全文获取类型
收费全文 | 63063篇 |
免费 | 4676篇 |
国内免费 | 3915篇 |
专业分类
化学 | 25974篇 |
晶体学 | 407篇 |
力学 | 4725篇 |
综合类 | 576篇 |
数学 | 16628篇 |
物理学 | 23344篇 |
出版年
2024年 | 103篇 |
2023年 | 595篇 |
2022年 | 1389篇 |
2021年 | 1450篇 |
2020年 | 1530篇 |
2019年 | 1528篇 |
2018年 | 1266篇 |
2017年 | 1470篇 |
2016年 | 1850篇 |
2015年 | 1465篇 |
2014年 | 2289篇 |
2013年 | 4126篇 |
2012年 | 2597篇 |
2011年 | 2882篇 |
2010年 | 2336篇 |
2009年 | 3558篇 |
2008年 | 3896篇 |
2007年 | 4325篇 |
2006年 | 3630篇 |
2005年 | 2852篇 |
2004年 | 2426篇 |
2003年 | 2690篇 |
2002年 | 3033篇 |
2001年 | 2263篇 |
2000年 | 2197篇 |
1999年 | 1855篇 |
1998年 | 1819篇 |
1997年 | 1135篇 |
1996年 | 1070篇 |
1995年 | 894篇 |
1994年 | 938篇 |
1993年 | 724篇 |
1992年 | 772篇 |
1991年 | 518篇 |
1990年 | 492篇 |
1989年 | 396篇 |
1988年 | 380篇 |
1987年 | 346篇 |
1986年 | 310篇 |
1985年 | 331篇 |
1984年 | 305篇 |
1983年 | 163篇 |
1982年 | 240篇 |
1981年 | 225篇 |
1980年 | 166篇 |
1979年 | 193篇 |
1978年 | 155篇 |
1977年 | 153篇 |
1976年 | 113篇 |
1973年 | 66篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
The current state of using expert system (ES) techniques for statistics is expounded through the difficulties of defining the notion of statistical expert system (SES) and through several well-known examples. The main characteristics of statistical expertise, and particularly the important concept of strategy are emphasized. The current state of SESs is seen in the context of the global trend of the evolution of ES, i.e. within the growth of second generation expert system (SGESs). The knowledge base of these ESs represent an attempt to represent not only experts' knowledge but also the way experts acquire their expertise, thanks to the existence of an underlying model. We show through many examples how far SESs are and/or will be able to be SGESs. 相似文献
162.
Pál Sohár Gábor Bernáth Géza Stájer Angela E. Szabo 《Magnetic resonance in chemistry : MRC》1989,27(9):872-876
Pentacyclic isoxazolines were obtained by the cycloaddition of benzonitrile oxide to norbornene-azetidinone-fused 3,1-oxazines. The constitutions of two of the isomers obtained, and the configurations and conformations of all products, were determined by means of 1H and 13C NMR spectroscopy and DNOE experiments. 相似文献
163.
在非线性项满足全局Lipschitz条件下,本文研究了一类It型非线性时滞关联随机大系统的分散鲁棒控制问题.系统的时滞是关于状态和控制输入的.基于Lyapunov泛函及线性矩阵不等式(LMI)的分析方法,得到了无记忆状态反馈控制器使整个时滞关联随机大系统可镇定的充分条件. 相似文献
164.
Characterizations of the containment of a convex set either in an arbitrary convex set or in the complement of a finite union
of convex sets (i.e., the set, described by reverse-convex inequalities) are given. These characterizations provide ways of
verifying the containments either by comparing their corresponding dual cones or by checking the consistency of suitable associated
systems. The convex sets considered in this paper are the solution sets of an arbitrary number of convex inequalities, which
can be either weak or strict inequalities. Particular cases of dual characterizations of set containments have played key
roles in solving large scale knowledge-based data classification problems where they are used to describe the containments
as inequality constraints in optimization problems. The idea of evenly convex set (intersection of open half spaces), which
was introduced by W. Fenchel in 1952, is used to derive the dual conditions, characterizing the set containments. 相似文献
165.
