全文获取类型
收费全文 | 49626篇 |
免费 | 128篇 |
国内免费 | 172篇 |
专业分类
系统科学 | 254篇 |
丛书文集 | 1052篇 |
教育与普及 | 108篇 |
理论与方法论 | 271篇 |
现状及发展 | 22246篇 |
研究方法 | 1881篇 |
综合类 | 23348篇 |
自然研究 | 766篇 |
出版年
2013年 | 361篇 |
2012年 | 670篇 |
2011年 | 1433篇 |
2010年 | 306篇 |
2008年 | 845篇 |
2007年 | 937篇 |
2006年 | 919篇 |
2005年 | 942篇 |
2004年 | 971篇 |
2003年 | 874篇 |
2002年 | 907篇 |
2001年 | 1409篇 |
2000年 | 1313篇 |
1999年 | 919篇 |
1992年 | 878篇 |
1991年 | 692篇 |
1990年 | 721篇 |
1989年 | 734篇 |
1988年 | 712篇 |
1987年 | 732篇 |
1986年 | 761篇 |
1985年 | 945篇 |
1984年 | 743篇 |
1983年 | 606篇 |
1982年 | 532篇 |
1981年 | 563篇 |
1980年 | 690篇 |
1979年 | 1549篇 |
1978年 | 1305篇 |
1977年 | 1271篇 |
1976年 | 954篇 |
1975年 | 996篇 |
1974年 | 1472篇 |
1973年 | 1241篇 |
1972年 | 1284篇 |
1971年 | 1511篇 |
1970年 | 2039篇 |
1969年 | 1602篇 |
1968年 | 1440篇 |
1967年 | 1510篇 |
1966年 | 1338篇 |
1965年 | 995篇 |
1964年 | 291篇 |
1959年 | 559篇 |
1958年 | 938篇 |
1957年 | 715篇 |
1956年 | 571篇 |
1955年 | 520篇 |
1954年 | 591篇 |
1948年 | 346篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
【目的】群的阶及群的共轭类的个数对群的结构有重要影响,探讨用群G的阶以及共轭类的个数k(G)刻画交错群A5,A6及射影特殊线性群L2(7)。 【方法】利用有限群的共轭类的长与元素的中心化子的阶的关系及群的素图与群的结构的关系, 使用分类讨论法及有限群的同构的判别法。 【结果】由|G|= 60,k(G)=5证明了G~=A5。同样地,由|G|=168,k(G)=6证明了G~=L2(7),由|G|=360,k(G)=7证明了G~=A6。【结论】通过给定群的阶以及共轭类的个数可以刻画某些有限单群,但是是否能够通过群的阶以及共轭类的个数共同刻画的单群都满足一些特殊的性质仍是一个开放的问题。 相似文献
992.
The aim of this study was to determine the structural, compositional, and mineralogical composition of carbonatitic copper sulfide concentrator plant streams. Three samples, each from a different stream(run of mine(ROM), concentrate, and tailings) of a copper concentrator were characterized using various techniques, including stereomicroscopy, X-ray fluorescence, X-ray diffraction, Fourier transform infrared(FTIR) spectroscopy, scanning electron microscopy(SEM) in conjunction with energy-dispersive X-ray spectroscopy(EDS), and optical microscopy. The results reveal that each stream possesses its own unique compositional features. Carbonate minerals associated with calcite and dolomite, followed by quartz, remain the major minerals in both the ROM and tails streams. In the ROM stream, chalcopyrite appears to occur as veins within the carbonatite-hosting ore body. Mineral phase mutation was discovered in the tails stream because magnetite formerly identified in the ROM as the primary iron oxide had evolved into hematite. This metamorphosis was likely promoted by the concentration process. The concentration process was effective, upgrading the chalcopyrite content from 2 wt% in the ROM stream to 58 wt% in the concentrate stream; it was accompanied by bornite(4 wt%), anilite(3 wt%), and digenite(2.5 wt%). In addition, the concentrate stream exhibited properties distinctive from those of the other streams. The FTIR analysis showed the existence of a sulfide group related to the chalcopyrite mineral. Free chalcopyrite grains were observed in the concentrate by SEM analysis, and their mineral presence was supported by the EDS analysis results. All characterization techniques corresponded well with each other regarding the structure, chemistry, and composition of the samples. 相似文献
993.
