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   5933篇
  免费   581篇
  国内免费   464篇
化学   533篇
晶体学   12篇
力学   234篇
综合类   54篇
数学   603篇
物理学   2026篇
综合类   3516篇
  2024年   21篇
  2023年   42篇
  2022年   98篇
  2021年   93篇
  2020年   97篇
  2019年   110篇
  2018年   106篇
  2017年   162篇
  2016年   190篇
  2015年   164篇
  2014年   279篇
  2013年   314篇
  2012年   405篇
  2011年   424篇
  2010年   297篇
  2009年   337篇
  2008年   351篇
  2007年   425篇
  2006年   396篇
  2005年   353篇
  2004年   269篇
  2003年   266篇
  2002年   263篇
  2001年   255篇
  2000年   207篇
  1999年   178篇
  1998年   156篇
  1997年   134篇
  1996年   90篇
  1995年   76篇
  1994年   71篇
  1993年   47篇
  1992年   55篇
  1991年   50篇
  1990年   45篇
  1989年   41篇
  1988年   31篇
  1987年   15篇
  1986年   12篇
  1985年   4篇
  1984年   8篇
  1983年   3篇
  1982年   2篇
  1981年   5篇
  1980年   7篇
  1979年   8篇
  1978年   7篇
  1977年   3篇
  1976年   2篇
  1955年   1篇
排序方式: 共有6978条查询结果,搜索用时 15 毫秒
1.
We construct models for the level by level equivalence between strong compactness and supercompactness in which for κ the least supercompact cardinal and δκ any cardinal which is either a strong cardinal or a measurable limit of strong cardinals, 2δ > δ + and δ is < 2δ supercompact. In these models, the structure of the class of supercompact cardinals can be arbitrary, and the size of the power set of κ can essentially be made as large as desired. This extends and generalizes [5, Theorem 2] and [4, Theorem 4]. We also sketch how our techniques can be used to establish a weak indestructibility result. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
2.
The Fermi and Coulomb holes of the 21 S state of the helium isoelectronic sequence are investigated. Several interesting differences between the results obtained and those which might be expected on the basis of the corresponding 23 S state are pointed out and discussed.  相似文献   
3.
Weakly bound linear and bent dimers, FH—X (where X = CO, OC, CNH, NCH, N2O and ON2), are investigated using the DFT B3LYP and ab initio MP2 methods with the same basis sets (6–311++G(3df,2pd)). The strengths of the H—C or H—N H‐bonds in dimers FH—CO, FH—CNH, and FH—N2O are compared with those of the H—O or H—N H‐bonds in dimers FH—OC, FH—NCH, and FH—ON2. The results obtained for the H‐bond distances, the elongation effect of the HF bond, the red shift of the HF stretching frequency, and the energy difference between the dimer and the charge transfer reveal that the H‐bonds of the first group of dimers are stronger than those of the second. The Gibbs energies calculated for the six dimer formations indicate that the weakly bound dimers are unstable at room temperature (T = 298 K) (FH—X's → FH + X's, ΔG < 0).  相似文献   
4.
Selected group frequencies of the IR spectra of a number of 2-substituted 4-methylene-1,3-dioxolanes (a) and their endocyclic isomers, 2-substituted 4-methyl-1,3-dioxoles (b), have been studied to establish their usefulness as a measure of the strength of p– conjugation in the O—C=C moieties of the title compounds. In the exo compounds (a), the C=C stretching frequencies C=C and the in-phase, out-of-plane bending (wagging) frequencies CH2 w of the =CH2 group were found to be linearly related, besides to each other, also to the electron-donating character of the substituents and the 13C NMR chemical shift of the C atom of the exocyclic CH2 group. This suggests that the two IR group frequencies of the exo isomers are linearly related to the strength of p– conjugation in the O—C=C moiety. The C=C stretching absorption of the exo compounds in the 1600–1700 cm–1 region appeared as a doublet, shown to arise from Fermi resonance of the C=C fundamental with the CH2 w overtone. The C=C frequencies of the endo compounds support some previous findings of an unexpected strength of p– conjugation in the 2-alkoxy derivatives. Finally, DFT calculations at the B3LYP/6-31G* level of theory are shown to give accurate predictions of the effect of 2-substituents on the group frequencies.  相似文献   
5.
介绍了快速消费品行业信息化的现状,指出ERP对提供支持决策信息能力不足的缺点,进而提出在ERP广泛应用的快速消费品行业里,建立基于ERP系统的综合数据仓库决策支持系统的重要性以及广阔前景。  相似文献   
6.
研究了一类基于个体等级结构差异的竞争种群系统最优控制问题。首先利用不动点定理证明了系统解的存在惟一性,其次证明了解对控制变量的连续依赖性,最后通过共轭系统及切锥-法锥的相关理论得出了最优控制的必要性条件。  相似文献   
7.
Cancer is a leading cause of mortality globally. Despite remarkable improvements in cancer-treatment approaches, disease recurrence and progression remain major obstacles to therapy. While chemotherapy is still a first-line treatment for a variety of cancers, the focus has shifted to the development and application of new approaches to therapy. Nevertheless, the relationship between immune response, neoplastic diseases and treatment efficiency is not fully understood. Therefore, the aim of the study was to investigate the immunopharmacological effects of methacrylic acid homopolymer in an in vivo tumor model. Materials and methods: Monomeric methacrylic acid was used to synthesize polymers. Methacrylic acid was polymerized in dioxane in the presence of 4-Cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid. To study the molecular weight characteristics of PMAA by GPC, carboxyl groups were preliminarily methylated with diazomethane. An experimental cancer model was obtained by grafting RMK1 breast cancer cells. The serum levels of IL-6, IL-10, IL-17, transforming growth factor β1 (TGF-β1), and tumor necrosis factor α (TNF-α) were measured by ELISA. Results: The effect of PMAA on the serum concentrations of several cytokines was studied upon its single administration to laboratory animals in early neoplastic process. The IL-6, IL-17 and TGF-β1 concentrations were found to change significantly and reach the level observed in intact rats. The IL-10 concentration tended to normalize. Conclusion: The positive results obtained are the basis for further studies on the effect of methacrylic-acid polymers with different molecular-weight characteristics on the neoplastic process.  相似文献   
8.
蓄滞洪区平面二维干河床洪水演进数值模拟   总被引:15,自引:0,他引:15  
在地形较复杂的情况下,采用常用的处理干河床问题的方法有时会出现水流虚假流动的现象。本在洪水干河床演进模拟中提出了“水位平铺”假设进行处理,取得了比较好的模拟效果。模型建立过程中采用有限差分法的逆风格式离散控制方程,并用ADI方法求解代数方程组,模型计算速度快,节约计算内存,基本可以满足洪水实时预报要求。  相似文献   
9.
在铸铁平板像素检测法中,针对图像的不均匀特点,建立了铸铁平板图像的数学模型,并在此基础上提出了以二维插值为基础的铸铁平板图像均匀化方法.结果表明,该方法的均匀化效果良好,均匀化后的平板图像可直接用Otsu方法对其讲行分割.分割效果满足应用要求.  相似文献   
10.
双轴自动电子倾斜测试仪是一个高精度、大量程的角度倾斜测量仪器。本文介绍了该仪器的原理和性能,给出了技术指标和实测结果,最后阐明了它的应用领域。  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