全文获取类型
收费全文 | 6208篇 |
免费 | 907篇 |
国内免费 | 269篇 |
专业分类
化学 | 477篇 |
晶体学 | 15篇 |
力学 | 423篇 |
综合类 | 49篇 |
数学 | 287篇 |
物理学 | 1050篇 |
无线电 | 5083篇 |
出版年
2024年 | 41篇 |
2023年 | 111篇 |
2022年 | 181篇 |
2021年 | 202篇 |
2020年 | 201篇 |
2019年 | 133篇 |
2018年 | 165篇 |
2017年 | 222篇 |
2016年 | 229篇 |
2015年 | 289篇 |
2014年 | 396篇 |
2013年 | 474篇 |
2012年 | 444篇 |
2011年 | 490篇 |
2010年 | 368篇 |
2009年 | 333篇 |
2008年 | 407篇 |
2007年 | 430篇 |
2006年 | 410篇 |
2005年 | 320篇 |
2004年 | 276篇 |
2003年 | 276篇 |
2002年 | 222篇 |
2001年 | 160篇 |
2000年 | 117篇 |
1999年 | 85篇 |
1998年 | 77篇 |
1997年 | 56篇 |
1996年 | 49篇 |
1995年 | 46篇 |
1994年 | 35篇 |
1993年 | 25篇 |
1992年 | 22篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 9篇 |
1988年 | 12篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 7篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1974年 | 2篇 |
1972年 | 1篇 |
1969年 | 1篇 |
1959年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有7384条查询结果,搜索用时 15 毫秒
81.
Fuqiang Yao Jianzhao Zhang Hangsheng Zhao Yongxiang Liu 《Wireless Communications and Mobile Computing》2015,15(10):1444-1455
In the heterogeneous and unreliable channel environment of cognitive radio ad hoc networks (CRAHNs), a multipath route with channel assigned is preferable in both throughput and reliability. The cross‐layer multipath routing and channel assignment in CRAHNs is becoming a challenging issue. In this paper, this problem is characterized, formulated, and shown to be in the form of mixed integer programming. For this Non‐deterministic Polynomial‐time (NP)‐hard problem, the deficiency of the widely used linearization and sequential fixing algorithm is first analyzed. The main contribution of this paper is the development of a new backtracking algorithm with feasibility checking to search optimal solutions and a heuristic algorithm with high feasible solution‐obtained probability (HHFOP) for distributed application in CRAHNs. Through feasibility checking and solution bounds validating, backtracking algorithm with feasibility checking cuts off unnecessary searching space in early stage without loss of optimal solutions, making it much more efficient than brute searching. For practical application in CRAHNs with polynomial complexity, HHFOP first computes the maximal‐supported throughput through link‐channel assignment and link‐capacity coordination for each candidate path. Then the paths are combined, and the route throughput is optimized. Extensive simulation results demonstrate that HHFOP can achieve a high feasible solution‐obtained probability with little throughput degradation compared with linearization and sequential fixing algorithm, indicating its practicability for distributed applications in CRAHNs. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
82.
83.
This paper gives a brief and systematic presentation of the computer simulation techniques of satellite communications, especially for a mobile satellite channel, based on a summarization of the author's exploration and experience in this area. It includes the equivalent complex baseband notation (ECBN) method, a very convenient mathematical form for software simulation; development of a multi-functional and expandable channel simulation system on a computer from L-band to Ka-band; channel impulse or frequency response estimation in real time by the technique burst-mode link analyser with least-squares algorithm (BMLA/LS), modelling and simulation of the channel fast fading due to multipath and shadowing; acquisition of the equivalent normalized binary signal-to-noise ratio (Eb/N0) data at the channel input; and bit-error rate (BER) evaluation by the directly error-counting approach and the computational approach at the channel output in simulation. 相似文献
84.
