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   14766篇
  免费   385篇
  国内免费   134篇
化学   966篇
晶体学   38篇
力学   147篇
综合类   7篇
数学   12991篇
物理学   1136篇
  2023年   25篇
  2022年   44篇
  2021年   47篇
  2020年   79篇
  2019年   377篇
  2018年   439篇
  2017年   228篇
  2016年   145篇
  2015年   174篇
  2014年   436篇
  2013年   858篇
  2012年   515篇
  2011年   916篇
  2010年   802篇
  2009年   1080篇
  2008年   1125篇
  2007年   1146篇
  2006年   829篇
  2005年   570篇
  2004年   472篇
  2003年   412篇
  2002年   325篇
  2001年   302篇
  2000年   325篇
  1999年   388篇
  1998年   330篇
  1997年   240篇
  1996年   312篇
  1995年   308篇
  1994年   326篇
  1993年   262篇
  1992年   207篇
  1991年   112篇
  1990年   119篇
  1989年   112篇
  1988年   84篇
  1987年   80篇
  1986年   67篇
  1985年   78篇
  1984年   95篇
  1983年   49篇
  1982年   91篇
  1981年   68篇
  1980年   59篇
  1979年   45篇
  1978年   56篇
  1977年   49篇
  1976年   32篇
  1975年   18篇
  1973年   9篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
2.
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   
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.
5.
The composition of fluorescent polymer nanoparticles, commonly referred to as carbon dots, synthesized by microwave-assisted reaction of citric acid and ethylenediamine was investigated by 13C, 13C{1H}, 1H─13C, 13C{14N}, and 15N solid-state nuclear magnetic resonance (NMR) experiments. 13C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases. 15N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved 13C NMR peaks, including an unusual ═CH signal at 84 ppm (1H chemical shift of 5.8 ppm) and ═CN2 at 155 ppm, and two distinctive 15N NMR resonances near 80 and 160 ppm proved the presence of 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-a]pyridine-7-carboxylic acid (IPCA) or its derivatives. This molecular fluorophore with conjugated double bonds, formed by a double cyclization reaction of citric acid and ethylenediamine as first shown by Y. Song, B. Yang, and coworkers in 2015, accounts for the fluorescence of the carbon dots. Cross-peaks in a 1H─13C HETCOR spectrum with brief 1H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative 13C and 15N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in 13C chemical-shift perturbations and peak widths, with the broadest lines near the COO group of IPCA, indicated at least partial transformation of the carboxylic acid of IPCA by amide or ester formation.  相似文献   
6.
A manifold that contains small perturbations will induce a perturbed partial differential equation. The partial differential equation that we select is the Poisson equation – in order to explore the interplay between the geometry of the manifold and the perturbations. Specifically, we show how the problem of symmetry determination, for higher-order perturbations, can be elegantly expressed via geometric conditions.  相似文献   
7.
Abstract

In the present study, the reactions of 2,3-dichloro-1,4-naphthoquinone (DCNQ) with amino-1,2-propanediol and some thiols were investigated. Novel N-, N,S-, and S,O- substituted derivatives were obtained and the structures of all compounds were characterized by spectroscopic methods (FT-IR, 1H NMR, 13 Baker, R. A.; Tatum, J. H.; Nemec, S. Antimicrobial Activity of Naphthoquinones from Fusaria. Mycopathologia 1990, 111, 915. DOI: 10.1007/BF02277294.[Crossref], [PubMed], [Web of Science ®] [Google Scholar]C NMR, Mass spectroscopy) and microanalysis. The absorption behaviors of novel compounds were also investigated with UV-Vis spectroscopy in different solvents, such as ethanol, tetrahydrofuran and chloroform.  相似文献   
8.
9.
10.
Summary. The analytic treatment of problems related to the asymptotic behaviour of random dynamical systems generated by stochastic differential equations suffers from the presence of non-adapted random invariant measures. Semimartingale theory becomes accessible if the underlying Wiener filtration is enlarged by the information carried by the orthogonal projectors on the Oseledets spaces of the (linearized) system. We study the corresponding problem of preservation of the semimartingale property and the validity of a priori inequalities between the norms of stochastic integrals in the enlarged filtration and norms of their quadratic variations in case the random element F enlarging the filtration is real valued and possesses an absolutely continuous law. Applying the tools of Malliavin’s calculus, we give smoothness conditions on F under which the semimartingale property is preserved and a priori martingale inequalities are valid. Received: 12 April 1995 / In revised form: 7 March 1996  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