全文获取类型
收费全文 | 72557篇 |
免费 | 286篇 |
国内免费 | 640篇 |
专业分类
系统科学 | 1464篇 |
丛书文集 | 961篇 |
教育与普及 | 458篇 |
理论与方法论 | 549篇 |
现状及发展 | 36014篇 |
研究方法 | 1300篇 |
综合类 | 30547篇 |
自然研究 | 2190篇 |
出版年
2013年 | 808篇 |
2012年 | 740篇 |
2011年 | 3360篇 |
2010年 | 638篇 |
2009年 | 893篇 |
2008年 | 1066篇 |
2007年 | 1207篇 |
2006年 | 1208篇 |
2005年 | 1490篇 |
2004年 | 3338篇 |
2003年 | 2433篇 |
2002年 | 1924篇 |
2001年 | 2028篇 |
2000年 | 1684篇 |
1999年 | 1370篇 |
1998年 | 898篇 |
1997年 | 854篇 |
1994年 | 712篇 |
1993年 | 776篇 |
1992年 | 1179篇 |
1991年 | 1007篇 |
1990年 | 1100篇 |
1989年 | 884篇 |
1988年 | 876篇 |
1987年 | 888篇 |
1986年 | 899篇 |
1985年 | 1147篇 |
1984年 | 939篇 |
1983年 | 809篇 |
1982年 | 875篇 |
1981年 | 909篇 |
1980年 | 1045篇 |
1979年 | 1829篇 |
1978年 | 1591篇 |
1977年 | 1578篇 |
1976年 | 1332篇 |
1975年 | 1362篇 |
1974年 | 1486篇 |
1973年 | 1572篇 |
1972年 | 1626篇 |
1971年 | 1753篇 |
1970年 | 2096篇 |
1969年 | 1773篇 |
1968年 | 1629篇 |
1967年 | 1589篇 |
1966年 | 1368篇 |
1965年 | 1060篇 |
1959年 | 646篇 |
1958年 | 905篇 |
1957年 | 708篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Linear models are invariant under non-singular, scale-preserving linear transformations, whereas mean square forecast errors (MSFEs) are not. Different rankings may result across models or methods from choosing alternative yet isomorphic representations of a process. One approach can dominate others for comparisons in levels, yet lose to another for differences, to a second for cointegrating vectors and to a third for combinations of variables. The potential for switches in ranking is related to criticisms of the inadequacy of MSFE against encompassing criteria, which are invariant under linear transforms and entail MSFE dominance. An invariant evaluation criterion which avoids misleading outcomes is examined in a Monte Carlo study of forecasting methods. 相似文献
32.
Astacin, a digestive zinc-endopeptidase from the crayfish Astacus astacus L., is the prototype for the 'astacin family', which includes mammalian metallo-endopeptidases and developmentally regulated proteins of man, fruitfly, frog and sea urchin. Here we report the X-ray crystal structure of astacin, which reveals a deep active-site cleft, with the zinc at its bottom ligated by three histidines, a water molecule and a more remote tyrosine. The third histidine (His 102) forms part of a consensus sequence, shared not only by the members of the astacin family, but also by otherwise sequentially unrelated proteinases, such as vertebrate collagenases. It may therefore represent the elusive 'third' zinc ligand in these enzymes. The amino terminus of astacin is buried forming an internal salt-bridge with Glu 103, adjacent to His 102. Astacin pro-forms extended at the N terminus, as observed for some 'latent' mammalian astacin homologues, did not exhibit this 'active' conformation, indicating an activation mechanism reminiscent of trypsin-like serine proteinases. 相似文献
33.
M. Lazzari V. Franceschini G. Minelli F. Ciani 《Cellular and molecular life sciences : CMLS》1993,49(4):277-281
The corrosion cast technique provided for the first time an excellent three-dimensional visualization of the vascular pattern of the choroid and iris in the newt eye. The results show the presence of a single arterial afference to the choroidal and iris capillaries: the ophthalmic artery is the origin fo both ciliary arteries and the long posterior ciliary artery. Slightly behind the equatorial circumference of the eyeball the venous drainage consists of a single vessel on the dorsal side and two distinct vessels on the ventral one. It receives blood from both iris and choroid. The surface of the plastic endocasts shows some details of fine luminal structures of the endothelial cells. Shallow depressions may be regarded as imprints of endothelial cell nuclei, and they are distinctly different for arteries and capillaries. The angioarchitecture of the newt eye differs from that of brain in that hairpin-shaped capillary loops are not observed at all. 相似文献
34.
It was established that there is little diversity of bryophytes in the derived savanna. Mosses were found in the sampling sites, whereas liverworts were rarely observed. The reproductive methods of four dominant sexually reproducing savanna mosses —Archidium ohioense, Bryum coronatum, Fissidens minutifolius andTrachycarpidium tisserantii were monitored over two consecutive rainy seasons. Protonemal and gametophyte production were noticed in the field in March/April, and capsule dehiscence and spore dispersal occurred in September/October. The sequential stages of development, starting with gametangial production and ending with the falling of the dehisced capsules, occurred within the rainy season. However,A. ohioense andT. tisserantii did not discharge their spores easily (cleistocarpous), unlike the stegocarpous speciesB. coronatum andF. minutifolius. Water availability and possibly high humidity may have contributed to growth. The short period between sex organ formation and dehiscence of capsule seen in these studies, compared with the longer period in some temperate mosses, may be an advantage for bryophytes in a savanna environment. 相似文献
35.
36.
37.
38.
R. Poggioli A. V. Vergoni A. Bertolini 《Cellular and molecular life sciences : CMLS》1985,41(2):265-266
Summary Hydrochlorothiazide, acutely injected in rats, has a weak analgesic activity per se and potentiates and prolongs the antinociceptive effect of morphine.This work was supported in part by grants from Consiglio Nazionale delle Ricerche, and by Ministero della Pubblica Istruzione, Roma. 相似文献
39.
H R Brenner 《Nature》1985,317(6038):572-573
40.
1 Results This paper presents the development of platinum nanocomposites structures based on organically modified c.a.2 nm core platinum nanoparticles.The chemical modification of the 4-mercaptoaniline functionalized particles by various in coming molecules is evidenced and precisely quantified.The particles can be dissolved like molecules in various solvents depending on the features of the new crown and X-rays shows that the interparticle distance is affected by the crown modification.These platinum n... 相似文献