高新技术的智能交通系统(Intelligent Transport System,ITS)是改善世界性交通难题的希望所在。文中针对智能交通系统中的关键性问题的交通信息系统提出了构思,并深入研究了信息发布子系统,对无线数据传输提出了经济实用的方案,并予以实现。该文对提高我国城市交通系统的智能化水平具有参考意义。 相似文献
85.
在超声波流量计测量技术中, 雷诺修正系数相关的研究对于提高计量精度有重要作用. 为研究矩形流道的雷诺修正系数与雷诺数的关系, 对矩形流道在常温常压流量较小情况下进行仿真, 结果发现: 矩形流道层流状态下的雷诺修正系数与雷诺数呈线性相关. 保持压强、体积流量不变, 在不同温度下进行仿真及拟合, 结果表明: 在不同温度下雷诺修正系数与雷诺数的线性关系依然满足. 在上述实验基础上, 对矩形流道湍流状态下的雷诺修正系数与雷诺数关系进行研究, 通过改变温度、压强和体积流量进行仿真及拟合发现, 矩形流道湍流状态下雷诺修正系数与雷诺数呈非线性相关. 相似文献
86.
The Public key encryption scheme with keyword search (PEKS), firstly put forward by Boneh et al., can achieve the keyword searching without revealing any information of the initial data. However, the original PEKS scheme was required to construct a secure channel, which was usually expensive. Aimed at resolving this problem, Baek et al. put forward an improved scheme, which tried to construct a Secure channel free PEKS (SCF-PEKS). Subsequently, several SCF-PEKS schemes were proposed, however most of them turned out only secure in the random oracle model, which possibly lead to the construction of insecure schemes. Therefore, Fang et al. put forward an enhanced SCF-PEKS construction, which was provably secure in the standard model, however this construction needed a strong and complicated assumption. Then Yang et al. put forward an SCF-PEKS construction under simple assumption, but their construction had a big reduction in efficiency. In this article, we propose an SCF-PEKS construction, which is provably secure under the same assumption as that of Yang et al.'s scheme, however, with better performance. Then we give its full security proof, along with the performance analysis. Finally, we improve the SCF-PEKS construction to resist Keyword guessing attack (KGA) and give its security demonstration. 相似文献
87.
This work introduces channel-supermodular entropies, a subset of quasi-concave entropies. Channel-supermodularity is a property shared by some of the most commonly used entropies in the literature, including Arimoto–Rényi conditional entropies (which include Shannon and min-entropy as special cases), k-tries entropies, and guessing entropy. Based on channel-supermodularity, new preorders for channels that strictly include degradedness and inclusion (or Shannon ordering) are defined, and these preorders are shown to provide a sufficient condition for the more-capable and capacity ordering, not only for Shannon entropy but also regarding analogous concepts for other entropy measures. The theory developed is then applied in the context of query anonymization. We introduce a greedy algorithm based on channel-supermodularity for query anonymization and prove its optimality, in terms of information leakage, for all symmetric channel-supermodular entropies. 相似文献
88.
Lei Jia Jianzhu Wang Tianyuan Wang Xiaobao Li Haomin Yu Qingyong Li 《Entropy (Basel, Switzerland)》2022,24(4)
Vehicles carrying hazardous material (hazmat) are severe threats to the safety of highway transportation, and a model that can automatically recognize hazmat markers installed or attached on vehicles is essential for intelligent management systems. However, there is still no public dataset for benchmarking the task of hazmat marker detection. To this end, this paper releases a large-scale vehicle hazmat marker dataset named VisInt-VHM, which includes 10,000 images with a total of 20,023 hazmat markers captured under different environmental conditions from a real-world highway. Meanwhile, we provide an compact hazmat marker detection network named HMD-Net, which utilizes a revised lightweight backbone and is further compressed by channel pruning. As a consequence, the trained-model can be efficiently deployed on a resource-restricted edge device. Experimental results demonstrate that compared with some established methods such as YOLOv3, YOLOv4, their lightweight versions and popular lightweight models, HMD-Net can achieve a better trade-off between the detection accuracy and the inference speed. 相似文献
89.
90.