全文获取类型
收费全文 | 338241篇 |
免费 | 2424篇 |
国内免费 | 894篇 |
专业分类
化学 | 162087篇 |
晶体学 | 5036篇 |
力学 | 16557篇 |
综合类 | 2篇 |
数学 | 50600篇 |
物理学 | 107277篇 |
出版年
2020年 | 1764篇 |
2019年 | 2037篇 |
2018年 | 10976篇 |
2017年 | 11786篇 |
2016年 | 7782篇 |
2015年 | 3437篇 |
2014年 | 4010篇 |
2013年 | 11266篇 |
2012年 | 12249篇 |
2011年 | 22150篇 |
2010年 | 13530篇 |
2009年 | 13499篇 |
2008年 | 19310篇 |
2007年 | 22855篇 |
2006年 | 8549篇 |
2005年 | 14618篇 |
2004年 | 10404篇 |
2003年 | 9530篇 |
2002年 | 7508篇 |
2001年 | 7357篇 |
2000年 | 5660篇 |
1999年 | 4248篇 |
1998年 | 3594篇 |
1997年 | 3593篇 |
1996年 | 3455篇 |
1995年 | 3228篇 |
1994年 | 3029篇 |
1993年 | 3071篇 |
1992年 | 3359篇 |
1991年 | 3401篇 |
1990年 | 3211篇 |
1989年 | 3172篇 |
1988年 | 3245篇 |
1987年 | 3107篇 |
1986年 | 2996篇 |
1985年 | 4062篇 |
1984年 | 4244篇 |
1983年 | 3477篇 |
1982年 | 3828篇 |
1981年 | 3720篇 |
1980年 | 3594篇 |
1979年 | 3643篇 |
1978年 | 3882篇 |
1977年 | 3627篇 |
1976年 | 3864篇 |
1975年 | 3435篇 |
1974年 | 3555篇 |
1973年 | 3898篇 |
1972年 | 2363篇 |
1971年 | 1807篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Robert M. O'Keefe 《The Journal of the Operational Research Society》1989,40(5):415-422
Cognitive style refers to differences that can be perceived in individuals due to differing cognitive structures. There has been a large amount of research on how cognitive style affects managerial decision-making, particularly in the context of management information systems, but little or no attempt to relate these findings to operational research. This paper reviews the findings of cognitive-style research from the perspective of operational research, and draws a number of useful inferences concerning the matching of OR aids to managers, the presentation of information and recommendations, the use and acceptance of OR, and the structure of OR groups. Further, it is argued that cognitive-style findings help to give a justification to the move towards ‘soft’ approaches to analysis. 相似文献
62.
63.
C. Ghezzi E. Gombia R. Mosca M. Pillan 《Applied Physics A: Materials Science & Processing》1989,48(5):457-463
Capacitance DLTS measurements have been performed in VPE GaAs MESFETs prepared on Bridgman Cr-doped and LEC undoped semi-insulating substrates. A band of electron traps not intrinsically related to the VPE growth process and accumulating near the metal (gate) — semiconductor interface was detected in all the samples. Deeper regions into the channel were free from any detectable trap. Near pinch-off conditions, a positive capacitance signal was found to dominate the DLTS spectra only in the case of samples prepared on Cr-doped substrates. The hypothesis of this positive transient being related to changes in the occupation of surface states in the ungated surface access regions has been checked by comparing experimental and calculated dependencies of the signal amplitude on reverse gate voltage. Unexplained discrepancies, together with the absence of positive signal in MESFETs prepared on LEC undoped substrates, suggest the possibility of hole emission from hole traps within the bulk of the device. 相似文献
64.
