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   7575篇
  免费   238篇
  国内免费   38篇
化学   4622篇
晶体学   42篇
力学   326篇
数学   1458篇
物理学   1403篇
  2023年   56篇
  2022年   65篇
  2021年   157篇
  2020年   192篇
  2019年   188篇
  2018年   166篇
  2017年   152篇
  2016年   329篇
  2015年   253篇
  2014年   280篇
  2013年   505篇
  2012年   468篇
  2011年   506篇
  2010年   325篇
  2009年   316篇
  2008年   481篇
  2007年   454篇
  2006年   361篇
  2005年   350篇
  2004年   266篇
  2003年   228篇
  2002年   173篇
  2001年   73篇
  2000年   68篇
  1999年   58篇
  1998年   45篇
  1997年   46篇
  1996年   76篇
  1995年   66篇
  1994年   59篇
  1993年   48篇
  1992年   49篇
  1991年   43篇
  1990年   51篇
  1989年   49篇
  1988年   45篇
  1987年   57篇
  1986年   35篇
  1985年   59篇
  1984年   65篇
  1983年   39篇
  1982年   42篇
  1981年   42篇
  1980年   48篇
  1979年   33篇
  1978年   43篇
  1977年   55篇
  1976年   22篇
  1975年   23篇
  1974年   18篇
排序方式: 共有7851条查询结果,搜索用时 15 毫秒
1.
2.
In this cross-sectional study, the relationship between noninvasively measured neurocardiovascular signal entropy and physical frailty was explored in a sample of community-dwelling older adults from The Irish Longitudinal Study on Ageing (TILDA). The hypothesis under investigation was that dysfunction in the neurovascular and cardiovascular systems, as quantified by short-length signal complexity during a lying-to-stand test (active stand), could provide a marker for frailty. Frailty status (i.e., “non-frail”, “pre-frail”, and “frail”) was based on Fried’s criteria (i.e., exhaustion, unexplained weight loss, weakness, slowness, and low physical activity). Approximate entropy (ApEn) and sample entropy (SampEn) were calculated during resting (lying down), active standing, and recovery phases. There was continuously measured blood pressure/heart rate data from 2645 individuals (53.0% female) and frontal lobe tissue oxygenation data from 2225 participants (52.3% female); both samples had a mean (SD) age of 64.3 (7.7) years. Results revealed statistically significant associations between neurocardiovascular signal entropy and frailty status. Entropy differences between non-frail and pre-frail/frail were greater during resting state compared with standing and recovery phases. Compared with ApEn, SampEn seemed to have better discriminating power between non-frail and pre-frail/frail individuals. The quantification of entropy in short length neurocardiovascular signals could provide a clinically useful marker of the multiple physiological dysregulations that underlie physical frailty.  相似文献   
3.
4.
Methodology and Computing in Applied Probability - The family of the multivariate conditional hazard rate functions often reveals to be a convenient tool to describe the joint probability...  相似文献   
5.
Reactivity studies of the GeII→B complex L(Cl)Ge⋅BH3 ( 1 ; L=2-Et2NCH2-4,6-tBu2-C6H2) were performed to determine the effect on the GeII→B donation. N-coordinated compounds L(OtBu)Ge⋅BH3 ( 2 ) and [LGe⋅BH3]2 ( 3 ) were prepared. The possible tuning of the GeII→B interaction was proved experimentally, yielding compounds 1-PPh2-8-(LGe)-C10H6 ( 4 ) and L(Cl)Ge⋅GaCl3 ( 5 ) without a GeII→B interaction. In 5 , an unprecedented GeII→Ga coordination was revealed. The experimental results were complemented by a theoretical study focusing on the bonding in 1 − 5 . The different strength of the GeII→E (E=B, Ga) donation was evaluated by using energy decomposition analysis. The basicity of different L(X)Ge groups through proton affinity is also assessed.  相似文献   
6.
Journal of Thermal Analysis and Calorimetry - Two samples (“O” and “C”) of aluminium-based powders were calorimetrically investigated with respect to the self-heating risk...  相似文献   
7.
Journal of Optimization Theory and Applications - Pollution management models of Shallow Lake type identify a well-known class of infinite horizon optimal control problems, characterized by the...  相似文献   
8.
对于空间环境中近乎无碰撞的等离子体,可采用Vlasov方程进行理论描述,基于Vlasov方程,讨论了等离子体湍流能量传输和耗散的过程:由亚格子应力引起的尺度间的能量传输,电场做功,压强张量做功(压强张量与速度梯度张量的相互作用).通过混合Vlasov-Maxwell(HVM)数值模拟,进一步研究了能量传输通道之间的联系与区别.不同能量传输通道尽管在不同尺度起主要作用,但其空间的分布非常相似,即各能量传输通道之间存在一定的空间相关.结合近年数值模拟和卫星观测的结果,可以大致概括等离子体湍流从磁流体动力学(magnetohydrodynamic,MHD)尺度到动理学尺度的能量传输过程.   相似文献   
9.
10.
Numerical Algorithms - We introduce a modulus-based formulation for vertical linear complementarity problems (VLCPs) with an arbitrary number ? of matrices. This formulation can be used to...  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