全文获取类型
收费全文 | 15473篇 |
免费 | 53篇 |
国内免费 | 92篇 |
专业分类
系统科学 | 97篇 |
丛书文集 | 110篇 |
教育与普及 | 95篇 |
理论与方法论 | 80篇 |
现状及发展 | 6182篇 |
研究方法 | 677篇 |
综合类 | 8096篇 |
自然研究 | 281篇 |
出版年
2012年 | 232篇 |
2011年 | 566篇 |
2010年 | 118篇 |
2009年 | 159篇 |
2008年 | 268篇 |
2007年 | 313篇 |
2006年 | 285篇 |
2005年 | 332篇 |
2004年 | 380篇 |
2003年 | 297篇 |
2002年 | 302篇 |
2001年 | 483篇 |
2000年 | 485篇 |
1999年 | 337篇 |
1992年 | 298篇 |
1991年 | 235篇 |
1990年 | 254篇 |
1989年 | 240篇 |
1988年 | 249篇 |
1987年 | 260篇 |
1986年 | 228篇 |
1985年 | 338篇 |
1984年 | 251篇 |
1983年 | 198篇 |
1982年 | 174篇 |
1981年 | 169篇 |
1980年 | 212篇 |
1979年 | 465篇 |
1978年 | 381篇 |
1977年 | 338篇 |
1976年 | 301篇 |
1975年 | 306篇 |
1974年 | 399篇 |
1973年 | 374篇 |
1972年 | 325篇 |
1971年 | 442篇 |
1970年 | 566篇 |
1969年 | 421篇 |
1968年 | 398篇 |
1967年 | 398篇 |
1966年 | 425篇 |
1965年 | 276篇 |
1964年 | 104篇 |
1959年 | 132篇 |
1958年 | 250篇 |
1957年 | 146篇 |
1956年 | 155篇 |
1955年 | 118篇 |
1954年 | 99篇 |
1948年 | 135篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
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
22.
23.
24.
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
25.
It is well known that transmitter secretion requires specialized secretory organelles, the synaptic vesicles, for the packaging, storage and exocytotic release of the transmitter. Here we report that when acetylcholine (ACh) is loaded into an isolated Xenopus myocyte, there is spontaneous quantal release of ACh from the myocyte which results in activation of its own surface ACh channels and the appearance of membrane currents resembling miniature endplate currents. This myocyte secretion probably reflects Ca(2+)-regulated exocytosis of ACh-filled cytoplasmic compartments. Furthermore, step depolarization of the myocyte membrane triggers evoked ACh release from the myocyte with a weak excitation-secretion coupling. These findings suggest that quantal transmitter secretion does not require secretory pathways unique to neurons and that the essence of presynaptic differentiation may reside in the provision of transmitter supply and modification of the preexisting secretion pathway. 相似文献
26.
A. F. McDonagh Y. -M. Pu D. A. Lightner 《Cellular and molecular life sciences : CMLS》1992,48(3):246-248
The characteristic circular dichroism of bilirubin bound to human serum albumin undergoes a remarkable sign inversion on addition of halothane, chloroform and other volatile anesthetics. This sign inversion, which is completely reversed by removal of the anesthetic, reflects a pronounced conformational change of the bound ligand; probably a complete inversion of chirality. The observation suggests that association of volatile anesthetics with proteins can markedly alter the internal topography of receptor sites and potentially influence the stereoselectivity of ligand binding. 相似文献
27.
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
28.
K K Leong K M Yu W B Lee 《厦门大学学报(自然科学版)》2002,(Z1)
Product data management (PDM) has been accepted as an important tool for the manufacturing industries. In recent years, more and mor e researches have been conducted in the development of PDM. Their research area s include system design, integration of object-oriented technology, data distri bution, collaborative and distributed manufacturing working environment, secur ity, and web-based integration. However, there are limitations on their rese arches. In particular, they cannot cater for PDM in dis... 相似文献
29.
Summary Host cell reactivation capacity for ozone T3 phage was determined for differentE. coli strains deficient in one or more of the DNA repair mechanisms. The results indicate that DNA polymerase I appears to play a key role in the repair of damage produced on the DNA by ozone while thelexA gene product seems to play a minor one.This research was sponsored by the National Sciences and Engineering Council of Canada. One of us (PLH) acknowledges a scholarship from the NSERCof Canada. 相似文献
30.
Chromosomal effects of adeno-associated virus vector integration. 总被引:17,自引:0,他引:17
Adeno-associated virus (AAV) vectors are currently being used in several clinical gene-therapy trials (see the NIH OBA Human Gene Transfer Clinical Trials Database); however, little is known about the chromosomal effects of vector integration. Here we report that integrated vector proviruses are associated with chromosomal deletions and other rearrangements and are frequently located on chromosome 19 (although not at the wildtype AAV integration site). 相似文献