全文获取类型
收费全文 | 2841篇 |
免费 | 89篇 |
国内免费 | 147篇 |
专业分类
系统科学 | 103篇 |
丛书文集 | 83篇 |
教育与普及 | 61篇 |
理论与方法论 | 10篇 |
现状及发展 | 10篇 |
研究方法 | 18篇 |
综合类 | 2792篇 |
出版年
2024年 | 12篇 |
2023年 | 41篇 |
2022年 | 37篇 |
2021年 | 52篇 |
2020年 | 25篇 |
2019年 | 18篇 |
2018年 | 16篇 |
2017年 | 38篇 |
2016年 | 44篇 |
2015年 | 62篇 |
2014年 | 60篇 |
2013年 | 64篇 |
2012年 | 86篇 |
2011年 | 107篇 |
2010年 | 112篇 |
2009年 | 132篇 |
2008年 | 132篇 |
2007年 | 133篇 |
2006年 | 105篇 |
2005年 | 97篇 |
2004年 | 69篇 |
2003年 | 73篇 |
2002年 | 82篇 |
2001年 | 76篇 |
2000年 | 85篇 |
1999年 | 170篇 |
1998年 | 138篇 |
1997年 | 173篇 |
1996年 | 143篇 |
1995年 | 107篇 |
1994年 | 112篇 |
1993年 | 87篇 |
1992年 | 68篇 |
1991年 | 79篇 |
1990年 | 64篇 |
1989年 | 57篇 |
1988年 | 56篇 |
1987年 | 37篇 |
1986年 | 17篇 |
1985年 | 8篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有3077条查询结果,搜索用时 70 毫秒
991.
目的 :探讨血清透明质酸 (HA)、 型胶原 ( - C)联合检测对肝纤维化诊断的准确性。方法 :运用放射免疫法 (RIA)测定 98例慢性肝炎患者血清 HA、 - C水平 ,并与临床 B型超声检查结果进行对比研究。结果 :二项指标升高与肝纤维组织增生程度呈良好的正相关 (P<0 .0 1)。二项指标分别在慢性肝炎中型、慢性肝炎重型以及肝硬化中渐次升高 ,有显著差异 (P<0 .0 1)。同期 B超检查结果准确率不高。结论 :二项指标的联合检测在肝纤维化和早期肝硬化的诊断上较 B超检查准确、可靠。 相似文献
992.
聚变等离子体中存在负磁剪切的情况下,研究发现在α(α=-q2Rdβ/dr,q是安全因子,β是等离子体压力与磁压力之比,R和r分别代表托卡马克的大半径和小半径)引起的第二个势井里面存在αTAEs(α-induced Alfvén eigenmodes)。这些模式是一种很危险的本征模,容易被激发从而破坏聚变等离子体的稳定状态。对该本征模的研究,为该领域的研究提供了一些参考。 相似文献
993.
针对GPS接收机在跟踪过程中由频率误差引起的位同步概率下降问题,提出了一种基于频率补偿的GPS位同步方法.分析了环路中频率误差引起相干积分累加值衰减的机理,利用不同频率的复信号分别对相干积分结果进行补偿,计算出20个候选位置的值,最大值对应的位置即为数据边界位置.该方法有效减少了由频率误差导致的相干累加损耗,提高了位同步的概率.在弱信号情况下,使用非相干积分方法可进一步提高信号增益和位同步概率.与其他位同步方法相比,所提方法可容忍更大的频率误差,即使跟踪环路处于失锁状态也能准确实现位同步,且在频率误差小于25 Hz的情况下表现出更优的位同步性能. 相似文献
994.
995.
Critical Systems Thinking on the Inefficiency in Post-Earthquake Relief: A Practice in Longmen Shan Fault Area 总被引:1,自引:1,他引:0
Jiuping Xu Jiuzhou Dai Renqiao Rao Huaidong Xie Yi Lu 《Systemic Practice and Action Research》2016,29(5):425-448
The Longmen Shan Fault area is one of the most active seismic zones in the world, has a relatively dense population and a low economic development level. Therefore, the provision of effective post-earthquake relief is very important for saving lives and restricting financial losses. However, providing urgent relief is a complex problem that requires efficient coordination between the organizations, and departments involved, and the specific environments, for the supply of urgent materials, energy and information. In practice, although there are sufficient relief supplies, because of the lack of systemic thinking, relief activities are often chaotic leading to inefficiencies and even increases in the damage and loss of life. In this study, a critical systems thinking approach is used to reflect on the system contexts, boundaries, structures and functions of the emergency relief systems. Using systems modeling we constructed simulations for different relief plans and their consequences, and a work shop of metasynthetic engineering was constructed to integrate the different views, knowledge and contexts. At the practical stage, after the model’s validity was verified from the 2008 Wenchuan earthquake data, based on the 2013 Lushan earthquake simulation results, some recommendations are made to improve future post-disaster relief operations. Finally, the significance of critical systems thinking is demonstrated to ensure effective and efficient of post-earthquake relief. Some limitations and future research also have been presented in conclusion. 相似文献
996.
王昌龄是唐代著名的边塞诗人,他对当时边塞战争的认识经历了一个不断深化的过程,从他的边塞诗中,我们可以清楚地看到其对当时边塞战争的态度。青年时代漫游边关,慷慨激昂,热情讴歌、积极支持保卫边关的战争;深入了解边塞和戍边士卒的现实情况后,他开始厌倦没完没了的开边战争;随着人生阅历的增加,他对边塞战争给人民带来的苦难有了更深的认识,到晚年他更是明确反对穷兵黩武的开边、扩边战争。 相似文献
997.
998.
Recurrent event data frequently occur in many longitudinal studies, and the observation on recurrent events could be stopped by a terminal event such as death. This paper considers joint modeling and analysis of recurrent event and terminal event data through a common subject-specific frailty, in which the proportional intensity model is used for modeling the recurrent event process and the additive hazards model is used for modeling the terminal event time. Estimating equation approaches are developed for parameter estimation and asymptotic properties of the resulting estimators are established. In addition, some procedures are presented for model checking. The finite sample behavior of the proposed estimators is evaluated through simulation studies, and an application to a heart failure study is provided. 相似文献
999.
戴建国 《湖南文理学院学报(自然科学版)》2017,29(2)
为构造最佳对数线性模型,通过λ系数构造λ相关系数矩阵,将其作为对数线性模型中交互项选择的方法,并用于分析肺癌治疗数据。结果表明,选出的交互项可以构造最佳的模型对数据进行准确的分析,因此在应用对数线性模型分析前,先用相关系数矩阵进行探索性研究分析是很有必要的。 相似文献
1000.
Nitrogen and sulfur doped titanium dioxide photocatalysts were prepared by the sol-gel method. The products were characterized
by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-visible diffuse reflectance spectra (DRS). Photocatalytic
activities of the samples were investigated on the degradation of methyl orange (MO). The effect of the dopants on the electronic
structure of TiO2 was studied by the first-principles calculations based on the density functional theory (DFT). The orbital hybridization
resulted in energy gap narrowing and electronic delocalization in the crystal of doped TiO2. Mobile electrons of varied energetic states could offer enhanced electron transfer, together with optical absorption improvement.
The results show that the doping elements of N and S play a cooperative role in the modification of electronic structure,
which enhances the photocatalytic performance. The experimentally observed absorption edges of N-doped TiO2, S-doped TiO2, and N, S-codoped TiO2 are 420, 413, and 429 nm, respectively, which can be explained by the theoretical calculation results. 相似文献