全文获取类型
收费全文 | 392785篇 |
免费 | 5262篇 |
国内免费 | 1175篇 |
专业分类
电工技术 | 6980篇 |
综合类 | 2900篇 |
化学工业 | 58027篇 |
金属工艺 | 15045篇 |
机械仪表 | 11879篇 |
建筑科学 | 10073篇 |
矿业工程 | 1519篇 |
能源动力 | 9761篇 |
轻工业 | 34731篇 |
水利工程 | 3872篇 |
石油天然气 | 4435篇 |
武器工业 | 8篇 |
无线电 | 48822篇 |
一般工业技术 | 74451篇 |
冶金工业 | 59294篇 |
原子能技术 | 5851篇 |
自动化技术 | 51574篇 |
出版年
2021年 | 2327篇 |
2019年 | 2179篇 |
2018年 | 17649篇 |
2017年 | 16452篇 |
2016年 | 13280篇 |
2015年 | 3176篇 |
2014年 | 4703篇 |
2013年 | 14559篇 |
2012年 | 10460篇 |
2011年 | 19516篇 |
2010年 | 16045篇 |
2009年 | 14606篇 |
2008年 | 16265篇 |
2007年 | 17082篇 |
2006年 | 8675篇 |
2005年 | 9091篇 |
2004年 | 8855篇 |
2003年 | 8657篇 |
2002年 | 7871篇 |
2001年 | 7671篇 |
2000年 | 7413篇 |
1999年 | 7625篇 |
1998年 | 17718篇 |
1997年 | 12895篇 |
1996年 | 10156篇 |
1995年 | 7898篇 |
1994年 | 7228篇 |
1993年 | 6947篇 |
1992年 | 5325篇 |
1991年 | 5121篇 |
1990年 | 4984篇 |
1989年 | 4824篇 |
1988年 | 4719篇 |
1987年 | 3930篇 |
1986年 | 4046篇 |
1985年 | 4765篇 |
1984年 | 4307篇 |
1983年 | 4055篇 |
1982年 | 3638篇 |
1981年 | 3807篇 |
1980年 | 3500篇 |
1979年 | 3468篇 |
1978年 | 3269篇 |
1977年 | 3864篇 |
1976年 | 4892篇 |
1975年 | 2823篇 |
1974年 | 2693篇 |
1973年 | 2708篇 |
1972年 | 2252篇 |
1971年 | 2006篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Artem Sokolov Darrell Whitley Andre’ da Motta Salles Barreto 《Genetic Programming and Evolvable Machines》2007,8(3):221-237
This paper evaluates different forms of rank-based selection that are used with genetic algorithms and genetic programming.
Many types of rank based selection have exactly the same expected value in terms of the sampling rate allocated to each member
of the population. However, the variance associated with that sampling rate can vary depending on how selection is implemented.
We examine two forms of tournament selection and compare these to linear rank-based selection using an explicit formula. Because
selective pressure has a direct impact on population diversity, we also examine the interaction between selective pressure
and different mutation strategies. 相似文献
972.
973.
The multi-homogeneous Bezout number is a bound for the number of solutions of a system of multi-homogeneous polynomial equations,
in a suitable product of projective spaces. Given an arbitrary, not necessarily multi-homogeneous, system, one can ask for
the optimal multi-homogenization that would minimize the Bezout number. In this paper it is proved that the problem of computing,
or even estimating, the optimal multi-homogeneous Bezout number is actually NP-hard. In terms of approximation theory for
combinatorial optimization, the problem of computing the best multi-homogeneous structure does not belong to APX, unless P
= NP. Moreover, polynomial-time algorithms for estimating the minimal multi-homogeneous Bezout number up to a fixed factor
cannot exist even in a randomized setting, unless BPP ⫆ NP. 相似文献
974.
