全文获取类型
收费全文 | 38572篇 |
免费 | 124篇 |
国内免费 | 149篇 |
专业分类
系统科学 | 180篇 |
丛书文集 | 537篇 |
教育与普及 | 63篇 |
理论与方法论 | 132篇 |
现状及发展 | 16447篇 |
研究方法 | 1566篇 |
综合类 | 19191篇 |
自然研究 | 729篇 |
出版年
2013年 | 345篇 |
2012年 | 549篇 |
2011年 | 1270篇 |
2010年 | 241篇 |
2008年 | 634篇 |
2007年 | 797篇 |
2006年 | 740篇 |
2005年 | 764篇 |
2004年 | 794篇 |
2003年 | 674篇 |
2002年 | 705篇 |
2001年 | 1283篇 |
2000年 | 1228篇 |
1999年 | 767篇 |
1992年 | 727篇 |
1991年 | 557篇 |
1990年 | 618篇 |
1989年 | 607篇 |
1988年 | 569篇 |
1987年 | 571篇 |
1986年 | 630篇 |
1985年 | 778篇 |
1984年 | 569篇 |
1983年 | 502篇 |
1982年 | 432篇 |
1981年 | 440篇 |
1980年 | 513篇 |
1979年 | 1243篇 |
1978年 | 953篇 |
1977年 | 922篇 |
1976年 | 753篇 |
1975年 | 770篇 |
1974年 | 1147篇 |
1973年 | 928篇 |
1972年 | 1002篇 |
1971年 | 1125篇 |
1970年 | 1522篇 |
1969年 | 1149篇 |
1968年 | 1069篇 |
1967年 | 1024篇 |
1966年 | 993篇 |
1965年 | 709篇 |
1964年 | 212篇 |
1959年 | 370篇 |
1958年 | 689篇 |
1957年 | 474篇 |
1956年 | 372篇 |
1955年 | 352篇 |
1954年 | 376篇 |
1948年 | 258篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
83.
84.
A. Farina A. ProtopapaRadar Dpt Selenia S. p. A. Via Tiburtina Km. - Rome Italy 《系统工程与电子技术(英文版)》1992,(3)
After a brief recall of the Sidelobe Canceler (SLC) working principle, including the derivation of a general formula for the Cancellation Ratio (CR), the effects of channel mismatching are investigated. In particular, curves providing CRvalues as a function of amplitude and phase channel mismatching, radar bandwidth, and jammer direction of arrival (JDOA) are provided for the cases of one and two auxiliary antennas. Subsequently, a time -space processor for performance restoration is analyzed in detail. In addition to the above mentioned quantities, the attainable CR value is expressed as a function of the space-time processor parameters. The contribution of the paper is related to the derivation of a number of mathematical equations of CR for several cases of practical interest for the radar engineer. In addition, several curves are presented to assist design of SLC systems. 相似文献
85.
86.
87.
88.
89.
Acquired immunity and epidemiology of Schistosoma haematobium 总被引:1,自引:0,他引:1
Human immune responses to schistosome infection have been characterized in detail. But there has been controversy over the relative importance of ecological factors (variation in exposure to infection) and immunological factors (acquired immunity) in determining the relationships between levels of infection and age typically found in areas where infection is endemic. Independent effects of exposure and age on the rates of reinfection with Schistosoma haematobium after chemotherapy have been demonstrated in the Gambia and Zimbabwe. This age effect could be the result of acquired immunity to infection. Indeed, allowing for variation in exposure and age, low rates of reinfection in the Gambia are correlated with high amounts of specific IgE antibodies--human IgE can kill S. mansoni schistosomulae in vitro. Further, animals can acquire immunologically mediated resistance to S. mansoni infection, although nonimmunological factors could also be involved. Acquisition of this immunity seems to be related to the cumulative effects of repeated infection and provides only partial protection. These characteristics are consistent with immuno-epidemiological data for both S. mansoni and S. haematobium infections of humans. We have now analysed age-prevalence data for human infection with S. haematobium, and find patterns of variation that are indeed consistent with the epidemiological effects of acquired immunity predicted by mathematical models. 相似文献
90.
Neurotransmission from mossy fibre terminals onto cerebellar granule cells is almost certainly mediated by L-glutamate. By taking advantage of the small soma size, limited number of processes and short dendrite length of granule cells, we have obtained high-resolution recordings of spontaneous miniature excitatory postsynaptic currents (m.e.p.s.cs) and evoked currents in thin cerebellar slices. Miniature currents have a similar time-course and pharmacology to evoked currents and consist of an exceptionally fast non-NMDA (N-methyl-D-aspartate) component (measured rise-time, 200 microseconds; estimated pre-filtered rise-time less than 100 microseconds; decay time constant, tau = 1.0 ms), followed by 50 pS NMDA channel openings that are directly resolvable. We could find no evidence for the recent proposal that miniature currents in granule cells are mediated solely by NMDA channels with a novel time course. The non-NMDA receptor component of m.e.p.s.cs has a skewed amplitude distribution, which suggests potential complications for quantal analysis. The difference in time course between the m.e.p.s.cs reported here and other synaptic currents in the brain could reflect differences in synaptic function or electrotonic filtering; the relative contribution of these possibilities has yet to be established. 相似文献