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   2720篇
  免费   226篇
  国内免费   334篇
化学   514篇
晶体学   5篇
力学   70篇
综合类   114篇
数学   2159篇
物理学   418篇
  2024年   4篇
  2023年   15篇
  2022年   50篇
  2021年   59篇
  2020年   66篇
  2019年   56篇
  2018年   66篇
  2017年   68篇
  2016年   75篇
  2015年   52篇
  2014年   119篇
  2013年   242篇
  2012年   119篇
  2011年   177篇
  2010年   143篇
  2009年   189篇
  2008年   194篇
  2007年   195篇
  2006年   162篇
  2005年   145篇
  2004年   154篇
  2003年   143篇
  2002年   136篇
  2001年   97篇
  2000年   73篇
  1999年   94篇
  1998年   80篇
  1997年   72篇
  1996年   41篇
  1995年   32篇
  1994年   30篇
  1993年   15篇
  1992年   17篇
  1991年   11篇
  1990年   17篇
  1989年   13篇
  1988年   11篇
  1987年   5篇
  1986年   6篇
  1985年   6篇
  1984年   7篇
  1983年   4篇
  1982年   3篇
  1981年   5篇
  1979年   2篇
  1977年   2篇
  1974年   2篇
  1970年   1篇
  1959年   1篇
  1936年   2篇
排序方式: 共有3280条查询结果,搜索用时 15 毫秒
981.
胸腺五肽的NMR研究   总被引:8,自引:5,他引:3  
应用改进的DPFGSE 1D-TOCSY 和1D-NOESY 核磁共振方法测定胸腺五肽的氨基酸残基,从重叠的1H谱中分离出各自旋系统内氢的亚谱信号;继而由DPFGSE 1D-NOE 技术测定肽链氨基酸残基的连接顺序. 结合二维相关谱对胸腺五肽的碳、氢信号做了全归属. 并进行了计算机分子模拟,结果与NOESY 实验相符.  相似文献   
982.
图的谱矩序列与图的排序   总被引:1,自引:0,他引:1  
图的谱矩是代数图论中一个重要的代数不变量,本文通过计算图的第5、6阶谱矩,研究了图的结构与图的谱矩之阃的联系,动态地研究了图的结构变化(包括图的阶数的增大及同阶前提下所含圈长度的变化等)对谱矩序列排列的影响,给出了研究图依谱矩序列排序问题的新方法.  相似文献   
983.
Banach空间中p-严格渐近伪压缩映象的收敛性问题   总被引:7,自引:1,他引:7  
在p-一致凸的Banach空间中(1相似文献   
984.
We treat some fractal and statistical features of the DNA sequences. First, a fractal record model of DNA sequence is proposed by mapping,DNA sequences to integer sequences, followed by the R/S analysis of the model and computation of the Hurst exponents. Second, we consider the transition between the four kinds of bases within DNA. The transition matrix analysis of DNA sequences shows that some measures of complexity based on transition proportion matrices are of interest.  相似文献   
985.
1.IntroductionInthepresentpaper,wewillcontinuethestudyofHilbertmodules[1-5].LetHbeaHilbertspaceandAafunctionalgebra.WesaythatHisaHilbertmoduleoverAifthereisamultiplication(a,f)-affromAxHtoH,makingHintoanXmoduleandif,inaddition,theactionisjointlycontinuousinthesup-normonAandtheHilbertspacenormonH.ThecategoryHofallHilbertNmodulesisanaturalsettingfornumerousquestionsinoperatortheory.In[9],DouglasandPaulseninitiatedasystematicstudyofHilbertmodules'theywereabletotranslatemanyconceptsandpro…  相似文献   
986.
平稳NA序列的中偏差原理   总被引:1,自引:0,他引:1  
谭元兴  张峰  胡亦钧 《数学杂志》2000,20(2):222-226
设{Xn;n≥1}是平稳NA序列。假设X1具有有限的指数阶矩及其它适当条件,本文利用Cramer方法及分块技术,证明了{Xn;n≥1}的部分和序列满足中偏差原理。  相似文献   
987.
We show that the autocorrelation sequence of interarrival times for a Markovian arrival process (MAP) of order two is geometric. We determine the set of feasible values for the autocorrelation decay parameter and the first two or three moments of the interarrival time distribution. A method is derived for matching these parameters to a MAP of order two and some numerical examples are included to illustrate approximating higher dimensional MAPs by two dimensional ones. The numerical examples have helped us pose important questions regarding the significance of correlation in a MAP of order two when it is used as input to a queueing model.  相似文献   
988.
本文证明了无穷矩阵算子代数(λ,μ)在左(右)强、K收敛意义下的乘积定理成立,给出了(λ,μ)在弱收敛意义下乘积定理成立的充要条件。  相似文献   
989.
Complex modeling has received significant attention in recent years and is increasingly used to explain statistical phenomena with increasing and decreasing fluctuations, such as the similarity or difference of spike protein charge patterns of coronaviruses. Different from the existing covariance or correlation coefficient methods in traditional integer dimension construction, this study proposes a simplified novel fractional dimension derivation with the exact Excel tool algorithm. It involves the fractional center moment extension to covariance, which results in a complex covariance coefficient that is better than the Pearson correlation coefficient, in the sense that the nonlinearity relationship can be further depicted. The spike protein sequences of coronaviruses were obtained from the GenBank and GISAID databases, including the coronaviruses from pangolin, bat, canine, swine (three variants), feline, tiger, SARS-CoV-1, MERS, and SARS-CoV-2 (including the strains from Wuhan, Beijing, New York, German, and the UK variant B.1.1.7) which were used as the representative examples in this study. By examining the values above and below the average/mean based on the positive and negative charge patterns of the amino acid residues of the spike proteins from coronaviruses, the proposed algorithm provides deep insights into the nonlinear evolving trends of spike proteins for understanding the viral evolution and identifying the protein characteristics associated with viral fatality. The calculation results demonstrate that the complex covariance coefficient analyzed by this algorithm is capable of distinguishing the subtle nonlinear differences in the spike protein charge patterns with reference to Wuhan strain SARS-CoV-2, which the Pearson correlation coefficient may overlook. Our analysis reveals the unique convergent (positive correlative) to divergent (negative correlative) domain center positions of each virus. The convergent or conserved region may be critical to the viral stability or viability; while the divergent region is highly variable between coronaviruses, suggesting high frequency of mutations in this region. The analyses show that the conserved center region of SARS-CoV-1 spike protein is located at amino acid residues 900, but shifted to the amino acid residues 700 in MERS spike protein, and then to amino acid residues 600 in SARS-COV-2 spike protein, indicating the evolution of the coronaviruses. Interestingly, the conserved center region of the spike protein in SARS-COV-2 variant B.1.1.7 shifted back to amino acid residues 700, suggesting this variant is more virulent than the original SARS-COV-2 strain. Another important characteristic our study reveals is that the distance between the divergent mean and the maximal divergent point in each of the viruses (MERS > SARS-CoV-1 > SARS-CoV-2) is proportional to viral fatality rate. This algorithm may help to understand and analyze the evolving trends and critical characteristics of SARS-COV-2 variants, other coronaviral proteins and viruses.  相似文献   
990.
This is a survey of results on joint universality in Voroninʼs sense of various zeta-functions, when in the collection of these functions some of them have the Euler product and the others have not.  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