全文获取类型
收费全文 | 138257篇 |
免费 | 11242篇 |
国内免费 | 18082篇 |
专业分类
化学 | 95164篇 |
晶体学 | 3545篇 |
力学 | 4860篇 |
综合类 | 1320篇 |
数学 | 24669篇 |
物理学 | 38023篇 |
出版年
2024年 | 168篇 |
2023年 | 1126篇 |
2022年 | 1802篇 |
2021年 | 3687篇 |
2020年 | 3509篇 |
2019年 | 3548篇 |
2018年 | 2947篇 |
2017年 | 3568篇 |
2016年 | 4412篇 |
2015年 | 4290篇 |
2014年 | 5630篇 |
2013年 | 10090篇 |
2012年 | 7688篇 |
2011年 | 8345篇 |
2010年 | 7156篇 |
2009年 | 8588篇 |
2008年 | 9049篇 |
2007年 | 9391篇 |
2006年 | 8268篇 |
2005年 | 7012篇 |
2004年 | 6540篇 |
2003年 | 5812篇 |
2002年 | 7665篇 |
2001年 | 4310篇 |
2000年 | 3923篇 |
1999年 | 3251篇 |
1998年 | 3043篇 |
1997年 | 2507篇 |
1996年 | 2493篇 |
1995年 | 2179篇 |
1994年 | 1975篇 |
1993年 | 1679篇 |
1992年 | 1728篇 |
1991年 | 1152篇 |
1990年 | 918篇 |
1989年 | 827篇 |
1988年 | 743篇 |
1987年 | 570篇 |
1986年 | 489篇 |
1985年 | 633篇 |
1984年 | 504篇 |
1983年 | 266篇 |
1982年 | 456篇 |
1981年 | 657篇 |
1980年 | 591篇 |
1979年 | 574篇 |
1978年 | 484篇 |
1977年 | 378篇 |
1976年 | 327篇 |
1973年 | 206篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Under some general continuous and compact conditions, the existence problems of fiked points andd coupled fixed points for increasing operators are studied. an application, we utilize the results obtained to study the existence of solutions for differential inclusions in Banach spaces. 相似文献
982.
Vsevolod K. Malinovskii 《Acta Appl Math》1994,34(1-2):261-281
Edgeworth expansions for the distribution of a sequential least squares estimator in the random coefficient autoregressive (RCA) model are derived. The regenerative approach to second-order asymptotic analysis of Markov-type statistical models is developed. 相似文献
983.
Numerical methods for ordinary initial value problems that do not depend on special properties of the system are usually found in the class of linear multistage multivalue methods, first formulated by J.C. Butcher. Among these the explicit methods are easiest to implement. For these reasons there has been considerable research activity devoted to generating methods of this class which utilize independent function evaluations that can be performed in parallel. Each such group of concurrent function evaluations can be regarded as a stage of the method. However, it turns out that parallelism affords only limited opportunity for reducing the computing time with such methods. This is most evident for the simple linear homogeneous constant-coefficient test problem, whose solution is essentially a matter of approximating the exponential by an algebraic function. For a given number of stages and a given number of saved values, parallelism offers a somewhat enlarged set of algebraic functions from which to choose. However, there is absolutely no benefit in having the degree of parallelism (number of processors) exceed the number of saved values of the method. Thus, in particular, parallel one-step methods offer no speedup over serial one-step methods for the standard linear test problem. Although the implication of this result for general nonlinear problems is unclear, there are indications that dramatic speedups are not possible in general. Also given are some results relevant to the construction of methods.Work supported in part by National Science Foundation grants DMS 89 11410 and DMS 90 15533 and by US Department of Energy grant DOE DEFG02-87ER25026. Work of the second author was completed while at the University of Illinois. 相似文献
984.
985.
