全文获取类型
收费全文 | 108007篇 |
免费 | 8728篇 |
国内免费 | 6596篇 |
出版年
2023年 | 1276篇 |
2022年 | 1444篇 |
2021年 | 5099篇 |
2020年 | 3568篇 |
2019年 | 4374篇 |
2018年 | 4352篇 |
2017年 | 3232篇 |
2016年 | 4599篇 |
2015年 | 6677篇 |
2014年 | 7863篇 |
2013年 | 8315篇 |
2012年 | 9960篇 |
2011年 | 8870篇 |
2010年 | 5445篇 |
2009年 | 4749篇 |
2008年 | 5586篇 |
2007年 | 4923篇 |
2006年 | 4370篇 |
2005年 | 3331篇 |
2004年 | 2933篇 |
2003年 | 2532篇 |
2002年 | 2205篇 |
2001年 | 2001篇 |
2000年 | 1855篇 |
1999年 | 1831篇 |
1998年 | 991篇 |
1997年 | 1115篇 |
1996年 | 992篇 |
1995年 | 918篇 |
1994年 | 943篇 |
1993年 | 666篇 |
1992年 | 993篇 |
1991年 | 838篇 |
1990年 | 613篇 |
1989年 | 559篇 |
1988年 | 485篇 |
1987年 | 411篇 |
1986年 | 388篇 |
1985年 | 390篇 |
1984年 | 211篇 |
1983年 | 197篇 |
1982年 | 137篇 |
1981年 | 114篇 |
1980年 | 107篇 |
1979年 | 115篇 |
1978年 | 77篇 |
1977年 | 60篇 |
1975年 | 57篇 |
1974年 | 74篇 |
1972年 | 62篇 |
排序方式: 共有10000条查询结果,搜索用时 14 毫秒
61.
62.
63.
The financing channels, investors and operators of urban rail transit are becoming more and more diversified, and public private partnership pattern has been increasingly suggested in financing and investment field of urban rail transit in China. The diversification of investors of urban rail transit will no doubt lead to the diversification of operators of urban rail transit network. To legitimately distribute the cooperation profits among operators, a model is developed based on passenger’s path choice behavior by considering travel period, travel time, transfer convenience and the comprehensive proportion of different service types provided by operators. In accordance with the features of urban rail transit network and origin-destination (OD) pairs of transferring among lines of different operators, a scheme of improved rail transit network is proposed. On the basis of the algorithm of breadth-first search and depth-first search, an algorithm of searching effective paths based on backtracking and traversing along the shortest path is established by considering the factor of transfer. Taking the example of Shenzhen’s rail transit network, three typical OD pairs are selected to measure and calculate, compare and analyze by six different conditions. The result shows that travel period, travel time, transfer convenience, and service types provided by operators exert great influence on the distribution of cooperation profits. Therefore, it is advisable to comprehensively consider all of these factors to improve the accuracy of cooperation profits distribution. Moreover, the proposed algorithm can search effective paths efficiently. 相似文献
64.
65.
66.
67.
68.
69.
L Ghoda M A Phillips K E Bass C C Wang P Coffino 《The Journal of biological chemistry》1990,265(20):11823-11826
Ornithine decarboxylase (ODC) is a key enzyme in polyamine biosynthesis. Mouse ODC is rapidly degraded in mouse cells, whereas ODC within Trypanosoma brucei, a protozoan parasite infesting cattle, is stable. We have expressed cloned ODC genes of both T. brucei and mouse in ODC-deficient Chinese hamster ovary (CHO) cells. The T. brucei enzyme is stable, whereas the mouse ODC similarly expressed in CHO cells is unstable. This shows that the observed difference in intracellular stability is a property of the ODC protein itself, rather than the cellular environment in which it is expressed. A chimeric ODC composed of the amino terminus of trypanosome and the carboxyl terminus of mouse ODC is rapidly degraded in CHO cells, suggesting that peptide sequences in the mouse ODC carboxyl terminus determine its stability. 相似文献
70.
S T Chen L C Lo S H Wu K T Wang 《International journal of peptide and protein research》1990,35(1):52-54
2-Oxazolidone derivatives formed through an intramolecular reaction in the process of alkaline treatment of urethane-type N-protected peptides of which the N-terminal residues were Ser or Thr having unprotected hydroxyl groups. In oder to avoid this side reaction, the esters of these peptides could be cleaved by enzymatic hydrolyses instead of saponification. 相似文献