全文获取类型
收费全文 | 2123篇 |
免费 | 248篇 |
国内免费 | 92篇 |
专业分类
化学 | 196篇 |
晶体学 | 3篇 |
力学 | 352篇 |
综合类 | 52篇 |
数学 | 1377篇 |
物理学 | 483篇 |
出版年
2024年 | 3篇 |
2023年 | 23篇 |
2022年 | 39篇 |
2021年 | 57篇 |
2020年 | 48篇 |
2019年 | 44篇 |
2018年 | 55篇 |
2017年 | 63篇 |
2016年 | 77篇 |
2015年 | 58篇 |
2014年 | 93篇 |
2013年 | 125篇 |
2012年 | 129篇 |
2011年 | 105篇 |
2010年 | 97篇 |
2009年 | 98篇 |
2008年 | 98篇 |
2007年 | 127篇 |
2006年 | 114篇 |
2005年 | 105篇 |
2004年 | 100篇 |
2003年 | 80篇 |
2002年 | 104篇 |
2001年 | 82篇 |
2000年 | 76篇 |
1999年 | 78篇 |
1998年 | 52篇 |
1997年 | 43篇 |
1996年 | 33篇 |
1995年 | 28篇 |
1994年 | 39篇 |
1993年 | 35篇 |
1992年 | 28篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1983年 | 3篇 |
1982年 | 8篇 |
1980年 | 13篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1973年 | 2篇 |
1957年 | 3篇 |
排序方式: 共有2463条查询结果,搜索用时 15 毫秒
1.
研究来源于多元统计分析中的一类矩阵迹函数最小化问题$\min c+ tr(AX)+\sum\limits_{j=1}^{m}tr(B_j X C_jX^{T}),\ \ {\rm s. t.} \ X^TX=I_p,$其中$c$为常数, $A\in R^{p\times n}\ (n\geq p)$, $B_j\in R^{n\times n}, C_j\in R^{p\times p}$为给定系数矩阵. 数值实验表明已有的Majorization算法虽可行, 但收敛速度缓慢且精度不高. 本文从黎曼流形的角度重新研究该问题, 基于Stiefel流形的几何性质, 构造一类黎曼非单调共轭梯度迭代求解算法, 并给出算法收敛性分析.数值实验和数值比较验证所提出的算法对于问题模型是高效可行的. 相似文献
2.
Shigeaki Koike Takahiro Kosugi Makoto Naito 《Journal of Mathematical Analysis and Applications》2018,457(1):436-460
The rate of convergence of approximate solutions via penalization for free boundary problems are concerned. A key observation is to obtain global bounds of penalized terms which give necessary estimates on integrations by the nonlinear adjoint method by L.C. Evans. 相似文献
3.
《中国化学快报》2020,31(9):2353-2357
NiS2 has become a research hotspot of anode materials for Na-ion batteries due to its high theoretical specific capacity. However, the volume effect, the dissolution of polysulfide intermediates and the low conductivity during the charge/discharge process lead to the low specific capacity and poor cycling stability. NiS2/rGO nanocomposite was prepared by a facile two-step process: GO was prepared by modified Hummers method, and then NiS2/rGO nanocomposite was synthesized by l-cys assisted hydrothermal method. NiS2/rGO nanocomposite shows excellent cycle performance and rate performance, which could be attributed to the mesoporous structure on the graphene skeleton with high conductivity. Besides, the chemical constraint of a unique SO bond on NiS2 could inhibit the dissolution of intermediates and the loss of irreversible capacity. 相似文献
4.
ABSTRACT The theory of calculus of variations is a mathematical tool which is widely used in different scientific areas in particular in physics and chemistry. This theory is strongly related with optimisation. In fact the former seeks to optimise an integral related with some physical magnitude over some space to an extremum by varying a function of the coordinates. On the other hand, reaction paths and potential energy surfaces, in particular their stationary points, are the basis of many chemical theories, in particular reactions rate theories. We present a review where it is gathered together the variational nature of many types of reaction paths: steepest descent, Newton trajectories, artificial force induced reaction (AFIR) paths, gradient extremals, and gentlest ascent dynamics (GAD) curves. The variational basis permits to select the best optimisation technique in order to locate important theoretical objects on a potential energy surface. 相似文献
5.
Development of models for prediction of the antioxidant activity of derivatives of natural compounds
Antioxidants are important for maintaining the appropriate balance between oxidizing and reducing species in the body and thus preventing oxidative stress. Many natural compounds are being screened for their possible antioxidant activity. It was found that a mushroom pigment Norbadione A, which is a pulvinic acid derivative, shows an antioxidant activity; the same was found for other pulvinic acid derivatives and structurally related coumarines. Based on the results of in vitro studies performed on these compounds as a part of this study quantitative structure–activity relationship (QSAR) predictive models were constructed using multiple linear regression, counter-propagation artificial neural networks and support vector regression (SVR). The models have been developed in accordance with current QSAR guidelines, including the assessment of the models applicability domains. A new approach for the graphical evaluation of the applicability domain for SVR models is suggested. The developed models show sufficient predictive abilities for the screening of virtual libraries for new potential antioxidants. 相似文献
6.
多项式结构设计方法是主瓣指向可调宽带波束形成器设计的一类重要方法。多项式结构的阶数是有限的,导致主瓣实际指向与期望指向之间存在偏差,因而影响了波束形成器的指向性指数。针对这一问题,该文提出了一种基于插值点优化的多项式结构宽带波束形成器设计方法。首先,引入多项式结构插值点处阵列响应的空间导数约束,以减小主瓣指向偏差;进而利用粒子群优化算法对多项式结构中的插值点进行优化,以充分利用插值点位置提供的自由度进一步提升多项式结构宽带波束形成器的性能。优化设计结果表明,与现有设计方法相比,该文提出的方法不仅降低了主瓣的指向偏差,同时也提高了指向性指数,有效改善了多项式结构宽带波束形成器的性能。 相似文献
7.
8.
John Engbers 《Journal of Graph Theory》2017,85(4):780-787
For graphs G and H , an H‐coloring of G is a map from the vertices of G to the vertices of H that preserves edge adjacency. We consider the following extremal enumerative question: for a given H , which connected n‐vertex graph with minimum degree δ maximizes the number of H‐colorings? We show that for nonregular H and sufficiently large n , the complete bipartite graph is the unique maximizer. As a corollary, for nonregular H and sufficiently large n the graph is the unique k‐connected graph that maximizes the number of H‐colorings among all k‐connected graphs. Finally, we show that this conclusion does not hold for all regular H by exhibiting a connected n‐vertex graph with minimum degree δ that has more ‐colorings (for sufficiently large q and n ) than . 相似文献
9.
10.