全文获取类型
收费全文 | 1751篇 |
免费 | 94篇 |
国内免费 | 9篇 |
专业分类
化学 | 1318篇 |
晶体学 | 4篇 |
力学 | 27篇 |
数学 | 244篇 |
物理学 | 261篇 |
出版年
2023年 | 9篇 |
2022年 | 12篇 |
2021年 | 25篇 |
2020年 | 35篇 |
2019年 | 47篇 |
2018年 | 26篇 |
2017年 | 24篇 |
2016年 | 38篇 |
2015年 | 54篇 |
2014年 | 70篇 |
2013年 | 91篇 |
2012年 | 167篇 |
2011年 | 186篇 |
2010年 | 70篇 |
2009年 | 44篇 |
2008年 | 122篇 |
2007年 | 120篇 |
2006年 | 112篇 |
2005年 | 104篇 |
2004年 | 74篇 |
2003年 | 68篇 |
2002年 | 43篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 12篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 13篇 |
1995年 | 6篇 |
1994年 | 16篇 |
1993年 | 9篇 |
1992年 | 10篇 |
1990年 | 4篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1985年 | 8篇 |
1983年 | 5篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1978年 | 12篇 |
1977年 | 9篇 |
1976年 | 10篇 |
1975年 | 10篇 |
1974年 | 13篇 |
1973年 | 15篇 |
1972年 | 13篇 |
1968年 | 4篇 |
1967年 | 3篇 |
排序方式: 共有1854条查询结果,搜索用时 578 毫秒
1.
2.
Recently extremal double circulant self-dual codes have been classified for lengths n ≤ 62. In this paper, a complete classification
of extremal double circulant self-dual codes of lengths 64 to 72 is presented. Almost all of the extremal double circulant
singly-even codes given have weight enumerators for which extremal codes were not previously known to exist. 相似文献
3.
4.
Summary The optimality question for blockp-cyclic SOR iterations discussed in Young and Varga is answered under natural conditions on the spectrum of the block Jacobi matrix. In particular, it is shown that repartitioning a blockp-cyclic matrix into a blockq-cyclic form,q
, results in asymptotically faster SOR convergence for the same amount of work per iteration. As a consequence block 2-cyclic SOR is optimal under these conditions.Research supported in part by the US Air Force under Grant no. AFOSR-88-0285 and the National Science Foundation under grant no. DMS-85-21154 Present address: Boeing Computer Services, P.O. Box 24346, MS 7L-21, Seattle, WA 98124-0346, USA 相似文献
5.
Robyn Pierce Kaye Stacey 《International Journal of Computers for Mathematical Learning》2004,9(1):59-93
This article suggests a framework to organise a cluster of variables that are associated with students' effective use of computer algebra systems (CAS) in mathematics learning. Based on a review of the literature and from the authors' own teaching experience, the framework identifies the main characteristics of students' interactions with CAS technology and how these may be used to monitor students' developing use of CAS; from this, the framework may be used to plan teaching in order to gain greater benefit from the availability of CAS. Four case studies describing students' development over a semester are reported. These demonstrate a variety of combinations of technical competencies and personal attributes. They indicate the importance of both the technical and personal aspects but suggest that negative attitudes rather than technical difficulties can limit the effective use of CAS. Finally practical suggestions are given for teaching strategies which may promote effective use of CAS.This revised version was published online in September 2005 with corrections to the Cover Date. 相似文献
6.
Arndt Meier Aaron Goldman Thomas M. Stephen Nicholas B. Jones 《Journal of Quantitative Spectroscopy & Radiative Transfer》2004,83(1):109-113
High-resolution ground-based infrared solar spectra are routinely recorded at the Network for the Detection of Stratospheric Change (NDSC) stations. These data sets play a key role in providing a long-term record of atmospheric composition and their links to climate change. The analysis of observed infrared spectra involves comparison to a computer-modeled atmosphere where knowledge of the air mass distribution is an essential component. This note summarises improvements made to an existing and widely used computer code (FSCATM) to perform refractive ray-tracing and calculation of the air mass distribution. Changes were made towards higher vertical resolution in the troposphere and increased numerical precision. The revised FSCATM improves the analysis of infrared spectra mostly through the more accurate representation of the temperature profile. Air mass differences with respect to earlier versions are documented and are typically <0.7%, exceptions being extreme cases of inversion layers. The current version provides ray tracing and air mass calculations for any terrestrial observation site. The output files are reported in a format compatible with the SFIT and SFIT2 retrieval algorithms, which are widely used for NDSC infrared atmospheric studies. The improved computer code, documentation, reference profiles, and test cases are available electronically. 相似文献
7.
8.
Zhao Y Slepkov AD Akoto CO McDonald R Hegmann FA Tykwinski RR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,11(1):321-329
Monodisperse, cross-conjugated perphenylated iso-polydiacetylene (iso-PDA) oligomers, ranging from monomer 15 to pentadecamer 25, have been synthesized by using a palladium-catalyzed cross-coupling protocol. Structural characteristics elucidated by X-ray crystallographic analysis demonstrate a non-planar backbone conformation for the oligomers due to the steric interactions between alkylidene phenyl groups. The electronic absorption spectra of the oligomers show a slight red-shift of the maximum absorption wavelength as the chain length increases from dimer 17 b to pentadecamer 25, a trend that has saturated by the stage of nonamer 22. Fluorescence spectroscopy confirms that the pendent phenyl groups present on the oligomer framework enhance emission, and the relative emission intensity consistently increases as a function of chain length n. The molecular third-order nonlinearities, gamma, for this oligomer series have been measured via differential optical Kerr effect (DOKE) detection and show a superlinear increase as a function of the oligomer chain length n. Molecular modeling and spectroscopic studies suggest that iso-PDA oligomers (n>7) adopt a coiled, helical conformation in solution. 相似文献
9.
10.
Six methacrylate or acrylate derivatives of natural amino acids were synthesized and characterized. Based upon these monomers, six terpolymers [poly(acrylic) acid-co-itaconic acid-co-amino acid] were prepared and characterized. The synthesized polymers were used to formulate glass-ionomer cements (GICs) using Fuji II glass filler. The effects of the molecular weight (MW) and powder/liquid (P/L) ratio were evaluated. Scanning electron microscopy (SEM) was used to examine the fracture surfaces of the selected cement specimens. Results show that all the amino acid modified GICs exhibited higher compressive strengths (CS, 193-236 MPa) and much higher flexural strengths (FS, 55-71 MPa) as compared to commercial Fuji II GIC (191 in CS and 16 in FS). Both MW and P/L ratio affected the strength of the formed cement. It was important to find the optimal MW and P/L ratio to obtain the highest FS. In this study, optimized MW (number average) of the polyacids and P/L ratio were around 50,000 and 2.7/1, respectively. The microstructures of the fracture surfaces helped to explain the strength differences among the materials tested in the study. SEM analysis suggests that more integrated microstructures and fewer defects can lead to higher FS. 相似文献