Harold A. Linstone 《Systemic Practice and Action Research》1989,2(3):307-331
The evolution of the multiple perspective approach and its range of applications over the past decade are reviewed. The traditional technical perspective of systems analysis is augmented with organizational and personal perspectives. The three types of perspectives have inherently different characteristics and properties. The applications show that each perspective yields insights on a system that are not attainable with the others. The organizational and personal perspectives also focus more attention on problems of implementation. The concept is serving as an effective and practical vehicle to overcome the limitations of systems analysis in dealing with complex real-world situations. 相似文献
994.
995.
Respiration as the main determinant of carbon balance in European forests 总被引:106,自引:0,他引:106
Valentini R Matteucci G Dolman AJ Schulze ED Rebmann C Moors EJ Granier A Gross P Jensen NO Pilegaard K Lindroth A Grelle A Bernhofer C Grünwald T Aubinet M Ceulemans R Kowalski AS Vesala T Rannik U Berbigier P Loustau D Gudmundsson J Thorgeirsson H Ibrom A Morgenstern K Clement R 《Nature》2000,404(6780):861-865
Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha(-1) yr(-1), with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient. 相似文献
996.
Functional cartography of complex metabolic networks 总被引:16,自引:0,他引:16
High-throughput techniques are leading to an explosive growth in the size of biological databases and creating the opportunity to revolutionize our understanding of life and disease. Interpretation of these data remains, however, a major scientific challenge. Here, we propose a methodology that enables us to extract and display information contained in complex networks. Specifically, we demonstrate that we can find functional modules in complex networks, and classify nodes into universal roles according to their pattern of intra- and inter-module connections. The method thus yields a 'cartographic representation' of complex networks. Metabolic networks are among the most challenging biological networks and, arguably, the ones with most potential for immediate applicability. We use our method to analyse the metabolic networks of twelve organisms from three different superkingdoms. We find that, typically, 80% of the nodes are only connected to other nodes within their respective modules, and that nodes with different roles are affected by different evolutionary constraints and pressures. Remarkably, we find that metabolites that participate in only a few reactions but that connect different modules are more conserved than hubs whose links are mostly within a single module. 相似文献
997.
鱼尼丁受体类杀虫剂的研究进展 总被引:2,自引:0,他引:2
介绍了鱼尼丁受体类新型杀虫剂的创制现状、作用机制、生物活性、毒性及合成方法,由于此类杀虫剂的结构独特、作用方式新颖、对鳞翅目害虫效果好、杀虫广谱,该类药剂对各种益虫和天敌安全,对现用的杀虫剂无交互抗性,具有良好的发展前景. 相似文献
998.
根据实现差动保护功能所需信息类型的不同,提出分相电流差动和综合电流差动的新分类方法,并列举已有的综合电流差动保护方案.在对已有方案的性能评估和分析基础上,给出负序电流与正序电流故障分量相配合的综合电流差动新原理.理论分析和EMTP仿真表明,该方案可以对各种类型的区内故障灵敏动作. 相似文献
999.
Most recently produced wind generators use variable-speed systems for high-efficiency operation and low mechanical stress.In this study of a variable-speed standalone system that uses a synchronous machine,a power converter for the system was developed and tested.The maximum power points were approximated in a line according to the wind speed changes,and the linear approximation MPPT algorithm was used to determine the duty ratio that corresponds to the intersection point of the observed power and linearized line using the power generated from the generator at a point.With the fabricated converter,it was verified that the voltage,current and theta were normal with the varied load current. 相似文献
1000.
Mechanism of closure of the aortic valve 总被引:5,自引:0,他引:5