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   3167篇
  免费   104篇
  国内免费   27篇
化学   1974篇
晶体学   28篇
力学   85篇
数学   650篇
物理学   561篇
  2023年   14篇
  2022年   45篇
  2021年   85篇
  2020年   76篇
  2019年   88篇
  2018年   80篇
  2017年   69篇
  2016年   124篇
  2015年   112篇
  2014年   141篇
  2013年   237篇
  2012年   206篇
  2011年   218篇
  2010年   152篇
  2009年   150篇
  2008年   187篇
  2007年   181篇
  2006年   146篇
  2005年   123篇
  2004年   99篇
  2003年   73篇
  2002年   79篇
  2001年   35篇
  2000年   40篇
  1999年   30篇
  1998年   20篇
  1997年   22篇
  1996年   35篇
  1995年   27篇
  1994年   24篇
  1993年   21篇
  1992年   28篇
  1991年   13篇
  1990年   21篇
  1989年   14篇
  1988年   18篇
  1987年   12篇
  1986年   12篇
  1985年   15篇
  1984年   25篇
  1983年   15篇
  1982年   16篇
  1981年   15篇
  1980年   14篇
  1979年   11篇
  1978年   24篇
  1977年   22篇
  1976年   12篇
  1970年   7篇
  1968年   10篇
排序方式: 共有3298条查询结果,搜索用时 796 毫秒
241.

We construct a generalization of the Conley index for flows. The new index preserves information which in the classical case is lost in the process of collapsing the exit set to a point. The new index has most of the properties of the classical index. As examples, we study a flow with a knotted orbit in , and the problem of continuing two periodic orbits which are not homotopic as loops.

  相似文献   

242.
On Observables     
We propose a simple set-theoretic model of a generalized probability spaceadmitting intrinsic incompatible events and incompatible observables. It is acoproduct in the category D of D-posets and D-homomorphisms each factor ofwhich is a classical Kolmogorovian probability space. Since classical events,random functions, and probability measures can be treated within D in a canonicalway, the Kolmogorovian model becomes a special case. We show that -additivityand other -notions can be replaced in a natural way by sequential continuity.  相似文献   
243.
Oxidative-reductive properties on the acid-base surfaces of the oxide compositions Sn-Ce-Rh-O and Zr-Mg-Y-O, active as catalysts in the ketonization of secondary alcohols were determined based on the isopropanol conversion selectivity. The kinetics of isopropanol conversion was measured in the oxygen-free atmosphere. Activation energies for both directions of conversion (dehydration to propylene and dehydrogenation to acetone) were calculated. The results were compared with the kinetics over SnO2 and ZrO2. Both oxide compositions, Sn-Ce-Rh-O and Zr-Mg-Y-O are oxidative-reductive catalysts containing Lewis acid centers.  相似文献   
244.
A plenty of different programming techniques and instrument solutions are used in the development of Mössbauer spectrometers. Each of them should provide a faster spectrum accumulation process, increased productivity of measurements, decreased nonlinearity of the velocity scale, etc. The well known virtual instrumentation programming method has been used to design a computer-based Mössbauer spectrometer. Hardware solution was based on two commercially-available PCI modules produced by National Instruments Co. Virtual Mössbauer spectrometer is implemented by the graphical programming language LabVIEW 7 Express. This design environment allows to emulate the multichannel analyzer on the digital oscilloscope platform. This is a novel method based on Waveform Peak Detection function which allows detailed analysis of the acquired signal. The optimal treatment of the detector signal from various detector types is achieved by mathematical processing only. As a result, the possibility of an increase of signal/noise ratio is presented.  相似文献   
245.
A convenient synthesis and separation of α- and β-anomers of methyl (phenyl 3,5-dideoxy-2-thio-5-trifluoroacetamido-D-glycero-D-galacto-nonulopyranosid)onate (6a and 6b) on a multigram scale was developed. Both α- and β-isomers of 6 were obtained as crystals suitable for safe storage. The β-isomer forms a crystalline solvate with methanol. Fully O-trichloroacetylated and O-trifluoroacetylated N-trifluoroacetyl thiosialosides were synthesized in an efficient manner from the β-tetraol 6b.  相似文献   
246.
Polymer:fullerene blends have been widely studied as an inexpensive alternative to traditional silicon solar cells. Some polymer:fullerene blends, such as blends of poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene (pBTTT) with phenyl‐c71‐butyric acid methyl ester (PC71BM), form bimolecular crystals due to fullerene intercalation between the polymer side chains. Here we present the determination of the eutectic pBTTT:PC71BM phase diagram using differential scanning calorimetry (DSC) and two‐dimensional grazing incidence X‐ray scattering (2D GIXS) with in‐situ thermal annealing. The phase diagram explains why the most efficient pBTTT:PC71BM solar cells have 75–80 wt % PC71BM since these blends lie in the center of the only room‐temperature phase region containing both electron‐conducting (PC71BM) and hole‐conducting (bimolecular crystal) phases. We show that intercalation can be suppressed in 50:50 pBTTT:PC71BM blends by using rapid thermal annealing to heat the blends above the eutectic temperature, which forces PC71BM out of the bimolecular crystal, followed by quick cooling to kinetically trap the pure PC71BM phase. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011  相似文献   
247.
248.
The multipole-to-local (M2L) operator is the most time-consuming part of the far field computation in the fast multipole method for Laplace equation. Its natural expression, though commonly used, does not respect a sharp error bound: we here first prove the correctness of a second expression. We then propose a matrix formulation implemented with basic linear algebra subprograms (BLAS) routines in order to speed up its computation for these two expressions. We also introduce special data storages in memory to gain greater computational efficiency. This BLAS scheme is finally compared, for uniform distributions, to other M2L improvements such as block FFT, FFT with polynomial scaling, rotations and plane wave expansions. When considering runtime, extra memory storage, numerical stability and common precisions for Laplace equation, the BLAS version appears as the best one.  相似文献   
249.
We show that the conjectures by Matthews and Sumner (every 4-connected claw-free graph is Hamiltonian), by Thomassen (every 4-connected line graph is Hamiltonian) and by Fleischner (every cyclically 4-edge-connected cubic graph has either a 3-edge-coloring or a dominating cycle), which are known to be equivalent, are equivalent to the statement that every snark (i.e. a cyclically 4-edge-connected cubic graph of girth at least five that is not 3-edge-colorable) has a dominating cycle.We use a refinement of the contractibility technique which was introduced by Ryjá?ek and Schelp in 2003 as a common generalization and strengthening of the reduction techniques by Catlin and Veldman and of the closure concept introduced by Ryjá?ek in 1997.  相似文献   
250.
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