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   6609篇
  免费   288篇
  国内免费   404篇
化学   976篇
晶体学   9篇
力学   135篇
综合类   131篇
数学   5416篇
物理学   634篇
  2024年   5篇
  2023年   22篇
  2022年   68篇
  2021年   84篇
  2020年   96篇
  2019年   210篇
  2018年   233篇
  2017年   110篇
  2016年   107篇
  2015年   124篇
  2014年   307篇
  2013年   491篇
  2012年   351篇
  2011年   580篇
  2010年   484篇
  2009年   546篇
  2008年   533篇
  2007年   555篇
  2006年   440篇
  2005年   307篇
  2004年   293篇
  2003年   272篇
  2002年   176篇
  2001年   128篇
  2000年   101篇
  1999年   119篇
  1998年   105篇
  1997年   93篇
  1996年   52篇
  1995年   54篇
  1994年   42篇
  1993年   30篇
  1992年   24篇
  1991年   22篇
  1990年   22篇
  1989年   20篇
  1988年   13篇
  1987年   8篇
  1986年   10篇
  1985年   7篇
  1984年   11篇
  1983年   5篇
  1982年   5篇
  1981年   8篇
  1980年   4篇
  1979年   7篇
  1977年   3篇
  1974年   4篇
  1957年   2篇
  1936年   2篇
排序方式: 共有7301条查询结果,搜索用时 31 毫秒
1.
In this paper, let (Mn,g,dμ) be n-dimensional noncompact metric measure space which satisfies Poincaré inequality with some Ricci curvature condition. We obtain a Liouville theorem for positive weak solutions to weighted p-Lichnerowicz equation
p,fv+cvσ=0,
where c0,m>n1,1<p<m?1+(m?1)(m+3)2,σp?1 are real constants.  相似文献   
2.
3.
We relate the distribution characters and the wave front sets of unitary representation for real reductive dual pairs of type I in the stable range.  相似文献   
4.
The practical application of Shilov-type Pt catalysis to the selective hydroxylation of terminal aliphatic C−H bonds remains a formidable challenge, due to difficulties in replacing PtIV with a more economically viable oxidant, particularly O2. We report the potential of employing FeCl2 as a suitable redox mediator to overcome the kinetic hurdles related to the direct use of O2 in the Pt reoxidation. For the selective conversion of butyric acid to γ-hydroxybutyric acid (GHB), a significantly enhanced catalyst activity and stability (turnover numbers (TON)>30) were achieved under 20 bar O2 in comparison to current state-of-the-art systems (TON<10). In this regard, essential reaction parameters affecting the overall activity were identified, along with specific additives to attain catalyst stability at longer reaction times. Notably, deactivation by reduction to Pt0 was prevented by the addition of monodentate pyridine derivatives, such as 2-fluoropyridine, but also by introducing varying partial pressures of N2 in the gaseous atmosphere. Finally, stability tests revealed the involvement of PtII and FeCl2 in catalyzing the non-selective overoxidation of GHB. Accordingly, in situ esterification with boric acid proved to be a suitable strategy to maintain enhanced selectivities at much higher conversions (TON>60). Altogether, a useful catalytic system for the selective hydroxylation of primary aliphatic C−H bonds with O2 is presented.  相似文献   
5.
In this paper we give a realization of some symmetric space G/K as a closed submanifold P of G. We also give several equivalent representations of the submanifold P. Some properties of the set gKP are also discussed, where gK is a coset space in G.  相似文献   
6.
For X a metrizable space and (Y,ρ) a metric space, with Y pathwise connected, we compute the density of (C(X,(Y,ρ)),σ)—the space of all continuous functions from X to (Y,ρ), endowed with the supremum metric σ. Also, for (X,d) a metric space and (Y,‖⋅‖) a normed space, we compute the density of (UC((X,d),(Y,ρ)),σ) (the space of all uniformly continuous functions from (X,d) to (Y,ρ), where ρ is the metric induced on Y by ‖⋅‖). We also prove that the latter result extends only partially to the case where (Y,ρ) is an arbitrary pathwise connected metric space.To carry such an investigation out, the notions of generalized compact and generalized totally bounded metric space, introduced by the author and A. Barbati in a former paper, turn out to play a crucial rôle. Moreover, we show that the first-mentioned concept provides a precise characterization of those metrizable spaces which attain their extent.  相似文献   
7.
Given a graph Γ, we construct a simple, convex polytope, dubbed graph-associahedra, whose face poset is based on the connected subgraphs of Γ. This provides a natural generalization of the Stasheff associahedron and the Bott-Taubes cyclohedron. Moreover, we show that for any simplicial Coxeter system, the minimal blow-ups of its associated Coxeter complex has a tiling by graph-associahedra. The geometric and combinatorial properties of the complex as well as of the polyhedra are given. These spaces are natural generalizations of the Deligne-Knudsen-Mumford compactification of the real moduli space of curves.  相似文献   
8.
We introduce the notion of a partially selective ultrafilter and prove that (a) if G is an extremally disconnected topological group and p is a converging nonprincipal ultrafilter on G containing a countable discrete subset, then p is partially selective, and (b) the existence of a nonprincipal partially selective ultrafilter on a countable set implies the existence of a P-point in ω. Thus it is consistent with ZFC that there is no extremally disconnected topological group containing a countable discrete nonclosed subset.  相似文献   
9.
10.
在化工、造纸、制药、钢铁等工业生产中,一台设备或一条生产线可以生产多种产品的情况很常见。在生产中,如何安排各类产品的生产顺序以及生产数量显得十分重要。这类问题通常称作经济批量排产问题,这类问题是生产库存中的经典问题。本文研究的经济批量排产问题考虑了产品货架存放期因素,针对以往研究的不足,本文提出用批量变动方法求解该类问题,由计算结果显示,按照这种排产方法花费的成本要低于其他两种经济批量排产问题常用的方法。  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