全文获取类型
收费全文 | 20017篇 |
免费 | 49篇 |
国内免费 | 71篇 |
专业分类
系统科学 | 371篇 |
丛书文集 | 665篇 |
教育与普及 | 47篇 |
理论与方法论 | 91篇 |
现状及发展 | 8318篇 |
研究方法 | 987篇 |
综合类 | 9418篇 |
自然研究 | 240篇 |
出版年
2012年 | 358篇 |
2011年 | 702篇 |
2010年 | 161篇 |
2009年 | 103篇 |
2008年 | 459篇 |
2007年 | 547篇 |
2006年 | 481篇 |
2005年 | 527篇 |
2004年 | 513篇 |
2003年 | 501篇 |
2002年 | 483篇 |
2001年 | 616篇 |
2000年 | 623篇 |
1999年 | 355篇 |
1992年 | 333篇 |
1991年 | 256篇 |
1990年 | 302篇 |
1989年 | 276篇 |
1988年 | 262篇 |
1987年 | 277篇 |
1986年 | 287篇 |
1985年 | 340篇 |
1984年 | 246篇 |
1983年 | 228篇 |
1982年 | 210篇 |
1981年 | 247篇 |
1980年 | 264篇 |
1979年 | 581篇 |
1978年 | 473篇 |
1977年 | 474篇 |
1976年 | 353篇 |
1975年 | 376篇 |
1974年 | 587篇 |
1973年 | 462篇 |
1972年 | 417篇 |
1971年 | 516篇 |
1970年 | 662篇 |
1969年 | 575篇 |
1968年 | 494篇 |
1967年 | 531篇 |
1966年 | 440篇 |
1965年 | 332篇 |
1959年 | 198篇 |
1958年 | 293篇 |
1957年 | 192篇 |
1956年 | 172篇 |
1955年 | 167篇 |
1954年 | 160篇 |
1952年 | 122篇 |
1951年 | 100篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
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
2.
Synthesis is a creative and mysterious process in which a chaotic array of concepts, what might be called a mess, is cognitively ‘engaged’ with a problem domain to produce as many interpretations as can validly be derived for effective decision‐making purposes. The benefits of engaging concepts into a mess are that it allows the problem to be viewed through multiple lenses simultaneously, which affords a holistic vantage point for richer problem interpretation. By deliberately shifting the underlying concept held by actors and provoking the movement of concepts, better interpretations might lead to more useful decision making. Although research has discussed the possibility of using synthesis through case studies, we argue that there is a need to formally develop methodologies in this area for the purpose of fostering and solving complex problems. To demonstrate this, we provide a speculative case study to illustrate how this approach may be usefully applied. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
3.
Summary Larvae of the cabbage white butterfly,Pieris brassicae, have a dietary requirement for linolenic acid (C183n3) and were found to accumulate two other members of the n-3 family, C203n3 and C205n3 (eicosapentaenoic acid) especially in testicular phospholipids. Arachidonic acid was observed in trace amounts only. During diapause the relative titer of eicosapentaenoic acid increased in testicular phospholipids to about 4.2% of the fatty acids. Eicosapentaenoic acid is a possible precursor of prostaglandins, suggesting that prostaglandins of the 3-series predominate in this insect. 相似文献
4.
F. J. Castillo P. R. Miller H. Greppin 《Cellular and molecular life sciences : CMLS》1987,43(2):111-115
Summary Peroxidase and superoxide dismutase activities, and ascorbic acid content, were measured in both intercellular fluid and cell material of current and 1-year-old needles of Norway spruce saplings treated with ozone, ambient air and activated carbon-filtered air in outdoor fumigation chambers. Ethylene evolution was also compared. Plants from carbon-filtered air treatments had significantly lower enzyme activities and higher ascorbic acid content. These changes were more marked in intercellular fluid than in cell material. Significant changes were noted at ozone levels typical of ambient air quality in a typical urban area. These results suggest the need for simultaneous screening of several biochemical markers as a way of overcoming the lack of specificity of any single marker for the identification of a perturbation by a particular stress, such as ozone.Acknowledgments. This work was supported in part by Grant Number 4849-0.85 (NFP 14) from the Swiss National Fund of Scientific Research. We thank Dr. F. Cupelin and Dr. J. Cl. Landry from the Service d'Ecotoxicologie, Genève, who provided the facilities for the monitoring of ozone levels. We also thank A. Mesrobian, A. Rossier and D. Voluntaru for technical assistance. Du Pont de Nemours International is gratefully acknowledged for the gift of TEDLAR (polyvinyl fluoride film) to build up the fumigation chambers. 相似文献
5.
Stereochemical analysis has long been recognised as a powerful tool for elucidating the mechanisms of chemical and enzyme-catalysed reactions. Although much is known about the stereochemical course of reactions at saturated carbon, phosphate and thiophosphate esters whose ligands to phosphorus are also tetrahedrally disposed, are capable in principle of revealing sterochemical information about events at the active site of enzymes that transform such substrates. Nucleotidyl transferases are a group of enzymes which in general selectively use one of the diastereoisomers of a nucleoside 5'(1-thiotriphosphate), such as isomers A and B of adenosine 5'(1-thiotriphosphate), designated ATP alpha S-A and ATP alpha S-B, and allow investigation of the stereochemical course of nucleotidyl transfer. We have developed a simple method based on 31P nuclear magnetic resonance spectroscopy for determining the stereochemical course of these reactions, and using this method show here that the nucleotidyl transfer step in two aminoacyl-tRNA synthetases from Escherichia coli occurs with inversion of configuration at phosphorus. These observations greatly constrain the mechanistic possibilities for these enzymes, and are interpreted most simply as a direct 'in line' transfer from ATP to the amino acid. 相似文献
6.
7.
ATransportProtocolandItsFormalDescriptionforLocalNetworksLiLayuanWuhanTransportationUniversity,430063,P.R.China(ReceivedMay4... 相似文献
8.
9.
Zusammenfassung Winterschlafende Fledermäuse wurden mit Ringen markiert, die radioaktives Antimonium (124Sb) enthielten.Dadurch konnte gezeigt werden, dass in unzugänglilichen Felsspalten ein unsichtbares Fledermausreservoir vorhanden ist. Ferner wurde gezeigt, dass im Verlaufe des Winters Wechsel von Höhle zu Höhle stattfinden.Im Januar 1956 wurden in einer Höhle 26 Fledermäuse gefangen, mit radioaktiven Ringen markiert und in einer 6 km entfernten Höhle ausgesetzt. Nach einem Tag konnte in der Ausgangshöhle der erste Rückkehrer festgestellt werden und kurz hernach 7 weitere Tiere. 相似文献
10.