全文获取类型
收费全文 | 16906篇 |
免费 | 516篇 |
国内免费 | 709篇 |
专业分类
系统科学 | 616篇 |
丛书文集 | 409篇 |
教育与普及 | 576篇 |
理论与方法论 | 104篇 |
现状及发展 | 122篇 |
研究方法 | 75篇 |
综合类 | 16226篇 |
自然研究 | 3篇 |
出版年
2024年 | 104篇 |
2023年 | 192篇 |
2022年 | 293篇 |
2021年 | 311篇 |
2020年 | 164篇 |
2019年 | 79篇 |
2018年 | 131篇 |
2017年 | 162篇 |
2016年 | 147篇 |
2015年 | 306篇 |
2014年 | 495篇 |
2013年 | 462篇 |
2012年 | 539篇 |
2011年 | 568篇 |
2010年 | 599篇 |
2009年 | 630篇 |
2008年 | 761篇 |
2007年 | 861篇 |
2006年 | 654篇 |
2005年 | 616篇 |
2004年 | 477篇 |
2003年 | 404篇 |
2002年 | 461篇 |
2001年 | 502篇 |
2000年 | 529篇 |
1999年 | 899篇 |
1998年 | 868篇 |
1997年 | 875篇 |
1996年 | 768篇 |
1995年 | 699篇 |
1994年 | 681篇 |
1993年 | 524篇 |
1992年 | 488篇 |
1991年 | 449篇 |
1990年 | 390篇 |
1989年 | 361篇 |
1988年 | 286篇 |
1987年 | 208篇 |
1986年 | 107篇 |
1985年 | 38篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1974年 | 2篇 |
1972年 | 4篇 |
1971年 | 2篇 |
1970年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
431.
The authors investigated the heterogeneous size patterns and dynamic growth of the ramet population of Panicum virgatum, a clonal caespitose plant, limited to the space occupied by a ramet bunch and the time of the ramet yearly life cycle, to understand the ecology of clonal caespitose plants in the field, where the ramet bunch generally consisted of more than one genet. Dynamic life tables for ramet populations were established by the replacement of living ramets at the present time with "dead" ones in past time. These tables revealed stable coexisting patterns of isometric and allometric growing processes of ramets in mass and height respectively, which approximately followed the historic trajectory of a density-independent population. The ecology of clonal caespitose plants is further discussed based on the competitively random growth of ramet individuals, including the scale of foraging behavior. In the field, the ramet population ecol-ogy of switchgrass may be a statistical result of competitively random growth of ramet individuals. The foraging behavior of a ramet population could then be presented as a process in which ramet individuals competed with each other for light and grew randomly, whileat the same time a relatively stable dynamic growth pattern was apparent at the level of the ramet population, and the functional leaves were placed properly in time and space. 相似文献
432.
根据线性化的单模激光输出光强方程,运用计算光强的相关函数,得到光强的功率谱及信噪比的方法,计算了在δ函数形式关联的两白噪声驱动下单模激光线性模型瞬态的功率谱及信噪比,发现了瞬态的随机共振,研究了信噪比随时间的演化及到达定态的时间,分析了参数对随机共振的影响。 相似文献
433.
434.
运用模糊综合评判法和相关理论,对宁夏职业院校师资队伍现状进行了定性和定量分析,采用Delphi法及AHP法确立指标体系,进行了实证研究,以期为高等职业院校师资队伍建设提供参考. 相似文献
435.
针对模型不准确时,传统Kalman滤波目标跟踪算法精度有限甚至发散的问题,研究了基于新息协方差在线匹配技术的自适应Kalman滤波算法,提高跟踪精度;并以Kalman滤波估计的目标位置为基础,利用一步Kalman预测得到下一时刻目标可能的位置范围,避免对整幅后帧图像进行遍历搜索,减小了计算量;为了避免存在干扰时异常量测对目标跟踪的影响,研究了量测信息异常检测算法,以Kalman预测的量测代替异常量测,增强抗干扰能力。实验证明,所提算法能够有效提高目标跟踪的精度和鲁棒性。 相似文献
436.
The human brain is a huge, complex system generating brain activity. The exploration of human brain function using functional magnetic resonance imaging (fMRI) is a promising method to understand brain activity. However, the complexity of the big data generated by fMRI facilitates the analysis of various levels of human brain activity, such as the distribution of neural represen- tations, the interaction between different regions, and the dynamic interaction over time. These different levels can depict distinct prospects of the human brain activity, and considerable progress has been achieved. In the future, more big data analysis methods combining advances in computer science, including larger-scale computing, machine learning, and graph theory, will promote the understanding of the human brain. 相似文献
437.
The acute toxicity of Sodium dodecylbenzene sulfonate (SDBS) to subcamersed macrophyte Hydrilla verticillata (L.f) Royle was studied. Chlorophyll contents and the activities of 3 antioxidant enzymes: superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7) in the leaves were investigated under different concentrations of SDBS, The chlorophyll contents in leaves of experimental plants decreased gradually, while the electrical conductivity of culture solution increased gradually with the increased dose of SDBS. Under higher concentrations of SDBS (32 mg/L and 128 mg/L) treatment, SOD lost its activities completely after 24 h. The native PAGE analysis indicated the diversity of POD isoenzymes was obvious under different concentrations of SDBS stress. 相似文献
438.
应用专家系统开发工具,以P507-HCI-煤油体系分离镨钕为例,首次开发了稀土溶剂萃取分离过程的故障监测与诊断专家系统REMD。该系统可对分离过程进行日常监测及故障诊断,并能迅速指出故障原因及修复调整的办法,以指导生产实践,确保产品的质量和产量。在实践中经过了考证。 相似文献
439.
Eclogite from the Northern Dabie Mountain is a new finding by the authors. These eclogites in foliated perdotite are enveloped by banded gneiss and occur in the mafic-ultramafic rock belt. They are mainly composed of omphacite, garnet, diopside, orthopyroxene, amphibole, plagioclase and magnetite, and a small amount of rutile, spinel, olivin and 鏾rundum. The mineral association of peak metamorphism of the eclogite is omphacite+garnet+rutile. The existence of eclogite in the Northern Dabie Mountain implies that there was an eclogitic metamorphism prior to the granulitic facies one in the mafic-ultramafic rock belt. 相似文献
440.
本文研究了在18-冠(醚)-6存在下,苯乙烯和甲基丙烯酸丁酯无乳化剂乳液共聚。发现18-冠(醚)-6能起到相转移催化作用,加速共聚反应。对实验结果提出两相成核的解释。 相似文献