全文获取类型
收费全文 | 26895篇 |
免费 | 79篇 |
国内免费 | 112篇 |
专业分类
系统科学 | 138篇 |
丛书文集 | 140篇 |
教育与普及 | 74篇 |
理论与方法论 | 113篇 |
现状及发展 | 11191篇 |
研究方法 | 1169篇 |
综合类 | 13807篇 |
自然研究 | 454篇 |
出版年
2013年 | 204篇 |
2012年 | 398篇 |
2011年 | 854篇 |
2010年 | 171篇 |
2008年 | 476篇 |
2007年 | 542篇 |
2006年 | 539篇 |
2005年 | 556篇 |
2004年 | 561篇 |
2003年 | 521篇 |
2002年 | 482篇 |
2001年 | 868篇 |
2000年 | 865篇 |
1999年 | 538篇 |
1994年 | 354篇 |
1992年 | 529篇 |
1991年 | 393篇 |
1990年 | 461篇 |
1989年 | 422篇 |
1988年 | 392篇 |
1987年 | 457篇 |
1986年 | 444篇 |
1985年 | 583篇 |
1984年 | 427篇 |
1983年 | 372篇 |
1982年 | 333篇 |
1981年 | 319篇 |
1980年 | 337篇 |
1979年 | 862篇 |
1978年 | 645篇 |
1977年 | 595篇 |
1976年 | 538篇 |
1975年 | 560篇 |
1974年 | 695篇 |
1973年 | 622篇 |
1972年 | 566篇 |
1971年 | 708篇 |
1970年 | 981篇 |
1969年 | 716篇 |
1968年 | 719篇 |
1967年 | 690篇 |
1966年 | 697篇 |
1965年 | 488篇 |
1959年 | 245篇 |
1958年 | 433篇 |
1957年 | 259篇 |
1956年 | 264篇 |
1955年 | 231篇 |
1954年 | 214篇 |
1948年 | 241篇 |
排序方式: 共有10000条查询结果,搜索用时 43 毫秒
61.
62.
A latent class probit model for analyzing pick any/N data 总被引:3,自引:3,他引:0
A latent class probit model is developed in which it is assumed that the binary data of a particular subject follow a finite
mixture of multivariate Bermoulli distributions. An EM algorithm for fitting the model is described and a Monte Carlo procedure
for testing the number of latent classes that is required for adequately describing the data is discussed. In the final section,
an application of the latent class probit model to some intended purchase data for residential telecommunication devices is
reported.
Geert De Soete is supported as “Bevoegdverklaard Navorser” of the Belgian “Nationaal Fonds voor Wetenschappelijk Onderzoek.” 相似文献
63.
卢武星 《北京师范大学学报(自然科学版)》1994,(4)
1.0MeV208Pb离子在非晶Si中的投影射程RP和射程偏差ΔRP作为注量和温度二者的函数用背散射法进行测定.注量的变化范围为5×1013~7×1014cm-2.注入是在室温和t=-120℃下完成的.由由实验所确定的投影射程,射程偏差与注量或温度无关,并且分别等于295和72.2nm.与TRIM86的计算结果相比较,发现RP的偏离为18%,而ΔRP的偏离为36%.RP和ΔRP二者与注量及温度的无关性,排除了所观察到的与TRIM的矛盾是由于注入期间辐射增强扩散或离子束混合效应而引起的解释。 相似文献
64.
提出了一个关于氧化物半导瓷晶界势垒起源的新观点,认为晶界势垒起源于烧结过程中外界氧在晶界中的扩散,与材料的结构、化学缺陷、掺杂、外界气氛、烧结工艺、组成状态等有密切关系,并用此理论解释了许多实验现象。 相似文献
65.
论述了交织编码的工作原理,给出了一种交织扩展Golay码的改进型软件设计方案,用TMS320C25实现了其编译码器并给出了模拟实验结果。实验结果表明,它能准确地纠正不大于72比特位的差错;处理128×128×8比特位的一幅图像数据的运行时间为0.412s. 相似文献
66.
Aspirin-like drugs may block pain independently of prostaglandin synthesis inhibition 总被引:2,自引:0,他引:2
K. Brune W. S. Beck G. Geisslinger S. Menzel-Soglowek B. M. Peskar B. A. Peskar 《Cellular and molecular life sciences : CMLS》1991,47(3):257-261
Summary Using flurbiprofen, a chiral anti-inflammatory and analgesic 2-arylpropionic acid derivative, the enantiomers of which are not converted to each other (less than 5%) in rats or man, we obtained evidence that prostaglandin synthesis inhibition is primarily mediating the anti-inflammatory activity but prostaglandin synthesis independent mechanisms contribute to the analgesic effects. Thus, the S-form inhibited prostaglandin synthesis, inflammation and nociception in rats. The R-form had much less effect on prostaglandin synthesis and did not affect inflammation. It did, however, block nociception in rats almost as potently as the S-form. S-flurbiprofen, in contrast to the R-form, was clearly ulcerogenic in the gastrointestinal mucosa. These results indicate additional molecular mechanisms of analgesia and suggest the use of R-arylpropionic acids as analgesics. 相似文献
67.
史济群 《华中科技大学学报(自然科学版)》1994,(3)
报道一种新颖的激光开槽、埋槽电极硅太阳电池的结构、工艺流程及其研制结果。在AM1.5,25℃,100mW/cm2的条件下,以面积为45cm3的36片硅片研制的硅太阳电池的输出参数的平均值为JSC=36.1mA/cm2,V∞=633mV,F.F.=0.798,η=18.23%,最后分析了种高性能硅太阳电池的设计特点。 相似文献
68.
Low-molecular-weight GTP-binding proteins are strong candidates for regulators of membrane traffic. In yeast, mutations in the sec4 or ypt1 genes encoding small GTP-binding proteins inhibit constitutive membrane flow at the plasma membrane or Golgi complex, respectively. It has been suggested that membrane fusion-fission events are regulated by cycling of small GTP-binding proteins between a membrane-bound and free state, but although most of these small proteins are found in both soluble and tightly membrane-bound forms, there is no direct evidence to support such cycling. In rat brain a small GTP-binding protein, rab3A, is exclusively associated with synaptic vesicles, the secretory organelles of nerve terminals. Here we use isolated nerve terminals to study the fate of rab3A during synaptic vesicle exocytosis. We find that rab3A dissociates quantitatively from the vesicle membrane after Ca2(+)-dependent exocytosis and that this dissociation is partially reversible during recovery after stimulation. These results are direct evidence for an association-dissociation cycle of a small GTP-binding protein during traffic of its host membrane. 相似文献
69.
对目前的UBBE模型进行了适当的推广,使在解决估计问题时可以考虑那些可能是误差上界的数值,并在此基础上提出了一种方法,能够通过对估计精度和可靠性进行合理的权衡确定所需估计值,实际案例研究结果表明所提方法能够较好地解决估计精度和可靠性之间的矛盾。 相似文献
70.
We develop in this paper an efficient way to select the best subset threshold autoregressive model. The proposed method uses a stochastic search idea. Differing from most conventional approaches, our method does not require us to fix the delay or the threshold parameters in advance. By adopting the Markov chain Monte Carlo techniques, we can identify the best subset model from a very large of number of possible models, and at the same time estimate the unknown parameters. A simulation experiment shows that the method is very effective. In its application to the US unemployment rate, the stochastic search method successfully selects lag one as the time delay and five best models from more than 4000 choices. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献