全文获取类型
收费全文 | 63536篇 |
免费 | 5264篇 |
国内免费 | 3974篇 |
专业分类
化学 | 5033篇 |
晶体学 | 102篇 |
力学 | 4979篇 |
综合类 | 517篇 |
数学 | 13108篇 |
物理学 | 12449篇 |
综合类 | 36586篇 |
出版年
2024年 | 253篇 |
2023年 | 706篇 |
2022年 | 1349篇 |
2021年 | 1469篇 |
2020年 | 1567篇 |
2019年 | 1386篇 |
2018年 | 1382篇 |
2017年 | 1769篇 |
2016年 | 2003篇 |
2015年 | 1992篇 |
2014年 | 3192篇 |
2013年 | 3799篇 |
2012年 | 3683篇 |
2011年 | 3986篇 |
2010年 | 3125篇 |
2009年 | 3760篇 |
2008年 | 3434篇 |
2007年 | 4224篇 |
2006年 | 3681篇 |
2005年 | 3312篇 |
2004年 | 2942篇 |
2003年 | 2590篇 |
2002年 | 2373篇 |
2001年 | 1923篇 |
2000年 | 1792篇 |
1999年 | 1594篇 |
1998年 | 1312篇 |
1997年 | 1164篇 |
1996年 | 994篇 |
1995年 | 881篇 |
1994年 | 806篇 |
1993年 | 680篇 |
1992年 | 599篇 |
1991年 | 536篇 |
1990年 | 494篇 |
1989年 | 365篇 |
1988年 | 338篇 |
1987年 | 266篇 |
1986年 | 181篇 |
1985年 | 178篇 |
1984年 | 150篇 |
1983年 | 87篇 |
1982年 | 110篇 |
1981年 | 66篇 |
1980年 | 37篇 |
1979年 | 55篇 |
1978年 | 36篇 |
1977年 | 39篇 |
1976年 | 21篇 |
1973年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
孟良义 《中国科学B辑(英文版)》1993,(4)
According to the features of geophysics, experimental petrology, granite geology and geochemistry, the differentiations of ore-bearing granitic magmas are discussed. It is put forward that only the supergene differentiations of ore-bearing granitic magma might exist, instead of the hypogene one of the magma. Based on it, a supergene differentiation model for the ore-bearlng granitic magma is presented. 相似文献
62.
Linear and nonlinear optical properties of racemic (±)2-(α-methylbenzylamino)-5-nitropyridine ((±)MBANP) single crystals have been comprehensively investigated and compared with those of the enantiomorph (–)2-(α-methylbenzylamino)-5-nitropyridine ((–)MBANP) crystals. (±)MBANP crystal exhibits very high chemical and physical stability, but relatively small nonlinear optical coefficients (d31 = 6.8 pm/V, d32 = 4.7 pm/V, d33 = 0.84 pm/V). A comparison between the nonlinear optical coefficients of (±)MBANP and (–)MBANP demonstrates the validity of the oriented-gas model in molecular crystals that neglects all the contributions from intermolecular interaction. 相似文献
63.
无偏的岭回归迭代算法 总被引:1,自引:0,他引:1
本文探讨线性模型的无偏的岭回归迭代算法,这种算法保持最小二乘法的性质,当存在较为严重的共线性时,它能给出较为精确的参数及其协差阵的估计值;当存在严格的共线性时,给出参数及其协差阵的无穷多解中的一个,这个解由初值决定。文章还给出了算法的收敛性及一些其它性质的证明。 相似文献
64.
Hidetoshi Shimodaira 《Annals of the Institute of Statistical Mathematics》1997,49(3):395-410
The asymptotic error probability of Linhart's model selection test isevaluated, and compared with the nominal significance level. We examine thecase where the expected discrepancies of the candidate models from the truemodel are asymptotically equal. The local alternatives method is employed inthe limiting operation of the asymptotic evaluation. Although the errorprobability under the null hypothesis is actually shown to be equal to orless than the level for most situations, intolerable violations of the errorcontrol are observed for nested models: It is often erroneously concludedthat the smaller model is significantly better than the larger model. Toprevent this violation, a modification of Linhart's test statistic isproposed. The effectiveness of the proposed test is confirmed throughtheoretical analysis and numerical simulations. 相似文献
65.
This research extends previous work with dynamic models to manage groundwater quality by using the consumptive nitrate use rate instead of the nitrate application rate. The analysis indicates that misspecification results in overestimation of economic benefits, and supra-optimum nitrogen fertilizer application rates and groundwater nitrate stocks at a steady state. 相似文献
66.
We construct a correspondence between the set of partitions of a finite set M and the set of pairs of walks to the same vertex on a graph giving the Bratteli diagram of the partition algebra on M. This is the precise analogue of the correspondence between the set of permutations of a finite set and the set of pairs of Young tableaux of the same shape, called the Robinson–Schensted correspondence. 相似文献
67.
祁明 《陕西师范大学学报(自然科学版)》1992,(2)
在 K-饱和的非标准模型中,研究了泛 Loeb 可测集 m(A),pns(~*X),cpt(~*X),fin(~*X),ns(~*X)的 S-饱和性.当 A 是紧致 Hausdorff 空间 X 的子集时,得到了 st~(-1)(A)是标准集的充分条件. 相似文献
68.
通过对国产与进D的同类型精密镗削头振动模态实验研究,找出了这两种精密镗削头的固有振动频率,给出了它们的各阶动画振型图.从这一侧面解释了国产精密镗削头回转精度偏低的原因。 相似文献
69.
Neutral Community Theory: How Stochasticity and Dispersal-Limitation Can Explain Species Coexistence
Neutral community theory explains biodiversity, i.e. the coexistence of several species, as the result of a stochastic balance
between immigration and extinction on a local level, and between speciation and extinction on a regional level. The most popular
model, presented by Hubbell in 2001, has seen many analytical developments in recent years, which can be used in model analysis,
model testing and model comparison. We review these developments here, and present alternative derivations and shine previously
unnoticed lights on them. 相似文献
70.
属性区间识别理论模型研究及其应用 总被引:15,自引:3,他引:12
根据程乾生教授创立的属性识别理论模型[1] ,提出了属性测度区间的概念 ,建立了属性区间识别理论模型 ,并在自然科学学术期刊质量评价中进行了应用 . 相似文献