全文获取类型
收费全文 | 38584篇 |
免费 | 125篇 |
国内免费 | 149篇 |
专业分类
系统科学 | 178篇 |
丛书文集 | 538篇 |
教育与普及 | 63篇 |
理论与方法论 | 132篇 |
现状及发展 | 16449篇 |
研究方法 | 1566篇 |
综合类 | 19203篇 |
自然研究 | 729篇 |
出版年
2013年 | 345篇 |
2012年 | 550篇 |
2011年 | 1273篇 |
2010年 | 241篇 |
2008年 | 637篇 |
2007年 | 802篇 |
2006年 | 740篇 |
2005年 | 764篇 |
2004年 | 806篇 |
2003年 | 679篇 |
2002年 | 707篇 |
2001年 | 1284篇 |
2000年 | 1231篇 |
1999年 | 769篇 |
1992年 | 728篇 |
1991年 | 558篇 |
1990年 | 618篇 |
1989年 | 607篇 |
1988年 | 569篇 |
1987年 | 571篇 |
1986年 | 630篇 |
1985年 | 778篇 |
1984年 | 569篇 |
1983年 | 502篇 |
1982年 | 433篇 |
1981年 | 440篇 |
1980年 | 513篇 |
1979年 | 1245篇 |
1978年 | 954篇 |
1977年 | 922篇 |
1976年 | 753篇 |
1975年 | 770篇 |
1974年 | 1148篇 |
1973年 | 931篇 |
1972年 | 1003篇 |
1971年 | 1125篇 |
1970年 | 1521篇 |
1969年 | 1150篇 |
1968年 | 1068篇 |
1967年 | 1025篇 |
1966年 | 995篇 |
1965年 | 709篇 |
1964年 | 212篇 |
1959年 | 370篇 |
1958年 | 689篇 |
1957年 | 474篇 |
1956年 | 372篇 |
1955年 | 352篇 |
1954年 | 376篇 |
1948年 | 258篇 |
排序方式: 共有10000条查询结果,搜索用时 37 毫秒
11.
复数,实数及实对称数据下Bruun FFT算法的实现 总被引:3,自引:0,他引:3
本文在复数、实数以及实对称数据下研究和实现了Bruun FFT算法。文章分析和评价了各种数据下BruunFFT的运算量和特点,提出了算法的实现结构和方法。最后给了TMS320C30上的运算时间,我们实现的算法比基2FFT速度提高35%。 相似文献
12.
13.
Romeo S Pennacchio LA Fu Y Boerwinkle E Tybjaerg-Hansen A Hobbs HH Cohen JC 《Nature genetics》2007,39(4):513-516
Resequencing genes provides the opportunity to assess the full spectrum of variants that influence complex traits. Here we report the first application of resequencing to a large population (n = 3,551) to examine the role of the adipokine ANGPTL4 in lipid metabolism. Nonsynonymous variants in ANGPTL4 were more prevalent in individuals with triglyceride levels in the lowest quartile than in individuals with levels in the highest quartile (P = 0.016). One variant (E40K), present in approximately 3% of European Americans, was associated with significantly lower plasma levels of triglyceride and higher levels of high-density lipoprotein cholesterol in European Americans from the Atherosclerosis Risk in Communities Study and in Danes from the Copenhagen City Heart Study. The ratio of nonsynonymous to synonymous variants was higher in European Americans than in African Americans (4:1 versus 1.3:1), suggesting population-specific relaxation of purifying selection. Thus, resequencing of ANGPTL4 in a multiethnic population allowed analysis of the phenotypic effects of both rare and common variants while taking advantage of genetic variation arising from ethnic differences in population history. 相似文献
14.
F. Weygand A. Wacker V. Schmied-Kowarzik 《Cellular and molecular life sciences : CMLS》1948,4(11):427-428
Summary By condensing 2:4:5-triamino-6-hydroxy-pyrimidine with dihydroxyacetone (diacetate), diaminoacetone or acetone-1,3-di (p-formylaminobenzoic acid) not the expected 8- or 9-oxymethyl resp. -aminomethyl-pteridines but 8-or 9-methyl-pteridines were obtained. With p-tolyl-d-isoglucosamine not a tetrahydroxybutyl-pteridine but a trihydroxybutyl-pteridine was formed. For an explanation of these results it is supposed that from the dihydro-pteridines formed at first by intramolecular splitting off of H2O or R·NH2 aromatization takes place. 相似文献
15.
