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   310551篇
  免费   7273篇
  国内免费   5358篇
化学   152839篇
晶体学   4313篇
力学   17902篇
综合类   325篇
数学   56181篇
物理学   91622篇
  2022年   2111篇
  2021年   2893篇
  2020年   3355篇
  2019年   3603篇
  2018年   14301篇
  2017年   14059篇
  2016年   11546篇
  2015年   4594篇
  2014年   5880篇
  2013年   10898篇
  2012年   13145篇
  2011年   20992篇
  2010年   13548篇
  2009年   13927篇
  2008年   16128篇
  2007年   17753篇
  2006年   9077篇
  2005年   9242篇
  2004年   8673篇
  2003年   8478篇
  2002年   7563篇
  2001年   8103篇
  2000年   6095篇
  1999年   4691篇
  1998年   3726篇
  1997年   3564篇
  1996年   3430篇
  1995年   3005篇
  1994年   3015篇
  1993年   2777篇
  1992年   3207篇
  1991年   3337篇
  1990年   3169篇
  1989年   3054篇
  1988年   2923篇
  1987年   2936篇
  1986年   2719篇
  1985年   3207篇
  1984年   3219篇
  1983年   2656篇
  1982年   2624篇
  1981年   2389篇
  1980年   2369篇
  1979年   2758篇
  1978年   2840篇
  1977年   2759篇
  1976年   2818篇
  1975年   2600篇
  1974年   2600篇
  1973年   2627篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We describe the relationship between the fuzzy sets and the algebraic hyperstructures. In fact, this paper is a continuation of the ideas presented by Davvaz in (Fuzzy Sets Syst., 117: 477- 484, 2001) and Bhakat and Das in (Fuzzy Sets Syst., 80: 359-368, 1996). The concept of the quasicoincidence of a fuzzy interval value with an interval-valued fuzzy set is introduced and this is a natural generalization of the quasi-coincidence of a fuzzy point in fuzzy sets. By using this new idea, the concept of interval-valued (α,β)-fuzzy sub-hypermodules of a hypermodule is defined. This newly defined interval-valued (α,β)-fuzzy sub-hypermodule is a We shall study such fuzzy sub-hypermodules and sub-hypermodules of a hypermodule. generalization of the usual fuzzy sub-hypermodule. consider the implication-based interval-valued fuzzy  相似文献   
992.
In this paper,we present a random graph model with spatial reuse for a mobile ad hoc network(MANET) based on the dynamic source routing protocol.Many important performance parameters of theMANET are obtained,such as the average flooding distance (AFD),the probability generating function of theflooding distance,and the probability of a flooding route to be symmetric.Compared with the random graphmodel without spatial reuse,this model is much more effective because it has a smaller value of AFD and alarger probability for finding a symmetric valid route.  相似文献   
993.
The regression problem in learning theory is investigated with least square Tikhonov regularization schemes in reproducing kernel Hilbert spaces (RKHS). We follow our previous work and apply the sampling operator to the error analysis in both the RKHS norm and the L2 norm. The tool for estimating the sample error is a Bennet inequality for random variables with values in Hilbert spaces. By taking the Hilbert space to be the one consisting of Hilbert-Schmidt operators in the RKHS, we improve the error bounds in the L2 metric, motivated by an idea of Caponnetto and de Vito. The error bounds we derive in the RKHS norm, together with a Tsybakov function we discuss here, yield interesting applications to the error analysis of the (binary) classification problem, since the RKHS metric controls the one for the uniform convergence.  相似文献   
994.
A six level rate equation system was used to investigate the impact of photodepletion to amplified spontaneous emission (ASE) of intramolecular proton-transfer (IPT) dyes incorporated into polymeric hosts. The model includes the most important transitions for the normal and tautomer form of the molecule as well as intersystem crossing and triplet–triplet transitions. The experimentally observed pulse shape as well as photodepletion phenomena, i.e. a first order exponential decay of the ASE intensity and a shortening in ASE pulse width, have been simulated successfully. Additionally, the model was used to propose an explanation of the unexpected high photodepletion of proton-transfer dyes in solids. The results show that the emission cross section and reabsorption cross section of the tautomer form of the molecule are the most important parameter not only for efficiency but also for photodepletion. The model was tested by comparison with experimental results of 2-(2’-hydroxyphenyl)benzimidazole in polymethylmethacrylate (PMMA), polystyrene (PS) and a 1:1 alternating copolymer matrix PS-co-PMMA. PACS 78.45.th; 42.55.Mv; 42.70.Jk  相似文献   
995.
This paper considers efforts to improve in-patient flows, a particularly urgent issue in the National Health Service (NHS). The context is described and related to reasons why OR has been making relatively little contribution. The paper argues that large complex models may often be unnecessary and even get in the way of providing clear insight and guidance for problem owners. The importance of understanding the generic working of systems to lead to improvement, and the limitations of simply describing them, is stressed. It is demonstrated that some very simple models can be of significant practical value in understanding and managing complex systems, changing mindsets and driving collection and use of operationally valuable data. Recommendations for more effective engagement with the NHS are offered.  相似文献   
996.
997.
998.
Ohne Zusammenfassung Vortrag, gehalten in Wien am 24. April 1928 über Einladung des Komitees zur Veranstaltung von Gastvortr?gen ausl?ndischer Gelehrter der exakten Wissenschaften.  相似文献   
999.
We construct a new family of cyclic difference sets with parameters ((3 d – 1)/2, (3 d – 1 – 1)/2, (3 d – 2 – 1)/2) for each odd d. The difference sets are constructed with certain maps that form Jacobi sums. These new difference sets are similar to Maschietti's hyperoval difference sets, of the Segre type, in characteristic two. We conclude by calculating the 3-ranks of the new difference sets.  相似文献   
1000.
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