全文获取类型
收费全文 | 14649篇 |
免费 | 768篇 |
国内免费 | 66篇 |
专业分类
化学 | 10704篇 |
晶体学 | 41篇 |
力学 | 339篇 |
数学 | 2272篇 |
物理学 | 2127篇 |
出版年
2023年 | 116篇 |
2022年 | 138篇 |
2021年 | 248篇 |
2020年 | 343篇 |
2019年 | 367篇 |
2018年 | 181篇 |
2017年 | 197篇 |
2016年 | 519篇 |
2015年 | 574篇 |
2014年 | 587篇 |
2013年 | 803篇 |
2012年 | 1127篇 |
2011年 | 1124篇 |
2010年 | 690篇 |
2009年 | 604篇 |
2008年 | 874篇 |
2007年 | 851篇 |
2006年 | 818篇 |
2005年 | 773篇 |
2004年 | 660篇 |
2003年 | 518篇 |
2002年 | 486篇 |
2001年 | 238篇 |
2000年 | 189篇 |
1999年 | 179篇 |
1998年 | 190篇 |
1997年 | 187篇 |
1996年 | 147篇 |
1995年 | 135篇 |
1994年 | 124篇 |
1993年 | 117篇 |
1992年 | 95篇 |
1991年 | 91篇 |
1990年 | 69篇 |
1989年 | 91篇 |
1988年 | 63篇 |
1987年 | 46篇 |
1986年 | 48篇 |
1985年 | 67篇 |
1984年 | 56篇 |
1983年 | 55篇 |
1982年 | 62篇 |
1981年 | 65篇 |
1980年 | 51篇 |
1979年 | 71篇 |
1978年 | 58篇 |
1977年 | 56篇 |
1976年 | 56篇 |
1975年 | 46篇 |
1973年 | 46篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
31.
Accreditation and Quality Assurance - 相似文献
32.
Fabio A. C. C. Chalub Peter A. Markowich Benoît Perthame Christian Schmeiser 《Monatshefte für Mathematik》2004,142(1-2):123-141
Kinetic models for chemotaxis, nonlinearly coupled to a Poisson equation for the chemo-attractant density, are considered. Under suitable assumptions on the turning kernel (including models introduced by Othmer, Dunbar and Alt), convergence in the macroscopic limit to a drift-diffusion model is proven. The drift-diffusion models derived in this way include the classical Keller-Segel model. Furthermore, sufficient conditions for kinetic models are given such that finite-time-blow-up does not occur. Examples are given satisfying these conditions, whereas the macroscopic limit problem is known to exhibit finite-time-blow-up. The main analytical tools are entropy techniques for the macroscopic limit as well as results from potential theory for the control of the chemo-attractant density.Present address: Centro de Matemática e Aplicações Fundamentais, Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003, Lisboa, Portugal 相似文献
33.
34.
35.
36.
We present a systematic comparison of the correlation contribution at the level of the second-order polarization propagator approximation (SOPPA ) and MP 2 to the static dipole polarizability of (1) Be, BeH?, BH, CH+, MgH?, AIH, SiH+, and GeH+; (2) BH3, CH4, NH3, H2O, HF, BF, and F2; and (3) N2, CO, CN?, HCN, C2H2, and HCHO . Fairly extended basis sets were used in the calculations. We find that the agreement with experimental values is improved in SOPPA and MP .2 over the results at the SCF level. The signs and magnitudes of the correlation contribution in SOPPA are similar to those obtained in analytical derivative MP 2 calculations. However, it is not possible to say, in general, which method gives the largest correlation contribution or the best agreement with experiment, nor is it possible to make a priori prediction of the sign of the correlation contribution. For the first group of molecules, which have a quasi-degenerate ground state, additional CCDPPA and CCSDPPA calculations were performed and compared with polarizabilities obtained as analytical/numerical derivatives of the CCD and CCSD energies. The CCSDPPA results were found to be in better agreement with other calculations than were the SOPPA results, demonstrating the necessity of using methods based on infinite-order perturbation theory for these systems. © 1994 John Wiley & Sons, Inc. 相似文献
37.
Christian Prins 《The Journal of the Operational Research Society》1994,45(6):611-623
Satellite communications, like batches of work in a job shop, need to be scheduled in order to use their resources as efficiently as possible. The most common satellite communications system in use today is known as Time Division Multiple Access (TDMA), in which data from earth stations is buffered before being transmitted to the appropriate receiver on a satellite. Cycles of transmission are fixed for all stations. Since the same satellite will be used for routeing data in several different ways, a schedule must be devised to use the receivers, repeaters and transmitters on board to minimize the time needed for completion of a batch of work. This paper is a survey of current scheduling algorithms used for optimizing satellite communications resources. Apart from telecommunications, the methods presented here could be applied to more general scheduling problems with renewable resources but without precedence constraints. 相似文献
38.
Ingrid Lukkari Jaromir Ruzicka Gary D. Christian 《Fresenius' Journal of Analytical Chemistry》1993,346(6-9):813-818
Summary Automated methods for the determination of ammonium and ammonia are reviewed, and techniques based on gas diffusion using a semi-permeable membrane were selected for the determination of both total ammonium-nitrogen and free ammonia in fermentation samples. A simple and robust instrument based on sequential injection analysis (SIA) consisting of a piston pump and two selector valves was used. Two different methods of ammonia detection, the Berthelot method and detection using pH-indicators, have been evaluated and compared. The indicator method showed higher reproducibility and the range of determination could easily be adjusted to match the concentrations of the samples. The range of determination could be adjusted from 0.05 mmol/l to 350 mmol/l, depending on choice of acceptor solution. This method has been evaluated with fermentation medium samples and tested on-line in a yeast fermentation process.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday 相似文献
39.
Sudoku’s french ancestors 总被引:1,自引:0,他引:1
Christian Boyer 《Mathematical Intelligencer》2007,29(1):37-44
This column is a place for those bits of contagious mathematics that travel from person to person in the community, because
they are so elegant, suprising, or appealing that one has an urge to pass them on.
Contributions are most welcome. 相似文献
40.
Christian Remling 《Communications in Mathematical Physics》2007,271(1):275-287
I present an example of a discrete Schrödinger operator that shows that it is possible to have embedded singular spectrum and, at the same time, discrete eigenvalues that approach the edges of the essential spectrum (much) faster than exponentially. This settles a conjecture of Simon (in the negative). The potential is of von Neumann-Wigner type, with careful navigation around a previously identified borderline situation. 相似文献