctorial Fund 相似文献
72.
73.
Ken Ono 《Mathematische Annalen》1998,312(2):251-260
74.
This paper presents an analytic approach to the construction cost of fringe-balanced binary search trees. In [7], Mahmoud used a bottom-up approach and an urn model of Pólya. The present method is top-down and uses differential equations and Hwang's quasi-power theorem to derive the asymptotic normality of the number of rotations needed to construct such afringe balanced search tree. We also obtain the exact expectation and variance with this method. Although Pólya's urn model is no longer needed, we also present an elegant analysis of it based on an operator calculus as in [4].This research was supported by the Austrian Research Society (FWF) under the project number P12599-MAT. 相似文献
75.
Michael Demuth Peter Stollmann Günter Stolz Jan van Casteren 《Integral Equations and Operator Theory》1995,23(2):145-153
We give trace norm estimates for products of integral operators and for diffusion semigroups. These are applied to differences of heat semigroups. A natural example of an integral operator with finite trace which is not trace class is given. 相似文献
76.
首先将无线传感器网络的路由问题转化成求解最小Steiner树问题,然后给出了求解无线传感器网络路由的蚁群优化算法,并对算法的收敛性进行了证明.最后对找到最优解后信息素值的变化进行了分析.即在限制信息素取值的条件下,当迭代次数充分大时,该算法能以任意接近于1的概率找到最优解,并且当最优解找到后,最优树边上的信息素单调增加,而最优解以外边上的信息素在有限步达到最小值. 相似文献
77.
《中国科学A辑(英文版)》2007,(7)
The double loop network(DLN)is a circulant digraph with n nodes and outdegree 2.It is an important topological structure of computer interconnection networks and has been widely used in the designing of local area networks and distributed systems.Given the number n of nodes,how to construct a DLN which has minimum diameter?This problem has attracted great attention.A related and longtime unsolved problem is:for any given non-negative integer k,is there an infinite family of k-tight optimal DLN?In this paper,two main results are obtained:(1)for any k≥0,the infinite families of k-tight optimal DLN can be constructed,where the number n(k,e,c)of their nodes is a polynomial of degree 2 in e with integral coefficients containing a parameter c.(2)for any k≥0, an infinite family of singular k-tight optimal DLN can be constructed. 相似文献
78.
M. H. Sadr S. Astaraki S. Salehi 《Archive of Applied Mechanics (Ingenieur Archiv)》2007,77(11):795-807
In developing a neural network technique for a finite element model updating, researchers have been shown that the number
of training samples and their quality, significantly affect the accuracy of the NN predication. In this study, based on the
genetic algorithm (GA) method, we reduce the number of analyses required to develop the training pairs and reduce the amount
of time for training the NN. In the other words, a uniform distribution of design points inside the design space will be obtained
by means of this approach. To validate the efficiency of GA sample selection, random generation (RG) method is used for comparison.
Two comparisons are made based on a numerical and experimental example. One is updated from 10 degrees of freedom lumped parameters
system and the other is updated from a free–free beam using test data. The results indicate that the GA sample selection can
reduce the number of training samples without affecting the accuracy of the NN predication. In our present study, also, the
advantages of using frequency response function (FRF) data as input to the NN are kept and the drawback of having too many
frequency points is overcome by the application of principal component analyses (PCA). 相似文献
79.
电力市场中合同电量与竞争电量交易比例的研究 总被引:1,自引:0,他引:1
在单边开放的区域电力市场中,合理的合同电量与竞争电量交易比例是保证电力市场有效运行的一个重要环节。竞争电量所占的比例将主要取决于当前发电公司的市场行为。首先使用BP神经网络对电力需求弹性系数进行了预测,然后以长期电力市场均衡为目标函数,考虑贵州电网发电机组的可用容量与负荷预测的误差,以及贵州输电线路的可靠性诸因素,推导出合同电量与竞争电量交易比例,经过与南方区域电力市场目前运营规则规定的交易比例比较,该比例是合理的,可以规避电力市场价格波动等带来的风险。 相似文献
80.
By using the tools of statistical physics and recent investigations of the scaling properties of different complex networks,
the structural and evolving properties of the Chinese railway network (CRN) is studied. It has been verified that the CRN
has the same small-world properties of the Indian railway network (IRN). According to the class of small-world networks, we
believe the CRN is a single scale. In addition, a novel result is obtained. Measurements on the CRN indicate that the rate
at which nodes acquire links depends on the node’s degree and follows a power law.
相似文献