全文获取类型
收费全文 | 11846篇 |
免费 | 668篇 |
国内免费 | 574篇 |
专业分类
系统科学 | 691篇 |
丛书文集 | 177篇 |
教育与普及 | 83篇 |
理论与方法论 | 14篇 |
现状及发展 | 238篇 |
综合类 | 11883篇 |
自然研究 | 2篇 |
出版年
2024年 | 38篇 |
2023年 | 178篇 |
2022年 | 227篇 |
2021年 | 309篇 |
2020年 | 311篇 |
2019年 | 270篇 |
2018年 | 271篇 |
2017年 | 298篇 |
2016年 | 331篇 |
2015年 | 481篇 |
2014年 | 708篇 |
2013年 | 580篇 |
2012年 | 873篇 |
2011年 | 815篇 |
2010年 | 633篇 |
2009年 | 683篇 |
2008年 | 543篇 |
2007年 | 858篇 |
2006年 | 723篇 |
2005年 | 655篇 |
2004年 | 542篇 |
2003年 | 477篇 |
2002年 | 422篇 |
2001年 | 328篇 |
2000年 | 250篇 |
1999年 | 216篇 |
1998年 | 166篇 |
1997年 | 156篇 |
1996年 | 138篇 |
1995年 | 124篇 |
1994年 | 100篇 |
1993年 | 87篇 |
1992年 | 62篇 |
1991年 | 58篇 |
1990年 | 62篇 |
1989年 | 42篇 |
1988年 | 36篇 |
1987年 | 18篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1981年 | 3篇 |
1955年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
针对移动通信卫星射频载荷产生的系统内和系统间复杂电磁兼容性问题,以及批产化研制特点带来的非专门电磁兼容性试验验证评估需求,提出了一种基于近场扫描的电磁兼容性评估方法。针对卫星载荷的辐射影响,采用柱面近场扫描获得载荷的近场辐射特性,用于系统内的电磁兼容性分析。基于近远场变换技术获得远场辐射特性,用于星座系统间的电磁兼容性评估。给出了探头补偿方法和扫描步进优化方法来提高测试准确度和测试效率。仿真和试验校验证明了方法的有效性。将该方法应用到卫星系统内和系统间电磁兼容性(electromagnetic compatibility, EMC)分析评估中,结果表明方法可有效满足工程应用需求。 相似文献
32.
33.
34.
目前光纤通信系统正朝着超高速、大容量、动态化方向快速发展,新的维度和复用方式以及更加复杂的体系架构对光纤传送网安全可靠运行提出了巨大的挑战.多维光纤通信系统中的性能监测技术能对系统所受损伤进行监测与预警,为自适应补偿、传输质量评估和网络资源优化等提供信息来源和管理依据,对提升光纤传送网运行和管理水平具有重要的科学意义和社会意义.该文首先重点介绍色散参量监测、非线性参量监测、调制格式参量监测等多个参量监测的研究现状,随后讨论并分析了多维光纤通信系统性能监测技术的发展趋势.未来性能监测技术将朝着精细化、一体化、智能化、多参量同步监测等方向发展. 相似文献
35.
The availability of numerous modeling approaches for volatility forecasting leads to model uncertainty for both researchers and practitioners. A large number of studies provide evidence in favor of combination methods for forecasting a variety of financial variables, but most of them are implemented on returns forecasting and evaluate their performance based solely on statistical evaluation criteria. In this paper, we combine various volatility forecasts based on different combination schemes and evaluate their performance in forecasting the volatility of the S&P 500 index. We use an exhaustive variety of combination methods to forecast volatility, ranging from simple techniques to time-varying techniques based on the past performance of the single models and regression techniques. We then evaluate the forecasting performance of single and combination volatility forecasts based on both statistical and economic loss functions. The empirical analysis in this paper yields an important conclusion. Although combination forecasts based on more complex methods perform better than the simple combinations and single models, there is no dominant combination technique that outperforms the rest in both statistical and economic terms. 相似文献
36.
