全文获取类型
收费全文 | 20972篇 |
免费 | 757篇 |
国内免费 | 57篇 |
专业分类
系统科学 | 1271篇 |
丛书文集 | 40篇 |
教育与普及 | 42篇 |
理论与方法论 | 387篇 |
现状及发展 | 6556篇 |
研究方法 | 595篇 |
综合类 | 12574篇 |
自然研究 | 321篇 |
出版年
2018年 | 750篇 |
2017年 | 756篇 |
2016年 | 502篇 |
2015年 | 126篇 |
2013年 | 153篇 |
2012年 | 686篇 |
2011年 | 1648篇 |
2010年 | 918篇 |
2009年 | 565篇 |
2008年 | 731篇 |
2007年 | 996篇 |
2006年 | 273篇 |
2005年 | 499篇 |
2004年 | 945篇 |
2003年 | 861篇 |
2002年 | 460篇 |
2001年 | 431篇 |
2000年 | 499篇 |
1999年 | 307篇 |
1992年 | 242篇 |
1991年 | 196篇 |
1990年 | 211篇 |
1989年 | 217篇 |
1988年 | 207篇 |
1987年 | 212篇 |
1986年 | 194篇 |
1985年 | 288篇 |
1984年 | 213篇 |
1983年 | 166篇 |
1982年 | 134篇 |
1981年 | 143篇 |
1980年 | 161篇 |
1979年 | 409篇 |
1978年 | 312篇 |
1977年 | 281篇 |
1976年 | 272篇 |
1975年 | 276篇 |
1974年 | 367篇 |
1973年 | 338篇 |
1972年 | 291篇 |
1971年 | 411篇 |
1970年 | 529篇 |
1969年 | 398篇 |
1968年 | 392篇 |
1967年 | 366篇 |
1966年 | 412篇 |
1965年 | 276篇 |
1958年 | 210篇 |
1956年 | 130篇 |
1948年 | 133篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
41.
In October 1924, The Physical Review, a relatively minor journal at the time, published a remarkable two-part paper by John H. Van Vleck, working in virtual isolation
at the University of Minnesota. Using Bohr’s correspondence principle and Einstein’s quantum theory of radiation along with
advanced techniques from classical mechanics, Van Vleck showed that quantum formulae for emission, absorption, and dispersion
of radiation merge with their classical counterparts in the limit of high quantum numbers. For modern readers Van Vleck’s
paper is much easier to follow than the famous paper by Kramers and Heisenberg on dispersion theory, which covers similar
terrain and is widely credited to have led directly to Heisenberg’s Umdeutung paper. This makes Van Vleck’s paper extremely valuable for the reconstruction of the genesis of matrix mechanics. It also
makes it tempting to ask why Van Vleck did not take the next step and develop matrix mechanics himself.
This paper was written as part of a joint project in the history of quantum physics of the Max Planck Institut für Wissenschaftsgeschichte and the Fritz-Haber-Institut in Berlin. 相似文献
42.
43.
DING Mingtao WEI Fangqiang ZHANG Jinghong WANG Hongjuan 《武汉大学学报:自然科学英文版》2007,12(4):599-604
By analyzing the observation data from Dongchuan Debris Flow Observation and Research Station and historical data from year 1965 to 1990 gotten from National Astronomical Ob-servatories/Yunnan Observatory,the responding of debris flow in Jiangjia Ravine to Solar Proton Flare is studied. The following conclusion can be drawn. Solar Proton Flare,as one of most im-portant astronomical factors,affects the activity of debris flow in Yunnan. Generally,from 1965 to1990,the more active Solar Pro-ton Flare is,the greater the probability of high frequency and large runoff of debris flow is. On the contrary,the less active Solar Pro-ton Flare is,the greater the probability of low frequency,small runoff,and low sediment transport of debris flow is. 相似文献
44.
