全文获取类型
收费全文 | 446篇 |
免费 | 129篇 |
国内免费 | 18篇 |
专业分类
工业经济 | 39篇 |
计划管理 | 51篇 |
经济学 | 13篇 |
综合类 | 27篇 |
运输经济 | 3篇 |
旅游经济 | 7篇 |
贸易经济 | 228篇 |
农业经济 | 12篇 |
经济概况 | 10篇 |
水利工程 | 203篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 10篇 |
2021年 | 18篇 |
2020年 | 19篇 |
2019年 | 21篇 |
2018年 | 20篇 |
2017年 | 24篇 |
2016年 | 22篇 |
2015年 | 27篇 |
2014年 | 46篇 |
2013年 | 35篇 |
2012年 | 47篇 |
2011年 | 44篇 |
2010年 | 25篇 |
2009年 | 26篇 |
2008年 | 25篇 |
2007年 | 36篇 |
2006年 | 22篇 |
2005年 | 17篇 |
2004年 | 30篇 |
2003年 | 19篇 |
2002年 | 10篇 |
2001年 | 11篇 |
2000年 | 6篇 |
1999年 | 5篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
排序方式: 共有593条查询结果,搜索用时 15 毫秒
61.
A. V. Korzhov I. S. Okrainskaya A. I. Sidorov V. D. Kufel'd 《Power Technology and Engineering (formerly Hydrotechnical Construction)》2004,38(1):57-60
Data on the spectral composition and intensity of electromagnetic radiation of corona discharge are obtained in an experimental study performed on the outdoor switchgear of the Shagol 500-kV substation of the Chelyabinsk Enterprise of Trunk Transmission Grids and under a 500-kV Shagol – Kozyrevo overhead transmission line. The electromagnetic environment on the territory of the 500-kV outdoor switchgear is shown to be determined by narrow-band radiations (harmonics of the frequency of electric supply) and wide-band radiations due to corona discharges of high-voltage sources. This means that the personnel experience the action of a commercial-frequency electric field and electromagnetic radiation of a quite wide range, which is not allowed for by the existing guidelines. It is recommended to continue the study in cooperation with medical institutions in order to create guidelines that would allow for the joint action of commercial-frequency electric field and electromagnetic radiation and for the voltage in the line, the current load, the meteorological situation, and other factors. 相似文献
62.
Fluorescence depolarization is used to examine the effects of contaminants on the microviscosity of cell membranes of Ankistrodesmus sp. cells. One mg/L additions of Hg and Cd to the alga caused a change in the measured “index of viscosity” of the Ankistrodesmus cells in comparison to the index obtained by the addition of Zn and Pb. Elutriates from contaminated Niagara River sediment samples also caused an increase in the “index of viscosity,” whereas elutriate from uncontaminated sediments from the mid-lake station of Lake Ontario had no effect. This method has the potential for application in the assessment of the effects of contaminants on algal populations. 相似文献
63.
This paper evaluates the use of several parametric and nonparametric forecasting techniques for predicting tourism demand in selected European countries. We find that no single model can provide the best forecasts for any of the countries in the short-, medium- and long-run. The results, which are tested for statistical significance, enable forecasters to choose the most suitable model (from those evaluated here) based on the country and horizon for forecasting tourism demand. Should a single model be of interest, then, across all selected countries and horizons the Recurrent Singular Spectrum Analysis model is found to be the most efficient based on lowest overall forecasting error. Neural Networks and ARFIMA are found to be the worst performing models. 相似文献
64.
提出了联合测量超短激光脉冲混合啁啾量值的方法,这种方法是基于干涉自相关包络宽度及超短激光脉冲光谱宽度随啁啾量值的变化而变化.通过对干涉自相关信号包络宽度及超短激光脉冲光谱宽度的理论分析,得到干涉自相关包络宽度及超短激光脉冲光谱宽度与啁啾量值之间的对应关系,获得联合测量混合啁啾量值的方法.采用干涉自相关二次谐波检测系统对加浓染料激光器输出的脉冲进行测量,利用该方法测量出被测超短激光脉冲的线性啁啾值及平方啁啾值.根据干涉自相关包络信号及超短激光脉冲光谱,分别独立测量超短激光脉冲混合啁啾值,从而实现了对超短激光脉冲啁啾值的确定性测量. 相似文献
65.
Air transport demand forecasting is receiving increasing attention, especially because of intrinsic difficulties and practical applications. Total passengers are used as a proxy for air transport demand. However, the air passenger time series usually has a complex behavior due to their irregularity, high volatility and seasonality. This paper proposes a new hybrid approach, combining singular spectrum analysis (SSA), adaptive-network-based fuzzy inference system (ANFIS) and improved particle swarm optimization (IPSO), for short-term air passenger traffic prediction. The SSA is used for identifying and extracting the trend and seasonality of air transport demand and the artificial intelligence technologies, including ANFIS and IPSO, are utilized to deal with the irregularity and volatility of the demand. The HK air passenger data are collected to establish and validate the forecasting model. Empirical results clearly points to the enormous potential that the proposed approach possesses in air transport demand forecasting and can be considered as a viable alternative. 相似文献
66.
Andreas Grünwald 《Telecommunications Policy》2001,25(10-11)
Given its significant technological advantages compared to analog broadcasting, digital television (DTV) will be the television system of the future. However, it requires a full replacement of analog television sets by digital receivers, as DTV can only be watched with special equipment. In order to make this transition happen smoothly without losing the analog television audience, both signals have to be simulcasted until a substantial coverage with DTV broadcasting has been achieved. Australia and the US meanwhile have established a regulatory framework to lead this transition towards the end of analog broadcasting, the so-called analog switch-off. Part of the US regime is the FCC's obligation to reassign analog frequencies after the switch-off has taken place by means of auction. Spectrum auctions, however, originate not from the US but from New Zealand, where they have been used since the end of 1989 and ever since been subject to legal and political criticism. The article outlines the principles of both the DTV and spectrum auctions regulation and shows the links between both areas of telecommunications regulation. It concludes by suggesting that the analog switch-off is a unique opportunity to reconsider current spectrum policies, as it frees large amounts of the radio spectrum that are today occupied by analog broadcasting and will soon be subject to one of the biggest frequency reassignment processes in the history of telecommunications regulation. 相似文献
67.
《International Journal of Forecasting》2020,36(3):1128-1137
On 23 June, 2016, the UK held a referendum to decide whether to stay in the European Union or leave. The uncertainty surrounding the outcome of this referendum had major consequences for public policy, investment decisions, and currency markets. We discuss some of the subtleties involved in smoothing and disentangling poll data in light of the problem of tracking the dynamics of the intention to Brexit, and propose a multivariate singular spectrum analysis method that produces trendlines on the unit simplex. The trendline yield via multivariate singular spectrum analysis is shown to resemble that of local polynomial smoothing, and singular spectrum analysis presents the nice feature of disentangling the dynamics directly into components that can be interpreted as changes in public opinion or sampling error. The merits and disadvantages of some different approaches for obtaining smooth trendlines on the unit simplex are contrasted, in terms of both local polynomial smoothing and multivariate singular spectrum analysis. 相似文献
68.
69.
70.