全文获取类型
收费全文 | 874篇 |
免费 | 18篇 |
国内免费 | 2篇 |
专业分类
系统科学 | 35篇 |
理论与方法论 | 17篇 |
现状及发展 | 521篇 |
研究方法 | 51篇 |
综合类 | 214篇 |
自然研究 | 56篇 |
出版年
2020年 | 12篇 |
2019年 | 7篇 |
2018年 | 25篇 |
2017年 | 22篇 |
2016年 | 30篇 |
2015年 | 27篇 |
2014年 | 19篇 |
2013年 | 23篇 |
2012年 | 35篇 |
2011年 | 63篇 |
2010年 | 23篇 |
2009年 | 7篇 |
2008年 | 27篇 |
2007年 | 28篇 |
2006年 | 26篇 |
2005年 | 24篇 |
2004年 | 24篇 |
2003年 | 26篇 |
2002年 | 22篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 11篇 |
1993年 | 9篇 |
1992年 | 12篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1982年 | 10篇 |
1981年 | 11篇 |
1979年 | 23篇 |
1978年 | 16篇 |
1977年 | 16篇 |
1976年 | 17篇 |
1975年 | 8篇 |
1974年 | 13篇 |
1973年 | 20篇 |
1972年 | 10篇 |
1971年 | 12篇 |
1970年 | 23篇 |
1969年 | 20篇 |
1968年 | 20篇 |
1967年 | 14篇 |
1966年 | 12篇 |
1965年 | 8篇 |
1964年 | 10篇 |
1963年 | 7篇 |
1962年 | 7篇 |
1961年 | 8篇 |
1959年 | 5篇 |
排序方式: 共有894条查询结果,搜索用时 15 毫秒
891.
José M. F. CRAVEIRINHA 《系统科学与系统工程学报(英文版)》2006,15(3):314-329
1. Introduction Nowadays the exponential and convergent development of informatics and telecommunications, including Internet, makes that, in many communication networks, important decisions of technical nature have to be made in short time periods or even in real-time. This is particularly relevant in the context of routing models involving the calculation and selection of routes (corresponding to loopless paths) for node to node traffic flows, which seek to optimize certain metric(s) while s… 相似文献
892.
It has commonly been thought that snakes underwent progressive loss of their limbs by gradual diminution of their use. However, recent developmental and palaeontological discoveries suggest a more complex scenario of limb reduction, still poorly documented in the fossil record. Here we report a fossil snake with a sacrum supporting a pelvic girdle and robust, functional legs outside the ribcage. The new fossil, from the Upper Cretaceous period of Patagonia, fills an important gap in the evolutionary progression towards limblessness because other known fossil snakes with developed hindlimbs, the marine Haasiophis, Pachyrhachis and Eupodophis, lack a sacral region. Phylogenetic analysis shows that the new fossil is the most primitive (basal) snake known and that all other limbed fossil snakes are closer to the more advanced macrostomatan snakes, a group including boas, pythons and colubroids. The new fossil retains several features associated with a subterranean or surface dwelling life that are also present in primitive extant snake lineages, supporting the hypothesis of a terrestrial rather than marine origin of snakes. 相似文献
893.
Assembly of multi-component supramolecular machines is fundamental to biology, yet in most cases, assembly pathways and their control are poorly understood. An example is the type III secretion machine, which mediates the transfer of bacterial virulence proteins into host cells. A central component of this nanomachine is the needle complex or injectisome, an organelle associated with the bacterial envelope that is composed of a multi-ring base, an inner rod, and a protruding needle. Assembly of this organelle proceeds in sequential steps that require the reprogramming of the secretion machine. Here we provide evidence that, in Salmonella typhimurium, completion of the assembly of the inner rod determines the size of the needle substructure. Assembly of the inner rod, which is regulated by the InvJ protein, triggers conformational changes on the cytoplasmic side of the injectisome, reprogramming the secretion apparatus to stop secretion of the needle protein. 相似文献
894.
Velásquez-Rodríguez Camilo Payán-Durán Luisa 《Systemic Practice and Action Research》2022,35(1):33-53
Systemic Practice and Action Research - An organizational design proposal is made for Creo en el Agro, an organization made up of small coffee growers who were victims of the armed conflict in the... 相似文献