全文获取类型
收费全文 | 25201篇 |
免费 | 58篇 |
国内免费 | 72篇 |
专业分类
系统科学 | 108篇 |
丛书文集 | 295篇 |
教育与普及 | 65篇 |
理论与方法论 | 104篇 |
现状及发展 | 10283篇 |
研究方法 | 1064篇 |
综合类 | 12984篇 |
自然研究 | 428篇 |
出版年
2013年 | 233篇 |
2012年 | 353篇 |
2011年 | 783篇 |
2010年 | 154篇 |
2008年 | 402篇 |
2007年 | 486篇 |
2006年 | 477篇 |
2005年 | 480篇 |
2004年 | 449篇 |
2003年 | 434篇 |
2002年 | 445篇 |
2001年 | 882篇 |
2000年 | 862篇 |
1999年 | 494篇 |
1992年 | 467篇 |
1991年 | 376篇 |
1990年 | 404篇 |
1989年 | 435篇 |
1988年 | 403篇 |
1987年 | 383篇 |
1986年 | 419篇 |
1985年 | 493篇 |
1984年 | 373篇 |
1983年 | 349篇 |
1982年 | 278篇 |
1981年 | 280篇 |
1980年 | 335篇 |
1979年 | 818篇 |
1978年 | 630篇 |
1977年 | 615篇 |
1976年 | 482篇 |
1975年 | 525篇 |
1974年 | 778篇 |
1973年 | 616篇 |
1972年 | 647篇 |
1971年 | 763篇 |
1970年 | 963篇 |
1969年 | 734篇 |
1968年 | 764篇 |
1967年 | 669篇 |
1966年 | 649篇 |
1965年 | 452篇 |
1964年 | 112篇 |
1959年 | 245篇 |
1958年 | 411篇 |
1957年 | 310篇 |
1956年 | 257篇 |
1955年 | 234篇 |
1954年 | 229篇 |
1948年 | 186篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
32.
F. Acerbi 《Archive for History of Exact Sciences》2003,57(6):465-502
33.
Contemporary developments in economicmethodology have produced a vibrant agenda ofcompeting positions. These include, amongothers, constructivism, critical realism andrhetoric, with each contributing to the Realistvs. Pragmatism debate in the philosophies of thesocial sciences. A major development in theneo-pragmatist contribution to economicmethodology has been Quine's pragmatic assaulton the dogmas of empiricism, which are nowclearly acknowledged within contemporaryeconomic methodology. This assault isencapsulated in the celebrated Duhem-Quinethesis, which according to a number ofcontemporary leading philosophers of economics,poses a particularly serious methodologicalproblem for economics. This problem, asreflected in Hausman's analysis, consists ofthe inability of economics to learn fromexperience, thereby subverting the capacity totest economic theories. In this paper wedispute this position. Our argument is basedon a combination of Quine's holism with VanFraassen's constructive empiricism, especiallythe latter's analysis of empirical adequacy andhis pragmatic approach to explanation. Theresulting reorientation of economic methodologyrestores the capacity of economics to learnfrom experience and reinstates the imperativeof developing alternatives to orthodoxtheorizing in economics. 相似文献
34.
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
35.
36.
The genetic engineering of production traits in domestic animals 总被引:1,自引:0,他引:1
37.
M E MacDonald A Novelletto C Lin D Tagle G Barnes G Bates S Taylor B Allitto M Altherr R Myers 《Nature genetics》1992,1(2):99-103
Analysis of 78 Huntington's disease (HD) chromosomes with multi-allele markers revealed 26 different haplotypes, suggesting a variety of independent HD mutations. The most frequent haplotype, accounting for about one third of disease chromosomes, suggests that the disease gene is between D4S182 and D4S180. However, the paucity of an expected class of chromosomes that can be related to this major haplotype by assuming single crossovers may reflect the operation of other mechanisms in creating haplotype diversity. Some of these mechanisms sustain alternative scenarios that do not require a multiple mutational origin for HD and/or its positioning between D4S182 and D4S180. 相似文献
38.
Structure of the detoxification catalyst mercuric ion reductase from Bacillus sp. strain RC607 总被引:10,自引:0,他引:10
Several hundred million tons of toxic mercurials are dispersed in the biosphere. Microbes can detoxify organo-mercurials and mercury salts through sequential action of two enzymes, organomercury lyase and mercuric ion reductase (MerA). The latter, a homodimer with homology to the FAD-dependent disulphide oxidoreductases, catalyses the reaction NADPH + Hg(II)----NADP+ + H+ + Hg(0), one of the very rare enzymic reactions with metal substrates. Human glutathione reductase serves as a reference molecule for FAD-dependent disulphide reductases and between its primary structure and that of MerA from Tn501 (Pseudomonas), Tn21 (Shigella), p1258 (Staphylococcus) and Bacillus, 25-30% of the residues have been conserved. All MerAs have a C-terminal extension about 15 residues long but have very varied N termini. Although the enzyme from Streptomyces lividans has no addition, from Pseudomonas aeruginosa Tn501 and Bacillus sp. strain RC607 it has one and two copies respectively of a domain of 80-85 residues, highly homologous to MerP, the periplasmic component of proteins encoded by the mer operon. These domains can be proteolytically cleaved off without changing the catalytic efficiency. We report here the crystal structure of MerA from the Gram-positive bacterium Bacillus sp. strain RC607. Analysis of its complexes with nicotinamide dinucleotide substrates and the inhibitor Cd(II) reveals how limited structural changes enable an enzyme to accept as substrate what used to be a dangerous inhibitor. Knowledge of the mode of mercury ligation is a prerequisite for understanding this unique detoxification mechanism. 相似文献
39.
40.
Genotoxic effects of dithane M-45 were studied on the bone marrow cells of male albino mice (Lacca strain) in vivo. Different doses (30 mg, 40 mg and 300 mg/kg b.wt) of dithane M-45 were injected intraperitoneally and their effects were investigated after time intervals of 1, 2, 5 and 10 days. The chromosomal aberrations observed in the bone marrow cells of male mice after treatment with dithane M-45 were fragments, rings, dicentric chromosomes, terminal chromatid deletions, chromatid gaps and breaks. In addition to these chromosomal aberrations, physiological effects such as uneven stretching of chromatin material, end-to-end chromosomal associations, exchange configurations, clumping, stickiness and centromeric associations were also observed. 相似文献