全文获取类型
收费全文 | 7328篇 |
免费 | 629篇 |
国内免费 | 2篇 |
专业分类
系统科学 | 1121篇 |
理论与方法论 | 292篇 |
现状及发展 | 2082篇 |
研究方法 | 95篇 |
综合类 | 4295篇 |
自然研究 | 74篇 |
出版年
2020年 | 17篇 |
2018年 | 745篇 |
2017年 | 746篇 |
2016年 | 465篇 |
2015年 | 62篇 |
2014年 | 21篇 |
2013年 | 30篇 |
2012年 | 319篇 |
2011年 | 1037篇 |
2010年 | 842篇 |
2009年 | 467篇 |
2008年 | 551篇 |
2007年 | 801篇 |
2006年 | 71篇 |
2005年 | 101篇 |
2004年 | 177篇 |
2003年 | 196篇 |
2002年 | 100篇 |
2001年 | 32篇 |
2000年 | 21篇 |
1999年 | 20篇 |
1992年 | 18篇 |
1991年 | 20篇 |
1985年 | 14篇 |
1984年 | 27篇 |
1983年 | 14篇 |
1982年 | 16篇 |
1981年 | 21篇 |
1979年 | 36篇 |
1978年 | 31篇 |
1977年 | 40篇 |
1976年 | 37篇 |
1975年 | 31篇 |
1974年 | 36篇 |
1973年 | 46篇 |
1972年 | 42篇 |
1971年 | 38篇 |
1970年 | 65篇 |
1969年 | 56篇 |
1968年 | 86篇 |
1967年 | 67篇 |
1966年 | 50篇 |
1965年 | 47篇 |
1964年 | 42篇 |
1963年 | 20篇 |
1962年 | 24篇 |
1961年 | 17篇 |
1959年 | 15篇 |
1958年 | 12篇 |
1957年 | 13篇 |
排序方式: 共有7959条查询结果,搜索用时 281 毫秒
91.
92.
ZHUHuanzhang CHENXiaoguang LIFeng GONGJuli XUEJinglun 《科学通报(英文版)》2003,48(20):2196-2200
To explore the expression of human clotting factor Ⅸ (hFⅨ) cDNA in vitro and the feasibility of gene therapy for hemophilia B mice mediated by recombinant lentiviral vector, a recombinant hFⅨ lentiviral vector driven by ubiquitin-C promoter, FUXW, and by ABP liver specific promoter, FAXW, was constructed respectively. Recombinant lentivirus was harvested from 293T cells by calcium phosphate-mediated transient cotransfection of three plasmids (transgene vector, CMV腞8.2, VSV-G). hFⅨ expression was detected in supernatant of 293T, BHK and L-02 cells infected with FUXW virus, whereas higher expression of hFⅨ levels (630 ng/106 cells/48 h) was detected only in L-02 cells infected with FAXW virus. Serum hFⅨ antigen was detected in all hemophilia B mice treated with FAXW virus by tail vein injection, an efficiency level of hFⅨ was observed (45 ng/mL, approximately 1% of normal human levels), the expression lasted for more than 60 d. The results indicated that HIV-based lentiviral vectors offer a promising approach to the gene therapy of hemophilia B. 相似文献
93.
基于数学形态学的图像处理方法 总被引:7,自引:0,他引:7
从图像的描述方法出发,在二值形态变换的基础上,通过引入图像的阀集合和本影,导出了多值形态学基本形态变换建立的方法.并揭示了二值形态学与多值形态学在结构算法上的相互联系.在图像处理的形态学方法上,提出了基于形态变换的图像边缘提取、多种形态梯度、形态骨架表示方法,以及具有去噪和形态平滑功能的开、闭形态变换在图像分析中的应用. 相似文献
94.
95.
以小本体聚丙烯为原料,通过添加成核剂和相对分子质量调节剂,制备了具有良好透明性能的小本体聚丙烯,探讨了成核剂、相对分子质量调节剂对透明性的影响。通过XRD谱图分析表明,成核剂的加入能有效地降低体系β晶型的含量,从而有效提高其透明性。 相似文献
96.
NPP distribution related to the terrains along the North-South Transect of Eastern China 总被引:2,自引:0,他引:2
LI Yan LIAO Shendong CHI Guobin & LIAO Qifang . Spatial Information Research Center South China Normal University Guangzhou China . Guangdong Key Laboratory of RS GIS Applications Guangzhou China 《科学通报(英文版)》2004,49(6):617-624
Vegetation net primary productivity (NPP) is defined as the accumulative amount of organic matters per unit area and time, which is calculated from the difference between accumulative photosynthesis and accumulative autotrophic respiration by green plants. Since the NPP reflects the physiological and ecological characteristics of the plants and the interactive results of the environmental factors, it is an important index to evaluate a structure of ecological system, functional features and … 相似文献
97.
TANGNi ZHANGBingqiang YANGe PUDan GAOXiaolin Tong-ChuanHe HUANGAilong 《科学通报(英文版)》2004,49(14):1470-1475
Persistent and recurrent infection of hepatitis B virus (HBV) represents one of the most common and severe viral infections of humans, and has caused a formidable health problem in the affected countries. Currently used antiviral drugs have a very limited success on controlling HBV replication and infection. RNA interference (RNAi), a process by which double-stranded RNA (dsRNA) directs sequence-specific degradation of target mRNA in mammalian and plant cells, has recently been used to knockdown gene expression in various species. In this study, we sought to determine whether RNAi-mediated silencing of HBV viral gene expression could lead to the effective inhibition of HBV replication. We first developed RNAi vectors that expressed small interfering RNA (siRNA) and targeted the HBV core or surface gene sequence. Our results demonstrated that these specific siRNAs efficiently reduced the levels of corresponding viral RNAs and proteins, and thus suppressed viral replication. Treatment with siRNA gave the greatest reduction in the levels of HBsAg (92%) and in HBeAg (85%) respectively in the cultured cell medium. Our findings further demonstrated that the RNAi-mediated antiviral effect was sequence-specific and dose-dependent. Therefore, our findings strongly suggest that RNAi-mediated silencing of HBV viral genes could effectively inhibit the replication of HBV, hence RNAi-based strategy should be further explored as a more efficacious antiviral therapy of HBV infection. 相似文献
98.
Nomenclature ki integral action coefficient[s-1]kp speed proportional gain[s-1]kv position proportional gain[s-1]kf feed forward factor of the speed loop[%]M axis mass[kg]V speed[m/s]F driving effort[N]Rc radius of curvature[m]d distance to the apex[m]o lateral offset[m]εcontouring error[m]ωm lowest mechanical resonant frequency[s-1]ζdamping coefficient[-]r inertia ratio[-]μmembership function[-]αcorner angle[rad]x,y horizontal and vertical coordinates[m]?angle to the normal line[rad]1.I… 相似文献
99.
Ibrahim M.AL-HARKAN 《系统科学与系统工程学报(英文版)》2005,14(2):187-206
1.Introduction Production sequencing and scheduling is one of the most important activities in production planning and control.Sequencing is defined as the order in which the jobs are processed through the machines.The allocation of machines over time to process a collection of jobs is defined as Scheduling.Several methods have been developed to solve the scheduling problem which can be classified as follows:1)efficient optimal methods,2)implicit and explicit,or complete enumerative methods,a… 相似文献
100.
GanShi-xin 《武汉大学学报:自然科学英文版》2004,9(1):13-17
This paper investigates some conditions which imply the strong laws of large numbers for Banach space valued random variable sequences. Some generalizations of the Marcinkiewicz- Zygmund theorem and the HoffmannJφrgensen and Pisier theorem are obtained. 相似文献