全文获取类型
收费全文 | 504篇 |
免费 | 66篇 |
国内免费 | 16篇 |
专业分类
化学 | 92篇 |
晶体学 | 2篇 |
力学 | 22篇 |
综合类 | 4篇 |
数学 | 287篇 |
物理学 | 179篇 |
出版年
2023年 | 3篇 |
2022年 | 24篇 |
2021年 | 17篇 |
2020年 | 15篇 |
2019年 | 12篇 |
2018年 | 13篇 |
2017年 | 20篇 |
2016年 | 15篇 |
2015年 | 11篇 |
2014年 | 26篇 |
2013年 | 50篇 |
2012年 | 21篇 |
2011年 | 22篇 |
2010年 | 18篇 |
2009年 | 25篇 |
2008年 | 23篇 |
2007年 | 30篇 |
2006年 | 32篇 |
2005年 | 26篇 |
2004年 | 24篇 |
2003年 | 19篇 |
2002年 | 16篇 |
2001年 | 17篇 |
2000年 | 8篇 |
1999年 | 14篇 |
1998年 | 9篇 |
1997年 | 10篇 |
1996年 | 13篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1978年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有586条查询结果,搜索用时 312 毫秒
81.
Alexey Kuz'min 《代数通讯》2013,41(8):3169-3189
Since 1976, it is known from the paper by V. P. Belkin that the variety RA2 of right alternative metabelian (solvable of index 2) algebras over an arbitrary field is not Spechtian (contains nonfinitely based subvarieties). In 2005, S. V. Pchelintsev proved that the variety generated by the Grassmann RA2-algebra of finite rank r over a field ?, for char(?) ≠ 2, is Spechtian iff r = 1. We construct a nonfinitely based variety 𝔐 generated by the Grassmann 𝒱-algebra of rank 2 of certain finitely based subvariety 𝒱 ? RA2 over a field ?, for char(?) ≠ 2, 3, such that 𝔐 can also be generated by the Grassmann envelope of a five-dimensional superalgebra with one-dimensional even part. 相似文献
82.
83.
全息光盘存储(HDS)技术作为一种非常具有潜力的新型信息存储技术, 是下一代光盘发展的目标。在全息光盘存储系统中, 为了实现数据精确快速地写入和读出, 降低误码率, 需要在整个高分辨率页面上实现空间光调制器(SLM)与光电阵列耦合器(CCD)之间1∶1像素匹配。在体全息存储理论和光学设计理论基础上, 研究实现SLM和CCD像素1∶1匹配的方法, 提出了实现像素匹配的要求和条件, 并且按照此要求完成该全息光盘存储器的光学系统。实验中, 分别使用随机数据掩膜版和SLM实现了对CCD的512×512精确像素匹配, 在光学系统中引入存储介质条件下, 图像质量良好, 掩膜版和SLM原始误码率分别为2.5×10-4和1.5×10-4。 相似文献
84.
85.
86.
Ricardo A. Luna-Mora Ángeles Torres-Reyes Oscar A. González-Cruz Fernando Ortega-Jiménez Hulme Ríos-Guerra Jessica V. González-Carrillo 《Green Chemistry Letters and Reviews》2018,11(4):371-378
The efficiency of conventional heating energy source compared with Infrared (IR), Ultrasound (US), Microwave and the simultaneous combination US–IR eco-friendly approaches for preparation of new N-(5-R1 -amino-2-nitrophenyl)acetamides and 5-R1-amino-2-nitroaniline by Nucleophilic Aromatic Substitution (SNAr) via addition–elimination reactions on the halogens F, Cl, Br, I, employing amines as nucleophiles were explored. Moreover, phenyldiazenyl derivatives in good yields by an oxidative one-pot SNAr-based amination reaction from an unusual oxidation of 2-phenylhydrazinyl derivatives in DMSO was prepared. 相似文献
87.
This paper investigates the relations between theorems of the alternative and the minimum norm duality theorem. A typical theorem of the alternative is associated with two systems of linear inequalities and/or equalities, a primal system and a dual one, asserting that either the primal system has a solution, or the dual system has a solution, but never both. On the other hand, the minimum norm duality theorem says that the minimum distance from a given point z to a convex set
is equal to the maximum of the distances from z to the hyperplanes separating z and
. We consider the theorems of Farkas, Gale, Gordan, and Motzkin, as well as new theorems that characterize the optimality conditions of discrete l
1-approximation problems and multifacility location problems. It is shown that, with proper choices of
, each of these theorems can be recast as a pair of dual problems: a primal steepest descent problem that resembles the original primal system, and a dual least–norm problem that resembles the original dual system. The norm that defines the least-norm problem is the dual norm with respect to that which defines the steepest descent problem. Moreover, let y solve the least norm problem and let r denote the corresponding residual vector. If r=0, which means that z
, then y solves the dual system. Otherwise, when r0 and z
, any dual vector of r solves both the steepest descent problem and the primal system. In other words, let x solve the steepest descent problem; then, r and x are aligned. These results hold for any norm on
. If the norm is smooth and strictly convex, then there are explicit rules for retrieving x from r and vice versa. 相似文献
88.
A. F. Voronin 《Siberian Mathematical Journal》2008,49(4):601-611
We derive some necessary and sufficient conditions for the well-posedness of a convolution equation of the second kind with even kernel on a finite interval. In order to check these conditions it suffices to compute a one-dimensional integral (of a given function) with precision less than 0.5. As an intermediate result we give a strengthening of the Fredholm alternative for the equation in question with an arbitrary kernel in L 1. 相似文献
89.
讨论了假设检验的教学和实践中可能会遇到的三个问题.分析得到三个结论:理论上原假设与备择假设可以任意设定,但实践中常常按照某种规则来设定;假设检验的两个常用的决策准则中,p值准则更精确,为决策者接受原假设提供了一个"风险"的度量;假设检验理论在处理利用样本推断总体的问题中是普遍有效的. 相似文献
90.
Shiran Aharon Dr. Shanti Gopal Patra Prof. Dr. Dan Meyerstein Dr. Eyal Tzur Dr. Dror Shamir Dr. Yael Albo Prof. Dr. Ariela Burg 《Chemphyschem》2023,24(22):e202300517
The Na3[Ru2(μ-CO3)4] complex is acting as a water oxidation catalyst in a homogeneous system. Due to the significance of heterogeneous systems and the effect of bicarbonate on the kinetic, we studied the bicarbonate effect on the heterogeneous electrocatalyst by entrapping the Na3[Ru2(μ-CO3)4] complex in a sol-gel matrix. We have developed two types of sol-gel electrodes, which differ by the precursor, and are demonstrating their stability over a minimum of 200 electrochemical cycles. The pH increases affected the currents and kcat for both types of electrodes, and their hydrophobicity, which was obtained from the precursor type, influenced the electrocatalytic process rate. The results indicate that NaHCO3 has an important role in the catalytic activity of the presented heterogeneous systems; without NaHCO3, the diffusing species is probably OH−, which undergoes diffusion via the Grotthuss mechanism. To the best of our knowledge, this is the first study to present a simple and fast one-step entrapment process for the Na3[Ru2(μ-CO3)4] complex by the sol-gel method under standard laboratory conditions. The results contribute to optimizing the WSP, ultimately helping expand the usage of hydrogen as a green and more readily available energy source. 相似文献