16.
Houlden H Johnson J Gardner-Thorpe C Lashley T Hernandez D Worth P Singleton AB Hilton DA Holton J Revesz T Davis MB Giunti P Giunti P Wood NW 《Nature genetics》2007,39(12):1434-1436
The microtubule-associated protein tau (encoded by MAPT) and several tau kinases have been implicated in neurodegeneration, but only MAPT has a proven role in disease. We identified mutations in the gene encoding tau tubulin kinase 2 (TTBK2) as the cause of spinocerebellar ataxia type 11. Affected brain tissue showed substantial cerebellar degeneration and tau deposition. These data suggest that TTBK2 is important in the tau cascade and in spinocerebellar degeneration. 相似文献
17.
18.
Tissue-specific and reversible RNA interference in transgenic mice 总被引:11,自引:0,他引:11
Dickins RA McJunkin K Hernando E Premsrirut PK Krizhanovsky V Burgess DJ Kim SY Cordon-Cardo C Zender L Hannon GJ Lowe SW 《Nature genetics》2007,39(7):914-921
Genetically engineered mice provide powerful tools for understanding mammalian gene function. These models traditionally rely on gene overexpression from transgenes or targeted, irreversible gene mutation. By adapting the tetracycline (tet)-responsive system previously used for gene overexpression, we have developed a simple transgenic system to reversibly control endogenous gene expression using RNA interference (RNAi) in mice. Transgenic mice harboring a tet-responsive RNA polymerase II promoter driving a microRNA-based short hairpin RNA targeting the tumor suppressor Trp53 reversibly express short hairpin RNA when crossed with existing mouse strains expressing general or tissue-specific 'tet-on' or 'tet-off' transactivators. Reversible Trp53 knockdown can be achieved in several tissues, and restoring Trp53 expression in lymphomas whose development is promoted by Trp53 knockdown leads to tumor regression. By leaving the target gene unaltered, this approach permits tissue-specific, reversible regulation of endogenous gene expression in vivo, with potential broad application in basic biology and drug target validation. 相似文献
19.
Risheg H Graham JM Clark RD Rogers RC Opitz JM Moeschler JB Peiffer AP May M Joseph SM Jones JR Stevenson RE Schwartz CE Friez MJ 《Nature genetics》2007,39(4):451-453
Opitz-Kaveggia syndrome (also known as FG syndrome) is an X-linked disorder characterized by mental retardation, relative macrocephaly, hypotonia and constipation. We report here that the original family for whom the condition is named and five other families have a recurrent mutation (2881C>T, leading to R961W) in MED12 (also called TRAP230 or HOPA), a gene located at Xq13 that functions as a thyroid receptor-associated protein in the Mediator complex. 相似文献
20.
Komen JC Distelmaier F Koopman WJ Wanders RJ Smeitink J Willems PH 《Cellular and molecular life sciences : CMLS》2007,64(24):3271-3281
Refsum disease is a rare, inherited neurodegenerative disorder characterized by accumulation of the dietary branched-chain
fatty acid phytanic acid in plasma and tissues caused by a defect in the alphaoxidation pathway. The accumulation of phytanic
acid is believed to be the main pathophysiological cause of the disease. However, the exact mechanism(s) by which phytanic
acid exerts its toxicity have not been resolved. In this study, the effect of phytanic acid on mitochondrial respiration was
investigated. The results show that in digitonin-permeabilized fibroblasts, phytanic acid decreases ATP synthesis, whereas
substrate oxidation per se is not affected. Importantly, studies in intact fibroblasts revealed that phytanic acid decreases both the mitochondrial
membrane potential and NAD(P)H autofluorescence. Taken together, the results described here show that unesterified phytanic
acid exerts its toxic effect mainly through its protonophoric action, at least in human skin fibroblasts.
Received 4 August 2007; received after revision 26 September 2007; accepted 10 October 2007
J. C. Komen, F. Distelmaier: These authors contributed equally to this work. 相似文献