全文获取类型
收费全文 | 16337篇 |
免费 | 56篇 |
国内免费 | 96篇 |
专业分类
系统科学 | 101篇 |
丛书文集 | 86篇 |
教育与普及 | 24篇 |
理论与方法论 | 51篇 |
现状及发展 | 7843篇 |
研究方法 | 780篇 |
综合类 | 7413篇 |
自然研究 | 191篇 |
出版年
2012年 | 257篇 |
2011年 | 433篇 |
2010年 | 105篇 |
2009年 | 95篇 |
2008年 | 298篇 |
2007年 | 369篇 |
2006年 | 352篇 |
2005年 | 319篇 |
2004年 | 287篇 |
2003年 | 275篇 |
2002年 | 333篇 |
2001年 | 554篇 |
2000年 | 560篇 |
1999年 | 350篇 |
1992年 | 310篇 |
1991年 | 254篇 |
1990年 | 272篇 |
1989年 | 248篇 |
1988年 | 244篇 |
1987年 | 283篇 |
1986年 | 255篇 |
1985年 | 361篇 |
1984年 | 290篇 |
1983年 | 229篇 |
1982年 | 238篇 |
1981年 | 238篇 |
1980年 | 232篇 |
1979年 | 524篇 |
1978年 | 444篇 |
1977年 | 368篇 |
1976年 | 402篇 |
1975年 | 378篇 |
1974年 | 410篇 |
1973年 | 335篇 |
1972年 | 336篇 |
1971年 | 450篇 |
1970年 | 585篇 |
1969年 | 401篇 |
1968年 | 439篇 |
1967年 | 449篇 |
1966年 | 357篇 |
1965年 | 261篇 |
1964年 | 143篇 |
1959年 | 151篇 |
1958年 | 231篇 |
1957年 | 172篇 |
1956年 | 140篇 |
1955年 | 126篇 |
1954年 | 122篇 |
1948年 | 103篇 |
排序方式: 共有10000条查询结果,搜索用时 828 毫秒
1.
R.S.K. Barnes 《Journal of Natural History》2018,52(1-2):87-113
Areas of the Knysna estuarine bay in the Western Cape are dominated by three endemic South African truncatelloid microgastropods, temporarily known as ‘Hydrobia’ knysnaensis (Krauss), ‘Assiminea’ capensis (Sowerby) and ‘Assiminea’ globulus Connolly. Although first described 80–170 years ago and present in abundance (up to 100,000 m?2), they remain surrounded by confusion and still await taxonomic assignment, largely because they appear most atypical members of their groups by virtue of anatomy and/or biogeography and/or habitat. This study contributes in-life perspectives to morphological and phylogenetic analyses known to be on-going. At Knysna, they are syntopic: at least two occurring in >85% and all three in >40% of individual 0.0026 m2 samples from their region of dominance. Nevertheless, they tend to greater abundance in divergent microhabitats; ‘A.’ globulus dominating higher tidal levels, and ‘A.’ capensis and ‘Hydrobia’ lower ones; the former especially unvegetated sediment, the latter, if anything, seagrass. Interspecific feeding interactions appear unlikely to be responsible for these patterns, other evidence suggesting that all are maintained below carrying capacity. Field biology of ‘H.’ knysnaensis generally appears equivalent to that of northern-hemisphere intertidal hydrobiids and that of ‘A.’ globulus is typically assimineid, albeit at atypically low shore height. Unlike assimineids, however, ‘A.’ capensis is truly aquatic. The success of these truncatelloids in unusual circumstances may be consequent on the absence from South Africa of other microgastropod groups that fill their niches elsewhere in the southern hemisphere. 相似文献
2.
This paper undertakes a comprehensive examination of 10 measures of core inflation and evaluates which measure produces the best forecast of headline inflation out‐of‐sample. We use the Personal Consumption Expenditure Price Index as our measure of inflation. We use two sets of components (17 and 50) of the Personal Consumption Expenditure Price Index to construct these core inflation measures and evaluate these measures at the three time horizons (6, 12 and 24 months) most relevant for monetary policy decisions. The best measure of core inflation for both sets of components and over all time horizons uses weights based on the first principal component of the disaggregated (component‐level) prices. Interestingly, the results vary by the number of components used; when more components are used the weights based on the persistence of each component is statistically equivalent to the weights generated by the first principal component. However, those forecasts using the persistence of 50 components are statistically worse than those generated using the first principal component of 17 components. The statistical superiority of the principal component method is due to the fact that it extracts (in the first principal component) the common source of variation in the component level prices that accurately describes trend inflation over the next 6–24 months. 相似文献
3.
4.
复数,实数及实对称数据下Bruun FFT算法的实现 总被引:3,自引:0,他引:3
本文在复数、实数以及实对称数据下研究和实现了Bruun FFT算法。文章分析和评价了各种数据下BruunFFT的运算量和特点,提出了算法的实现结构和方法。最后给了TMS320C30上的运算时间,我们实现的算法比基2FFT速度提高35%。 相似文献
5.
6.
7.
F. Weygand A. Wacker V. Schmied-Kowarzik 《Cellular and molecular life sciences : CMLS》1948,4(11):427-428
Summary By condensing 2:4:5-triamino-6-hydroxy-pyrimidine with dihydroxyacetone (diacetate), diaminoacetone or acetone-1,3-di (p-formylaminobenzoic acid) not the expected 8- or 9-oxymethyl resp. -aminomethyl-pteridines but 8-or 9-methyl-pteridines were obtained. With p-tolyl-d-isoglucosamine not a tetrahydroxybutyl-pteridine but a trihydroxybutyl-pteridine was formed. For an explanation of these results it is supposed that from the dihydro-pteridines formed at first by intramolecular splitting off of H2O or R·NH2 aromatization takes place. 相似文献
8.
9.
V.A.帕瑟根 《国外科技新书评介》2007,(2):7-7
分子间的范德华力是十分微弱的,但可以说普遍的存在着,在很多体系中起着巨大的作用,在物理学、化学、生物学等学科中得到了广泛的研究。但怎样计算范德华力?如何考虑各种条件对范德华力的影响? 相似文献
10.