Georgios D. Styliaras Georgios K. Tsolis Chris M. Papaterpos 《International Journal on Digital Libraries》2007,8(1):61-78
In this paper, AssetCollector is presented, which is a system for managing collections of cultural assets. AssetCollector
covers the needs of collection curators towards defining, populating and searching a collection in a flexible way, while supporting
them in generating reports based on the collection’s assets and reusing them in order to build web sites and CD-ROMs. In order
to support the above functionality, the system provides the content structuring subsystem, the content input subsystem, the
search subsystem and the report subsystem. The use of the subsystems is straightforward and requires no technical skills from
the curators. AssetCollector has been successfully applied for organizing various collections of cultural assets in Greece,
such as archaeological sites, museums and published books. In the future, an evaluation procedure is planned in order to further
refine the use of the system according to the targeted users’ needs. Furthermore, more import and export facilities will be
provided, which will make the system compliant with widely accepted standards. 相似文献
975.
976.
Load bearing capacity of cracked specimens can be improved following pre-loading procedures like warm pre-stressing (WPS). In this paper, the effects of modes I and II pre-loading on fracture load of a central angled crack specimen are studied by finite element analysis. The specimen is reloaded either in mode I or in mode II. To predict the fracture load of cracked specimen after pre-loading, the maximum tensile stress (MTS) and maximum shear stress (MSS) criteria are used. It is shown that mode II fracture load is independent of pre-loading but the mode I fracture load is highly improved after warm pre-stressing. The mode mixity of the specimen is also changed when the pre-loading and reloading modes are not identical. An increase in the fracture load of the specimen following pre-loading may be due to change of failure mode as predicted by finite element analysis in this work. 相似文献
977.
Jules Sadefo Kamdem 《Computing and Visualization in Science》2007,10(4):197-210
In this paper, we generalize the Linear VaR method from portfolios with normally distributed risk factors to portfolios with
mixture of elliptically distributed ones. We treat both the Expected Shortfall and the Value-at-Risk of such portfolios. Special
attention is given to the particular case of a mixture of multivariate t-distributions.
This is a part of J. SADEFO-KAMDEM PhD Thesis[12] of the Université de Reims, France . It has been presented at the workshop
on modelling and computation in Financial Engineering at Bad Herrenalb, Germany May 6-8, 2003. The author is an associate
professor at the Department of mathematics, université d’Evry Val d’Essonne. 相似文献
978.
C. Janke R. Jones S. Öberg P. R. Briddon 《Journal of Materials Science: Materials in Electronics》2007,18(7):775-780
Boron is observed to diffuse very slowly in germanium, as opposed to its behaviour in silicon where it exhibits Transient
Enhanced Diffusion effects in implanted samples. As a result of this slow diffusion, boron is a very stable dopant, allowing
devices to be created with very well-defined doping regions. To understand this superior performance, calculations were performed
on a variety of boron diffusion paths, within vacancy and interstitial mediated methods. It was found that the vacancy mediated
diffusion which is associated with the fast diffusion of many other species in germanium exhibits a total barrier of 5.8 eV.
Interstitial-mediated diffusion had a total barrier for migration of 3.4 eV in the neutral and singly negative charge states,
3.2 eV for the positive charge state, but a formation barrier of 4.1 eV. Thus the barrier for interstitial-mediated diffusion
is dominated by the formation energy of the self-interstitial. 相似文献
979.
Unicode is rapidly becoming the preferred means for representing symbols used in creating multimedia content, especially for information that's presented in multiple languages. This article discusses a unicode vulnerability that makes such content susceptible to being used for creation of covert channel communications. We also developed a solution architecture, the unified secure message augmentation (USMA) service. The USMA service incorporates rules (in an XML vocabulary) that we can apply to unicode transmissions that will detect an attempt to transmit a potential exploit, alert network managers to the presence of the unicode anomaly, and take action to mitigate the exploit. 相似文献
980.
The US Centers for Disease Control and Prevention (CDC) created the Public Health Information Network to advance fully capable, interoperable information systems in public health organizations. PHIN prioritizes public health information systems' functional requirements, capabilities, performance measures, and operational characteristics while letting the architects of those systems choose enabling approaches, methods, and concepts to meet the requirements. PHIN also provides a certification process for public health administrators to evaluate their information infrastructure's quality. The certification process is important because affected organizations often depend on certification for continued funding. 相似文献