全文获取类型
收费全文 | 170篇 |
免费 | 1篇 |
专业分类
系统科学 | 1篇 |
教育与普及 | 1篇 |
理论与方法论 | 1篇 |
现状及发展 | 78篇 |
研究方法 | 13篇 |
综合类 | 76篇 |
自然研究 | 1篇 |
出版年
2016年 | 1篇 |
2015年 | 1篇 |
2013年 | 1篇 |
2012年 | 8篇 |
2011年 | 12篇 |
2010年 | 3篇 |
2009年 | 4篇 |
2008年 | 4篇 |
2007年 | 9篇 |
2006年 | 5篇 |
2005年 | 7篇 |
2004年 | 5篇 |
2003年 | 5篇 |
2002年 | 4篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1992年 | 2篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1984年 | 1篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1972年 | 6篇 |
1971年 | 7篇 |
1970年 | 6篇 |
1969年 | 2篇 |
1968年 | 3篇 |
1967年 | 5篇 |
1966年 | 6篇 |
1965年 | 2篇 |
1964年 | 2篇 |
1963年 | 1篇 |
1962年 | 5篇 |
1959年 | 1篇 |
1957年 | 1篇 |
1947年 | 1篇 |
1946年 | 2篇 |
1945年 | 1篇 |
排序方式: 共有171条查询结果,搜索用时 31 毫秒
21.
22.
23.
24.
25.
26.
27.
AG Henry PS Ungar BH Passey M Sponheimer L Rossouw M Bamford P Sandberg DJ de Ruiter L Berger 《Nature》2012,487(7405):90-93
Specimens of Australopithecus sediba from the site of Malapa, South Africa (dating from approximately 2 million years (Myr) ago) present a mix of primitive and derived traits that align the taxon with other Australopithecus species and with early Homo. Although much of the available cranial and postcranial material of Au. sediba has been described, its feeding ecology has not been investigated. Here we present results from the first extraction of plant phytoliths from dental calculus of an early hominin. We also consider stable carbon isotope and dental microwear texture data for Au. sediba in light of new palaeoenvironmental evidence. The two individuals examined consumed an almost exclusive C(3) diet that probably included harder foods, and both dicotyledons (for example, tree leaves, fruits, wood and bark) and monocotyledons (for example, grasses and sedges). Like Ardipithecus ramidus (approximately 4.4 Myr ago) and modern savanna chimpanzees, Au. sediba consumed C(3) foods in preference to widely available C(4) resources. The inferred consumption of C(3) monocotyledons, and wood or bark, increases the known variety of early hominin foods. The overall dietary pattern of these two individuals contrasts with available data for other hominins in the region and elsewhere. 相似文献
28.
Soderberg AM Kulkarni SR Nakar E Berger E Cameron PB Fox DB Frail D Gal-Yam A Sari R Cenko SB Kasliwal M Chevalier RA Piran T Price PA Schmidt BP Pooley G Moon DS Penprase BE Ofek E Rau A Gehrels N Nousek JA Burrows DN Persson SE McCarthy PJ 《Nature》2006,442(7106):1014-1017
Over the past decade, long-duration gamma-ray bursts (GRBs)--including the subclass of X-ray flashes (XRFs)--have been revealed to be a rare variety of type Ibc supernova. Although all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated with supernova SN 2006aj), the second-nearest GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 10(48) erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218. 相似文献
29.
Decisions on ass et allocations are often determined by covariance estimates from historical market data. In this paper, we introduce a wavelet-based portfolio algorithm, distinguishing between newly embedded news and long-run information that has already been fully absorbed by the market. Exploiting the wavelet decomposition into short- and long-run covariance regimes, we introduce an approach to focus on particular covariance components. Using generated data, we demonstrate that short-run covariance regimes comprise the relevant information for periodical portfolio management. In an empirical application to US stocks and other international markets for weekly, monthly, quarterly, and yearly holding periods (and rebalancing), we present evidence that the application of wavelet-based covariance estimates from short-run information outperforms portfolio allocations that are based on covariance estimates from historical data. 相似文献
30.
Correlation between immunoglobulin light chain expression and variant translocation in Burkitt's lymphoma 总被引:35,自引:0,他引:35
Burkitt's-type lymphomas-leukaemias (BL) are monoclonal proliferations of malignant B lymphocytes. Irrespective of whether they carry the Epstein-Barr virus (EBV) genome, these tumour cells have been shown consistently to have one of the specific reciprocal chromosome translocations, t(8; 14), t(2; 8) or t(8; 22), involving the long arm of chromosome 8 (on 8q24) and chromosome 14, 2 or 22 (on 14q32, 2p12 and 22q11, respectively). The latter chromosomes have been shown recently to carry genes for immunoglobulin (Ig) heavy chains, and kappa and lambda light chains, respectively. Furthermore, the localization of kappa light chains within 2pcen-2p13 encompasses the breakpoint observed in Burkitt's translocation (2p12). It was therefore considered of interest to determine whether the expression of immunoglobulin chains in BL cells is related to the type of chromosomal anomalies observed. We report here that there is a direct relationship between expression of immunoglobulin light chains and specific type of translocation: BL cells with t(8; 22) express lambda chains, whereas those with t(2; 8) express kappa chains. 相似文献