Frederik Kunze 《Journal of forecasting》2020,39(2):313-333
By linking measures of forecast accuracy as well as testing procedures with regard to forecast rationality this paper investigates aggregated survey forecasts with forecast horizons of 3, 12, and 24 months for the exchange rates of the Chinese yuan, the Hong Kong dollar, the Japanese yen, and the Singapore dollar vis-à-vis the US dollar and, hence, for four different currency regimes. The rationality of the exchange rate predictions is initially assessed utilizing tests for unbiasedness and efficiency which indicate that the investigated forecasts are irrational in the sense that the predictions are biased. As one major contribution of this paper, it is subsequently shown that these results are not consistent with an alternative, less restrictive, measure of rationality. Investigating the order of integration of the time series as well as cointegrating relationships, this empirical evidence supports the conclusion that the majority of forecasts are in fact rational. Regarding forerunning properties of the predictions, the results are rather mediocre, with shorter term forecasts for the tightly managed USD/CNY FX regime being one exception. As one additional important and novel evaluation result, it can be concluded, that the currency regime matters for the quality of exchange rate forecasts. 相似文献
37.
The ability to improve out-of-sample forecasting performance by combining forecasts is well established in the literature. This paper advances this literature in the area of multivariate volatility forecasts by developing two combination weighting schemes that exploit volatility persistence to emphasise certain losses within the combination estimation period. A comprehensive empirical analysis of the out-of-sample forecast performance across varying dimensions, loss functions, sub-samples and forecast horizons show that new approaches significantly outperform their counterparts in terms of statistical accuracy. Within the financial applications considered, significant benefits from combination forecasts relative to the individual candidate models are observed. Although the more sophisticated combination approaches consistently rank higher relative to the equally weighted approach, their performance is statistically indistinguishable given the relatively low power of these loss functions. Finally, within the applications, further analysis highlights how combination forecasts dramatically reduce the variability in the parameter of interest, namely the portfolio weight or beta. 相似文献
38.
This paper is concerned with model averaging estimation for conditional volatility models. Given a set of candidate models with different functional forms, we propose a model averaging estimator and forecast for conditional volatility, and construct the corresponding weight-choosing criterion. Under some regulatory conditions, we show that the weight selected by the criterion asymptotically minimizes the true Kullback–Leibler divergence, which is the distributional approximation error, as well as the Itakura–Saito distance, which is the distance between the true and estimated or forecast conditional volatility. Monte Carlo experiments support our newly proposed method. As for the empirical applications of our method, we investigate a total of nine major stock market indices and make a 1-day-ahead volatility forecast for each data set. Empirical results show that the model averaging forecast achieves the highest accuracy in terms of all types of loss functions in most cases, which captures the movement of the unknown true conditional volatility. 相似文献
39.
Philip Hans Franses 《Journal of forecasting》2020,39(6):927-933
Each month, various professional forecasters give forecasts for next year's real gross domestic product (GDP) growth and unemployment. January is a special month, when the forecast horizon moves to the following calendar year. Instead of deleting the January data when analyzing forecast updates, I propose a periodic version of a test regression for weak-form efficiency. An application of this periodic model for many forecasts across a range of countries shows that in January GDP forecast updates are positive, whereas the forecast updates for unemployment are negative. I document that this January optimism about the new calendar year is detrimental to forecast accuracy. To empirically analyze Okun's law, I also propose a periodic test regression, and its application provides more support for this law. 相似文献
40.
The use of correlation between forecasts and actual returns is commonplace in the literature, often used as a measurement of investors' skill. A prominent application of this is the concept of the information coefficient (IC). Not only can the IC be used as a tool to rate analysts and fund managers but it also represents an important parameter in the asset allocation and portfolio construction process. Nevertheless, a theoretical understanding of it has typically been limited to the partial equilibrium context where the investing activities of each agent have no effect on other market participants. In this paper we show that this can be an undesirable oversimplification and we demonstrate plausible circumstances in which conventional empirical measurements of IC can be highly misleading. We suggest that improved understanding of IC in a general equilibrium setting can lead to refined portfolio decision making ex ante and more informative analysis of performance ex post. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献