This paper assesses the information content of two survey indicators for consumption developments in the near future for eight European countries in the period 1985–1998. Empirical work on this topic typically focuses on consumer confidence, the perceptions of buyers of consumption goods. This paper examines whether perceptions of sellers of consumption goods, measured by retail trade surveys, may also improve short‐term monitoring of consumption. We find that both consumer confidence and retailer confidence embody valuable information, when analysed in isolation. For France, Italy and Spain we conclude that adding retail confidence does not improve the indicator model once consumer confidence has been included. For the UK the reverse case is obtained. For the remaining four countries we show that combining consumer sentiment and retail trade confidence into a composite indicator leads to optimal results. Our results suggest that incorporating information from retail trade surveys may offer significant benefits for the analysis of short‐term prospects of consumption. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
45.
46.
The unified chaotic system contains the Lorenz system and the Chen system as two dual systems at the two extremes of its parameter
spectrum. This paper presents the design of bang bang controller for unified system and multitude of numerical experiments
under various control parameters. Numerical experiments meet the theoretic proof perfectly and convincingly demonstrated the
controller can be effectively used for unified systems with uncertainty of the equilibrium points. The method enriches the
applications of chaotic control.
Foundation item: Supported by the National Natural Science Foundation of China(50209012)
Biography: Deng Xiao-ming (1980-), male, Master candidate, research direction: chaos control. 相似文献
47.
48.
Obese postmenopausal female volunteers were given timed daily oral dosages of bromocriptine, and tested for reduction of body fat stores. This dopamine agonist has been shown to reset circadian rhythms that are altered in obese animals and to reduce body fat levels in several animal models. The participants were instructed not to alter their existing exercise and eating behavior during treatment. Skinfold measurements were taken on 33 subjects as indices of body fat. The measurements (e.g., suprailiac) were reduced after six weeks by about 25%, which represents a reduction of 11.7% of the total body fat. These dramatic decreases in body fat, which are equivalent to that produced by severe caloric restriction, were accompanied by more modest reductions of body weight (2.5%), indicating a possible conservation of protein that is usually lost as a consequence of such caloric restriction. The effects of bromocriptine treatment on body fat and hyperglycemia were also examined in non-insulin dependent diabetics being treated with oral hypoglycemics (7 subjects) or insulin (7 subjects). Total body fat was reduced by 10.7% and 5.1% in diabetics on oral hypoglycemics and insulin, respectively, without any significant reductions in body weight. Hyperglycemia was reduced in most of the 15 diabetic subjects treated leading to euglycemia and even cessation of hypoglycemic drugs in 3 of the 7 subjects during 4-8 weeks of bromocriptine treatment. These findings support the hypothesis that obesity and type II diabetes may be treated effectively with bromocriptine when administered at the proper times and dosages. 相似文献
49.
Short alanine peptides, containing 16 or 17 residues, appear to form alpha-helices in aqueous solution. But the main spectroscopic analyses used on helical peptides (circular dichroism and nuclear magnetic resonance) cannot distinguish between an alpha-helix (in which the ith residue is hydrogen-bonded to residue i + 4; ref. 9) and the next most common peptide helix, the 3(10)-helix10 (i-->i + 3 hydrogen-bonding). To address this problem we have designed single and doubly spin-labelled analogues of alanine-based peptides in which the nitroxide spin label forms an unbranched side chain extending from the sulphur atom of a cysteine residue. Here we report the circular dichroism, Fourier-transform infrared and electron-spin resonance spectra of these peptides under helix-forming conditions. The infrared absorbance gives an amide I' band with a frequency that is substantially different from that observed for alpha-helices. The electron-spin resonance spectra of doubly labelled helices show that the ranking of distances between side chains, around a single turn (residues 4-8), is inconsistent with an alpha-helical structure. Our experiments suggest that the more likely peptide geometry is a 3(10)-helix. 相似文献
50.
Ana Cuesta Laia Pedrola Teresa Sevilla Javier García-Planells María José Chumillas Fernando Mayordomo Eric LeGuern Ignacio Marín Juan J Vílchez Francesc Palau 《Nature genetics》2002,30(1):22-25
We identified three distinct mutations and six mutant alleles in GDAP1 in three families with axonal Charcot-Marie-Tooth (CMT) neuropathy and vocal cord paresis, which were previously linked to the CMT4A locus on chromosome 8q21.1. These results establish the molecular etiology of CMT4A (MIM 214400) and suggest that it may be associated with both axonal and demyelinating phenotypes. 相似文献