基于Prony算法的Delsuc谱预测是目前NMR LPSVD(Linear Prediction SingularValue Decomposition)法中比较实用的方法。但在高噪声的条件下,时有失峰和假峰出现且要事先知道谱的峰数。本文通过跟踪迭代过程发现Delsuc法的判别因子r_(mis) 很难选定。如果它太小会失峰,而过大又会出现假峰。同时,由于噪声的影响,弱信号的采样点之间的相关性比强信号小,从而在迭代过程中会增加数值不稳定性。文中引入峰面积判据来改善Delsuc法,经模拟与实验均证明结果有效。 相似文献
986.
当前,在医学核磁共振成象领域内,临床上广泛采用的是近似的自旋密度象、T1-和T2-加权密度象。但是,由于人体的正常组织和肿瘤之间的密度差别不大,从质子密度象很难区分人体的正常组织与肿瘤,而肿瘤与人体正常组织之间的在弛豫时间T1、T2的数值上差别较大。另外,自七十年代以来,大量关于离体(in vitro)核磁共振弛豫时间测量的文献表明,肿瘤的核磁共振弛豫时间T1,T2值具有明显的规律性,从而利用核磁共振弛豫时间成象的方法,及人体正常组织、良性及恶性肿瘤的活体(in vivo)T1,T2数值的测量,将有助于实现肿瘤识别的定量方法。 相似文献
987.
由于原子簇化合物在催化,生物活性,功能材料等方面所呈现的重要性,使其成为无机和物理化学最重要的研究领域之一。文章中和谱学方法对一些具有混配体的过渡金属簇合物的结构及其两者之间的关系进行. 相似文献
988.
The algebra dual to Woronowicz's deformation of the two-dimensional Euclidean group is constructed. The same algebra is obtained from SU
q
(2) via contraction on both the group and algebra levels. 相似文献
989.
Ryuhei Wakita Masanori Miyakoshi Yohji Nakatsuji Mitsuo Okahara 《Journal of inclusion phenomena and macrocyclic chemistry》1991,10(1):127-139
A series of noncyclic neutral ionophores has been synthesized by the reaction of oligoethylene glycol dihalides with 8-quinolinol. Complexation properties for alkali metal picrates were evaluated from solvent extraction and bulk liquid membrane transport experiments. Complexation profiles of the newly synthesized ionophores with a hexyl chain were similar to those of their homologues without the hexyl chain in the extraction experiments. Among them, the pentaethylene glycol derivatives showed the highest extraction efficiency and selectivity towards potassium ion. From the1H NMR spectra (400 MHz), the change in chemical shifts of the aromatic protons upon the addition of alkali metal thiocyanates suggested the existence of a stabilization effect which is caused by intramolecular stacking conformations between the quinoline rings during complexation. Aryl stacking interactions depend on the size of the cations and on the chain length of the oligoethylene glycol. The relationship between transport ability towards alkali metal cations and lipophilicity of these ionophores is also discussed. 相似文献
990.
Takuya Hashimoto Koichi Kitazawa Masaaki Nakabayashi Tadashi Shiraishi Youichi Suemune Takakazu Yamamoto Hideomi Koinuma 《应用有机金属化学》1991,5(4):325-330
For low-temperature deposition of oxide films relating to Bi-Sr-Ca-Cu-O superconductors, photo-absorption and -decomposition properties were examined with respect to copper and alkaline-earth ß-diketonates. It was confirmed that all ß-diketonates examined were promising as source materials for photochemical vapour deposition (photo-CVD) using a low-pressure mercury lamp, in view of their large light absorption coefficients at wavelength 254 nm. The light irradiation was effective for the formation of highly crystalline oxide films at temperatures below 600 °C. By combining two sources, Ca2CuO3 and SrCuO2 films were prepared. Photo-CVD of c-axis oriented Bi2Sr2CuOx film was achieved by the irradiation of ternary sources of Bi(C6H5)3 and strontium and copper ß-diketonates at 500 °C. 相似文献