全文获取类型
收费全文 | 31082篇 |
免费 | 4344篇 |
国内免费 | 2882篇 |
专业分类
化学 | 21144篇 |
晶体学 | 314篇 |
力学 | 2070篇 |
综合类 | 287篇 |
数学 | 3409篇 |
物理学 | 11084篇 |
出版年
2024年 | 105篇 |
2023年 | 596篇 |
2022年 | 933篇 |
2021年 | 988篇 |
2020年 | 1141篇 |
2019年 | 1117篇 |
2018年 | 931篇 |
2017年 | 837篇 |
2016年 | 1331篇 |
2015年 | 1317篇 |
2014年 | 1586篇 |
2013年 | 2134篇 |
2012年 | 2721篇 |
2011年 | 2745篇 |
2010年 | 1725篇 |
2009年 | 1688篇 |
2008年 | 1804篇 |
2007年 | 1711篇 |
2006年 | 1582篇 |
2005年 | 1294篇 |
2004年 | 978篇 |
2003年 | 826篇 |
2002年 | 778篇 |
2001年 | 590篇 |
2000年 | 588篇 |
1999年 | 687篇 |
1998年 | 621篇 |
1997年 | 604篇 |
1996年 | 699篇 |
1995年 | 542篇 |
1994年 | 493篇 |
1993年 | 400篇 |
1992年 | 377篇 |
1991年 | 336篇 |
1990年 | 269篇 |
1989年 | 214篇 |
1988年 | 160篇 |
1987年 | 140篇 |
1986年 | 134篇 |
1985年 | 121篇 |
1984年 | 91篇 |
1983年 | 64篇 |
1982年 | 42篇 |
1981年 | 36篇 |
1980年 | 18篇 |
1978年 | 19篇 |
1977年 | 23篇 |
1976年 | 20篇 |
1975年 | 28篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
本文利用主成分分析法分析了湖南各市州的经济发展水平和经济综合实力,对湖南省14个市州的经济发展情况做出综合评价,得出了各市州在湖南省的经济地位.最后用系统聚类分析法进行了验证,结果表明用多元统计分析法来评价地区经济发展水平是可行的. 相似文献
122.
123.
124.
四种多变量校准方法在FTIR多组分分析中的性能比较 总被引:1,自引:0,他引:1
本文对四种多变量校准方法--经典最小二乘法(CLS),偏最小二乘法(PLS),卡尔曼滤波法(KFM)以及人工神经网络法(ANN)--在多组分浓度分析方面的性能进行了比较。选择五种红外谱图严重混叠的大气有机毒物--1,3-丁二烯,苯,邻二甲苯,氯苯和丙烯醛--作为分析对象。分别计算各种方法对该5组分体系的平均预测误差MPE和平均相对误差MRE进行比较。结果表明,偏最小二乘法在处理这类问题中是最稳健的方法。 相似文献
125.
Dashan Huang Yoshitaka Kai Frank J. Fabozzi Masao Fukushima 《European Journal of Operational Research》2007
This paper presents a model for optimally designing a collateralized mortgage obligation (CMO) with a planned amortization class (PAC)-companion structure using dynamic cash reserve. In this structure, the mortgage pool’s cash flow is allocated by rule to the two bond classes such that PAC bondholders receive substantial prepayment protection, that protection being provided by the companion bondholders. The structure we propose provides greater protection to the PAC bondholders than current structures during periods of rising interest rates when this class of bondholders faces greater extension risk. We do so by allowing a portion of the cash flow from the collateral to be reserved to meet the PAC’s scheduled cash flow in subsequent periods. The greater protection is provided by the companion bondholders exposure to interest loss. To tackle this problem, we transform the problem of designing the optimal PAC-companion structure into a standard stochastic linear programming problem which can be solved efficiently. Moreover, we present an extended model by considering the quality of the companion bond and by relaxing the PAC bondholder shortfall constraint. Based on numerical experiments through Monte Carlo simulation, we show the utility of the proposed model. 相似文献
126.
To satisfy the volatile nature of today’s markets, businesses require a significant reduction in product development lead times. Consequently, the ability to develop precise product sales forecasts is of fundamental importance to decision-makers. Over the years, many forecasting techniques of varying capabilities have been introduced. The precise extent of their influences, and the interactions between them, has never been fully clarified, although various forecasting factors have been explored in previous studies. Accordingly, this study adopts the Taguchi method to calibrate the controllable factors of a forecasting model. An L9(34) inner orthogonal array is constructed for the controllable factors of data period, horizon length, and number of observations required. An experimental design is then performed to establish the appropriate levels for each factor. At the same time, an L4(23) outer orthogonal array is used to consider the inherited parameters of forecasting method as the noise factors of Taguchi method simultaneously. An illustrated example, employing data from a power company, serves to demonstrate the thesis. The results show that the proposed model permits the construction of a highly efficient forecasting model through the suggested data collection method. 相似文献
127.
128.
129.
Rhizopus arrhizus lipase (EC 3.1.1.3; triacylglycerol hydrolase) was used in this study to investigate the hydrolysis of palm kernel olein in AOT-isooctane-water reversed micelle system at Wo = 13, pH = 7, and T = 30?C. The hydrolytic reaction obeys Michaelis-Menten kinetics for substrate concentrations in the range (0.175 and 0.877M). The apparentK m and Vmax for the substrate were 0.397M (equivalent to 9.06% w/v) and 5523 U/mg protein, respectively. Product inhibition with a dissociation constant of the enzyme-product complex,K I = 9.74 mM, was confirmed. Experimental results from the change of product concentrations with respect to time correlated sufficiently with those predicted theoretically from the rate equation for a reaction time up to 100 min. However, the discrepancy between the observed results and the predicted ones would increase with reaction time. Possible reasons for this deviation were discussed. 相似文献
130.
介绍F-P干涉滤波器实现中心波长可调的两种机理,给出通过改变腔距来改变中心波工的滤波器的典型结构,并且给出了相应的电压驱动电路,实验表明选择合适的光放大器前置滤波器可以提高系统的灵敏度。 相似文献