全文获取类型
收费全文 | 55890篇 |
免费 | 268篇 |
国内免费 | 549篇 |
专业分类
系统科学 | 1243篇 |
丛书文集 | 190篇 |
教育与普及 | 288篇 |
理论与方法论 | 522篇 |
现状及发展 | 31713篇 |
研究方法 | 870篇 |
综合类 | 19689篇 |
自然研究 | 2192篇 |
出版年
2013年 | 807篇 |
2012年 | 617篇 |
2011年 | 2651篇 |
2009年 | 615篇 |
2008年 | 830篇 |
2007年 | 937篇 |
2006年 | 1027篇 |
2005年 | 1183篇 |
2004年 | 2271篇 |
2003年 | 1932篇 |
2002年 | 1579篇 |
2001年 | 1512篇 |
2000年 | 1052篇 |
1999年 | 1022篇 |
1998年 | 647篇 |
1997年 | 777篇 |
1996年 | 533篇 |
1994年 | 687篇 |
1993年 | 693篇 |
1992年 | 945篇 |
1991年 | 806篇 |
1990年 | 900篇 |
1989年 | 690篇 |
1988年 | 649篇 |
1987年 | 684篇 |
1986年 | 758篇 |
1985年 | 896篇 |
1984年 | 784篇 |
1983年 | 686篇 |
1982年 | 808篇 |
1981年 | 829篇 |
1980年 | 904篇 |
1979年 | 1416篇 |
1978年 | 1252篇 |
1977年 | 1232篇 |
1976年 | 1117篇 |
1975年 | 1081篇 |
1974年 | 932篇 |
1973年 | 1179篇 |
1972年 | 1206篇 |
1971年 | 1243篇 |
1970年 | 1323篇 |
1969年 | 1203篇 |
1968年 | 1186篇 |
1967年 | 1050篇 |
1966年 | 889篇 |
1965年 | 737篇 |
1964年 | 485篇 |
1958年 | 526篇 |
1957年 | 435篇 |
排序方式: 共有10000条查询结果,搜索用时 131 毫秒
381.
Summary Metabolism of 26-hydroxycholesterol to 3-hydroxychol-5-en-24-oic acid and other C24-bile acids has been expected to occur by way of 3-hydroxycholest-5-en-26-oic acid in studies in vitro. 3-Hydroxycholest-5-en-26-oic acid was infused intravenously into bile fistula hamsters and the following C24-bile acids were identified: 3-hydroxychol-5-en-24-oic acid, lithocholic acid, chenodeoxycholic acid, and a small amount of cholic acid. 相似文献
382.
刘金河 《厦门大学学报(自然科学版)》1990,29(4):472-474
不同组合巴西橡胶三合树在福建南部的生态条件下,其生长强度,生理代谢和抗逆能力都存在差异。本文中主要研究比较了几种橡胶三合树的生长强度,光合作用强度及其变化。研究结果表明,三合树的生长强度和光合强度明显比对照树高,不同组合三合树的生长强度和光合强度与它们的抗寒力成正相关。 相似文献
383.
384.
孙忠林 《山东科技大学学报(自然科学版)》1990,(1)
本文介绍的这种情报资料检索系统,采用了数据压缩存贮技术,并使用链表记录的存贮结构解决了变长信息的存贮问题,系统的功能以“菜单”和“快速”两种方式提供给用户,使用方便。 相似文献
385.
This paper focuses on the coupling among alliances, relationships, management, and activities, when major strategic changes are carried out in an old organization. The paper also points out hownew alliances andchanged relationships seem to be developing in a Norwegian company. As a conclusion, we point out six elements which, according to our research, have to be present to minimize the implementation of reversible processes in organizations where organizational change processes have been implemented so that continued change can go on in the company. 相似文献
386.
Barry W. Evans 《Systemic Practice and Action Research》1994,7(6):671-685
This paper develops the argument that in organizational redesign it is desirable to utilize user-designers as the architects for the redesign. A methodology is developed which emphasizes the use of ideal systems models by user-designers to plan for the organization's future. The methodology involves the creation of an ideal system model, then an analysis of the present system through the creation of root definitions and process models of the existing system. Then root definitions and process models for the idealized system are created. Finally, the two are compared and desirable changes are generated which bring the existing system toward the idealized. 相似文献
387.
In multivariate discrimination of several normal populations, the optimal classification procedure is based on quadratic discriminant functions. We compare expected error rates of the quadratic classification procedure if the covariance matrices are estimated under the following four models: (i) arbitrary covariance matrices, (ii) common principal components, (iii) proportional covariance matrices, and (iv) identical covariance matrices. Using Monte Carlo simulation to estimate expected error rates, we study the performance of the four discrimination procedures for five different parameter setups corresponding to standard situations that have been used in the literature. The procedures are examined for sample sizes ranging from 10 to 60, and for two to four groups. Our results quantify the extent to which a parsimonious method reduces error rates, and demonstrate that choosing a simple method of discrimination is often beneficial even if the underlying model assumptions are wrong.The authors wish to thank the editor and three referees for their helpful comments on the first draft of this article. M. J. Schmid supported by grants no. 2.724-0.85 and 2.038-0.86 of the Swiss National Science Foundation. 相似文献
388.
A. V. Edwards M. A. Ghatei S. R. Bloom 《Cellular and molecular life sciences : CMLS》1994,50(8):725-726
Mean plasma insulin concentration was reduced and mean plasma glucose concentration increased following the administration of N-nitro-L-arginine methyl ester (L-NAME; 100 mol kg–1 i.a.) in conscious calves given continuous infusions of exogenous glucose (30–60 mol min–1 kg–1 i.v.). It is concluded that the rise in plasma insulin concentration which occurs in these animals in response to glucose is mediated, at least in part, by a nitric oxide-related factor (NOx). 相似文献
389.
H. Tsuchiya M. Sato M. Iinuma J. Yokoyama M. Ohyama T. Tanaka I. Takase I. Namikawa 《Cellular and molecular life sciences : CMLS》1994,50(9):846-849
Phytoalexins, defensive compounds produced by plants against microbial infections, were purified fromSophora exigua (Leguminosae) and their growth inhibitory effects on oral cariogenic bacteria were determined in vitro. Among three isolated compounds, 5,7,2,4-tetrahydroxy-8-lavandulylflavanone completely inhibited the growth of oral bacteria including primary cariogenic mutans streptococci, other oral streptococci, actinomycetes, and lactobacilli, at concentrations of 1.56 to 6.25 g/ml. 相似文献
390.
Because of possible variation in venom composition, an understanding of venomous snake systematics is of great importance for the optimization of antivenom treatment of snakebite patients. Intraspecific variation in the morphology of many venomous snakes complicates the definition and indentification of some species when allopatric populations are involved. Selectively neutral or near-neutral mtDNA sequences can reveal evolutionary relationships obscured by ecogenetically-caused morphological variation. We use comparative sequencing of the cytochrome oxidase subunit 1 gene to reveal the existence of a widespread, cryptic species of spiting cobra from southeast Asia. This species,Naja siamensis, is widely sympatric with other Asiatic cobra species. This may be of considerable medical significance, and calls for further research into venom composition in Asiatic cobras. 相似文献