首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   71782篇
  免费   286篇
  国内免费   598篇
系统科学   1273篇
丛书文集   711篇
教育与普及   305篇
理论与方法论   542篇
现状及发展   37118篇
研究方法   1538篇
综合类   28984篇
自然研究   2195篇
  2013年   829篇
  2012年   813篇
  2011年   2924篇
  2009年   649篇
  2008年   1069篇
  2007年   1217篇
  2006年   1265篇
  2005年   1428篇
  2004年   2589篇
  2003年   2183篇
  2002年   1894篇
  2001年   1973篇
  2000年   1470篇
  1999年   1306篇
  1998年   662篇
  1997年   776篇
  1994年   688篇
  1993年   693篇
  1992年   1230篇
  1991年   1004篇
  1990年   1141篇
  1989年   949篇
  1988年   905篇
  1987年   986篇
  1986年   995篇
  1985年   1189篇
  1984年   991篇
  1983年   850篇
  1982年   910篇
  1981年   946篇
  1980年   1079篇
  1979年   1892篇
  1978年   1676篇
  1977年   1637篇
  1976年   1388篇
  1975年   1440篇
  1974年   1557篇
  1973年   1633篇
  1972年   1727篇
  1971年   1843篇
  1970年   2125篇
  1969年   1789篇
  1968年   1757篇
  1967年   1620篇
  1966年   1390篇
  1965年   1052篇
  1959年   611篇
  1958年   927篇
  1957年   728篇
  1956年   624篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Summary The tissue specificity of lactate dehydrogenase (EC 1.1.1.27) in brain and eye of the teleostPhallichthys amates was examined by acrylamide gel electrophoresis. It is suggested that subunit association is a function of gene product accessibility superimposed upon genetic restriction of assembly.  相似文献   
82.
Summary The administration of 15(R)-15-methyl prostaglandin E2 (15(R)-15-M-PGE2) in vivo significantly diminished the uptake of59Fe into blood, spleen, liver, femur and dried intestine of rats, whereas acetylsalicylic acid (ASA) increased the counts significantly. This effect of ASA was counteracted by 15(R)-15-M-PGE2. It is suggested that prostaglandins (PGs) might play an important role in inhibiting iron absorption at the intestinal level.This work was supported by grant No.6638 from CONICET (Argentina). The technical assistance of Mrs María E. Castro and Norma Rizzo is gratefully acknowleged.  相似文献   
83.
Summary Host cell reactivation capacity for ozone T3 phage was determined for differentE. coli strains deficient in one or more of the DNA repair mechanisms. The results indicate that DNA polymerase I appears to play a key role in the repair of damage produced on the DNA by ozone while thelexA gene product seems to play a minor one.This research was sponsored by the National Sciences and Engineering Council of Canada. One of us (PLH) acknowledges a scholarship from the NSERCof Canada.  相似文献   
84.
Summary The nature of antigen Dd, an antigen present in the extracts of human dandruff which precipitates human sera selectively, and antibodies reacting with it are reported.This work was supported through U.G.C. Grant No. F.23-230/75/SR II. H.K. participated in this study, first as J.R.F. of U.G.C. and then as S.R.F. of I.C.M.R. We are grateful to Dr Baruch S. Blumberg for his invaluable suggestions and to Professor H. Walter for the gift of IgG, IgM and IgA immune sera.  相似文献   
85.
比较1,1-双(2-叔丁基砜乙基)十二烷基苯基砜(1)双碳负离子在不同条件下的环合反应结果,控制反应条件,在铁催化下可以生成偶联糁烯化的环烯产物(2)为主要产物;不加催化剂,加热回流反应得到分子内亲核取代反应产物环丙烷化合物(3)。  相似文献   
86.
We discuss a generalization of the standard notion of probability space and show that the emerging framework, to be called operational probability theory, can be considered as underlying quantal theories. The proposed framework makes special reference to the convex structure of states and to a family of observables which is wider than the familiar set of random variables: it appears as an alternative to the known algebraic approach to quantum probability.  相似文献   
87.
Surface Preparation is very important in adhesive b on ding of spray coatings to the surface of a work piece. The common practice is gr it-blasting of the surface before subjecting it to the spray coating process. In this study, grit-blasting of an AISI 4130 steel (of different heat treatmen ts) with Al 2O 3 particles was studied. Various grit-blasting parameters such as blasting particle size, the distance between blasting nozzle and the work pi ece (25, 30 and 40 cm.), blasting pressure (3,4,5,6 and ...  相似文献   
88.
A detailed karyotypic analysis of two Egyptian species of bats,Taphozous perforatus andTaphozous nudiventris, was made on the basis of conventional data and G-band patterns. No detectable karyotypic difference was found between the two species (2n=42, F.N. 64). By comparison of G-band patterns, some differences between the two species were seen in the corresponding autosomal pairs. These results are reported for the first time in Egypt.  相似文献   
89.
90.
Recent years have seen an increasing cross-fertilization between the fields of decision analysis and forecasting. Decision-analytic models often require forecasts as inputs, and aspects of the Bayesian decision-theoretic framework underlying decision analysis have proved useful to forecasting, particularly in contexts where subjective judgemental inputs are required. This paper describes the use of decision tree analysis for forecasting and illustrates its use for corporate divisional forecasting and planning. A specialized decision-analytic technique, acts as events, is also described and illustrated to forecast a new product's earnings. Conclusions are drawn about the applicability of decision analysis for forecasting.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号