全文获取类型
收费全文 | 1759篇 |
免费 | 6篇 |
国内免费 | 24篇 |
专业分类
系统科学 | 34篇 |
丛书文集 | 3篇 |
教育与普及 | 18篇 |
理论与方法论 | 8篇 |
现状及发展 | 351篇 |
研究方法 | 140篇 |
综合类 | 1155篇 |
自然研究 | 80篇 |
出版年
2018年 | 8篇 |
2016年 | 7篇 |
2014年 | 8篇 |
2013年 | 14篇 |
2012年 | 50篇 |
2011年 | 137篇 |
2010年 | 17篇 |
2009年 | 25篇 |
2008年 | 64篇 |
2007年 | 80篇 |
2006年 | 99篇 |
2005年 | 103篇 |
2004年 | 138篇 |
2003年 | 89篇 |
2002年 | 66篇 |
2001年 | 83篇 |
2000年 | 96篇 |
1999年 | 51篇 |
1998年 | 10篇 |
1997年 | 9篇 |
1993年 | 7篇 |
1992年 | 30篇 |
1991年 | 18篇 |
1990年 | 25篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 20篇 |
1986年 | 20篇 |
1985年 | 19篇 |
1984年 | 16篇 |
1983年 | 12篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 12篇 |
1979年 | 19篇 |
1978年 | 19篇 |
1977年 | 21篇 |
1976年 | 18篇 |
1975年 | 21篇 |
1974年 | 27篇 |
1973年 | 17篇 |
1972年 | 21篇 |
1971年 | 31篇 |
1970年 | 47篇 |
1969年 | 17篇 |
1968年 | 28篇 |
1967年 | 27篇 |
1966年 | 22篇 |
1965年 | 11篇 |
1964年 | 8篇 |
排序方式: 共有1789条查询结果,搜索用时 710 毫秒
151.
152.
153.
154.
155.
156.
Inada N Oguri M Pindor B Hennawi JF Chiu K Zheng W Ichikawa S Gregg MD Becker RH Suto Y Strauss MA Turner EL Keeton CR Annis J Castander FJ Eisenstein DJ Frieman JA Fukugita M Gunn JE Johnston DE Kent SM Nichol RC Richards GT Rix HW Sheldon ES Bahcall NA Brinkmann J Ivezić Z Lamb DQ McKay TA Schneider DP York DG 《Nature》2003,426(6968):810-812
Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures (that is, clusters of galaxies and even larger assemblies) predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004 + 4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model. 相似文献
157.
158.
159.
Many natural habitats exist on privately owned land outside protected areas, but few governments can afford to enforce or subsidize conservation of this biodiversity. Even in some developed countries, conservation subsidy schemes have only achieved limited success. Fortunately, some landowners may be willing to accept management costs in return for other benefits, although this remains controversial when it involves the killing of charismatic species. For example, participants in British field sports, such as fox hunting and game-bird shooting, may voluntarily conserve important habitats that are required by quarry species. Here we report results from a multidisciplinary study that addressed this issue by focusing on three sites across central England. We found that landowners participating in field sports maintained the most established woodland and planted more new woodland and hedgerows than those who did not, despite the equal availability of subsidies. Therefore, voluntary habitat management appears to be important for biodiversity conservation in Britain. Current debates on the future of field sports in Britain, and similar activities globally, may benefit from considering their utility as incentives to conserve additional habitat on private land. 相似文献
160.
Nicastrin modulates presenilin-mediated notch/glp-1 signal transduction and betaAPP processing 总被引:26,自引:0,他引:26
Yu G Nishimura M Arawaka S Levitan D Zhang L Tandon A Song YQ Rogaeva E Chen F Kawarai T Supala A Levesque L Yu H Yang DS Holmes E Milman P Liang Y Zhang DM Xu DH Sato C Rogaev E Smith M Janus C Zhang Y Aebersold R Farrer LS Sorbi S Bruni A Fraser P St George-Hyslop P 《Nature》2000,407(6800):48-54
Nicastrin, a transmembrane glycoprotein, forms high molecular weight complexes with presenilin 1 and presenilin 2. Suppression of nicastrin expression in Caenorhabditis elegans embryos induces a subset of notch/glp-1 phenotypes similar to those induced by simultaneous null mutations in both presenilin homologues of C. elegans (sel-12 and hop-1). Nicastrin also binds carboxy-terminal derivatives of beta-amyloid precursor protein (betaAPP), and modulates the production of the amyloid beta-peptide (A beta) from these derivatives. Missense mutations in a conserved hydrophilic domain of nicastrin increase A beta42 and A beta40 peptide secretion. Deletions in this domain inhibit A beta production. Nicastrin and presenilins are therefore likely to be functional components of a multimeric complex necessary for the intramembranous proteolysis of proteins such as Notch/GLP-1 and betaAPP. 相似文献