Daniel Pa?ca 《Journal of Mathematical Analysis and Applications》2007,325(1):90-100
Some existence results are obtained for periodic solutions of nonautonomous second-order differential inclusions systems with p-Laplacian. 相似文献
166.
Joseph Y.-T. Leung 《Discrete Applied Mathematics》2007,155(8):945-970
We consider the problem of scheduling orders for multiple different product types in an environment with m dedicated machines in parallel. The objective is to minimize the total weighted completion time. Each product type is produced by one and only one of the m dedicated machines; that is, each machine is dedicated to a specific product type. Each order has a weight and may also have a release date. Each order asks for certain amounts of various different product types. The different products for an order can be produced concurrently. Preemptions are not allowed. Even when all orders are available at time 0, the problem has been shown to be strongly NP-hard for any fixed number (?2) of machines. This paper focuses on the design and analysis of efficient heuristics for the case without release dates. Occasionally, however, we extend our results to the case with release dates. The heuristics considered include some that have already been proposed in the literature as well as several new ones. They include various static and dynamic priority rules as well as two more sophisticated LP-based algorithms. We analyze the performance bounds of the priority rules and of the algorithms and present also an in-depth comparative analysis of the various rules and algorithms. The conclusions from this empirical analysis provide insights into the trade-offs with regard to solution quality, speed, and memory space. 相似文献
167.
168.
Electronic properties of model quantum-dot structures in zero and finite magnetic fields 总被引:1,自引:0,他引:1
H. Saarikoski E. Räsänen S. Siljamäki A. Harju M.J. Puska R.M. Nieminen 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(2):241-252
We have computed electronic structures and total energies of circularly confined two-dimensional quantum dots and their lateral
dimers in zero and finite uniform external magnetic fields using different theoretical schemes: the spin-density-functional
theory (SDFT), the current-and-spin-density-functional theory (CSDFT), and the variational quantum Monte Carlo (VMC) method.
The SDFT and CSDFT calculations employ a recently-developed, symmetry-unrestricted real-space algorithm allowing solutions
which break the spin symmetry. Results obtained for a six-electron dot in the weak confinement limit and in zero magnetic
field as well as in a moderate confinement and in finite magnetic fields enable us to draw conclusions about the reliability
of the more approximative SDFT and CSDFT schemes in comparison with the VMC method. The same is true for results obtained
for the two-electron quantum dot dimer as a function of inter-dot distance. The structure and role of the symmetry-breaking
solutions appearing in the SDFT and CSDFT calculations for the above systems are discussed.
Received 16 October 2001 and Received in final form 17 January 2002 相似文献
169.
Ben-Yuan Gu Yao Lu Tzong-Jer Yang 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(4):493-502
We present numerical investigations of the transmission properties of electrons in a normal quantum wire tangentially attached
to a superconductor ring threaded by magnetic flux. A point scatterer with a δ -function potential is placed at node to model
scattering effect. We find that the transmission characteristics of electrons in this structure strongly depend on the normal
or superconducting state of the ring. The transmission probability as a function of the energy of incident electrons, in the
case of a superconductor ring threaded by one quantum magnetic flux, emerges one deep dip, imposed upon the first broad bump
in spectrum. This intrinsic conductance dip originates from the superconductor state of the ring. When increasing the magnetic
flux from one quantum magnetic flux to two, the spectrum shifts toward higher energy region in the whole. This conductance
dip accordingly shifts and appears in the second bump. In the presence of a point-scatterer at the node, the spectrum is substantially
modified. Based on the condition of the formation of the standing wave functions in the ring and the broken of the time-reserve
symmetry of Schr?dinger equation after switching magnetic flux, the characteristics of transmission of electrons in this structure
can be well understood.
Received 6 November 2001 相似文献
170.