When solving large complex optimization problems, the user is faced with three major problems. These are (i) the cost in human time in obtaining accurate expressions for the derivatives involved; (ii) the need to store second derivative information; and (iii), of lessening importance, the time taken to solve the problem on the computer. For many problems, a significant part of the latter can be attributed to solving Newton-like equations. In the algorithm described, the equations are solved using a conjugate direction method that only needs the Hessian at the current point when it is multiplied by a trial vector. In this paper, we present a method that finds this product using automatic differentiation while only requiring vector storage. The method takes advantage of any sparsity in the Hessian matrix and computes exact derivatives. It avoids the complexity of symbolic differentiation, the inaccuracy of numerical differentiation, the labor of finding analytic derivatives, and the need for matrix store. When far from a minimum, an accurate solution to the Newton equations is not justified, so an approximate solution is obtained by using a version of Dembo and Steihaug's truncated Newton algorithm (Ref. 1).This paper was presented at the SIAM National Meeting, Boston, Massachusetts, 1986. 相似文献
65.
Dubrovin valuation rings and Henselization 总被引:4,自引:0,他引:4
Adrian R. Wadsworth 《Mathematische Annalen》1989,283(2):301-328
Supported in part by the National Science Foundation 相似文献
66.
B. R. Harvey M. B. Lovett S. J. Boggis 《Journal of Radioanalytical and Nuclear Chemistry》1987,115(2):357-368
Selected experiences in the control of contamination and the threat it poses to the quality of analytical date are discussed in the context of the whole analytical process from collection of marine environmental samples, through handling and radiochemical separation, to the final interpretation of results. Examples include a demonstration of the contamination introduced during sediment core sectioning, contamination of sea water by a ship's pumping system, and the effect of filtration on the apparent partitioning of radionuclides between solid and liquid phases of sea water. 相似文献
67.
William F. Bowlin 《The Journal of the Operational Research Society》1987,38(2):127-135
The objective of this research is to evaluate a new methodology, data envelopment analysis (DEA), as a tool for measuring and evaluating the operational efficiency of US Air Force organizations. This study involves the application of DEA to locate possible inefficiencies in the performance of US Air Force real-property maintenance activities. Dynamic behaviour, as exhibited via window analyses, is reviewed along with static counterparts. The testing was done in close coordination with Air Force officials, who reviewed the results for accuracy, validity and relevance. We conclude that this type of efficiency analysis does have value for the Air Force, where it can serve as a guide to auditors, budget programmers, managers and others in measuring, evaluating and enhancing operational efficiency. 相似文献
68.
69.
J.R. Cooper 《Journal of Molecular Spectroscopy》2006,239(1):51-58
The ν9 fundamental band of ethane occurs in the 12 μm region. It is the strongest band of ethane in a terrestrial window and is commonly used for the identification of ethane in the Jovian planets. The ν9 + ν4 − ν4 band occurs in the same region; neither can be analysed as an isolated band, since both are embedded in the torsional bath of the ground vibrational state. We report here two global fit models including data from both of these bands as well as the ν3 fundamental and the ν4, 2ν4 − ν4, and 3ν4 torsional transitions. The first is restricted to −5 ? KΔK ? 15 in the hot band and gives an excellent fit to the included data. Three resonant interactions are identified in this fit—a Coriolis interaction with two resonant cases between the ν9 torsional stack and that of the ground vibrational state (gs) and a resonant Fermi interaction between the ν3 fundamental and the gs. Hot band lines with KΔK < −5 are influenced by a fourth perturbation, with a crossing at −11 < KΔK < −10, which has been attributed to an interaction with the ν12 fundamental. A second fit, demonstrating a promising treatment of this interaction, is also presented. 相似文献
70.
E. V. Gromachevskaya A. G. Sakhabutdinov V. G. Kul'nevich I. S. Arustamova R. B. Valeev B. A. Bazhenov 《Chemistry of Heterocyclic Compounds》1991,27(5):502-503
It was shown that the reaction of o-hyroxyphenyldiphenylmethanol with benzaldehyde labeled with 17O and 18O isotopes proceeds in such a way that oxygen atoms of the starting phenoloalcohol are retained.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 632–633, May, 1991. 相似文献