全文获取类型
收费全文 | 1183篇 |
免费 | 155篇 |
国内免费 | 41篇 |
专业分类
化学 | 61篇 |
晶体学 | 2篇 |
力学 | 289篇 |
综合类 | 22篇 |
数学 | 744篇 |
物理学 | 261篇 |
出版年
2024年 | 2篇 |
2023年 | 13篇 |
2022年 | 19篇 |
2021年 | 24篇 |
2020年 | 23篇 |
2019年 | 29篇 |
2018年 | 31篇 |
2017年 | 39篇 |
2016年 | 43篇 |
2015年 | 35篇 |
2014年 | 63篇 |
2013年 | 70篇 |
2012年 | 81篇 |
2011年 | 60篇 |
2010年 | 56篇 |
2009年 | 53篇 |
2008年 | 51篇 |
2007年 | 78篇 |
2006年 | 64篇 |
2005年 | 61篇 |
2004年 | 61篇 |
2003年 | 37篇 |
2002年 | 66篇 |
2001年 | 41篇 |
2000年 | 44篇 |
1999年 | 45篇 |
1998年 | 29篇 |
1997年 | 18篇 |
1996年 | 16篇 |
1995年 | 18篇 |
1994年 | 23篇 |
1993年 | 22篇 |
1992年 | 12篇 |
1991年 | 6篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1978年 | 4篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1971年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有1379条查询结果,搜索用时 31 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.
Ieva
Dauickait Amos S. Lawless Jennifer A. Scott Peter Jan van Leeuwen 《Numerical Linear Algebra with Applications》2020,27(5)
We consider the large sparse symmetric linear systems of equations that arise in the solution of weak constraint four‐dimensional variational data assimilation, a method of high interest for numerical weather prediction. These systems can be written as saddle point systems with a 3 × 3 block structure but block eliminations can be performed to reduce them to saddle point systems with a 2 × 2 block structure, or further to symmetric positive definite systems. In this article, we analyse how sensitive the spectra of these matrices are to the number of observations of the underlying dynamical system. We also obtain bounds on the eigenvalues of the matrices. Numerical experiments are used to confirm the theoretical analysis and bounds. 相似文献
3.
4.
《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
5.
6.
Existence and limit behavior of prescribed L2‐norm solutions for Schrödinger‐Poisson‐Slater systems in
下载免费PDF全文
In this paper, we study constraint minimizers of the following L 2?critical minimization problem: where E (u ) is the Schrödinger‐Poisson‐Slater functional and N denotes the mass of the particles in the Schrödinger‐Poisson‐Slater system. We prove that e (N ) admits minimizers for and, however, no minimizers for N >N ?, where Q (x ) is the unique positive solution of in . Some results on the existence and nonexistence of minimizers for e (N ?) are also established. Further, when e (N ?) does not admit minimizers, the limit behavior of minimizers as N ↗N ? is also analyzed rigorously. 相似文献
7.
8.
9.
In many atmospheric and earth sciences, it is of interest to identify dominant spatial patterns of variation based on data observed at p locations and n time points with the possibility that p > n. While principal component analysis (PCA) is commonly applied to find the dominant patterns, the eigenimages produced from PCA may exhibit patterns that are too noisy to be physically meaningful when p is large relative to n. To obtain more precise estimates of eigenimages, we propose a regularization approach incorporating smoothness and sparseness of eigenimages, while accounting for their orthogonality. Our method allows data taken at irregularly spaced or sparse locations. In addition, the resulting optimization problem can be solved using the alternating direction method of multipliers, which is easy to implement, and applicable to a large spatial dataset. Furthermore, the estimated eigenfunctions provide a natural basis for representing the underlying spatial process in a spatial random-effects model, from which spatial covariance function estimation and spatial prediction can be efficiently performed using a regularized fixed-rank kriging method. Finally, the effectiveness of the proposed method is demonstrated by several numerical examples. 相似文献
10.