全文获取类型
收费全文 | 2951篇 |
免费 | 280篇 |
国内免费 | 74篇 |
专业分类
化学 | 303篇 |
晶体学 | 2篇 |
力学 | 50篇 |
综合类 | 43篇 |
数学 | 1408篇 |
物理学 | 232篇 |
无线电 | 1267篇 |
出版年
2024年 | 3篇 |
2023年 | 28篇 |
2022年 | 66篇 |
2021年 | 81篇 |
2020年 | 79篇 |
2019年 | 64篇 |
2018年 | 61篇 |
2017年 | 91篇 |
2016年 | 101篇 |
2015年 | 103篇 |
2014年 | 178篇 |
2013年 | 194篇 |
2012年 | 195篇 |
2011年 | 194篇 |
2010年 | 146篇 |
2009年 | 169篇 |
2008年 | 232篇 |
2007年 | 203篇 |
2006年 | 130篇 |
2005年 | 148篇 |
2004年 | 134篇 |
2003年 | 93篇 |
2002年 | 79篇 |
2001年 | 89篇 |
2000年 | 79篇 |
1999年 | 66篇 |
1998年 | 46篇 |
1997年 | 40篇 |
1996年 | 41篇 |
1995年 | 28篇 |
1994年 | 20篇 |
1993年 | 24篇 |
1992年 | 16篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 10篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1979年 | 4篇 |
1977年 | 8篇 |
1976年 | 4篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有3305条查询结果,搜索用时 15 毫秒
171.
随着信息时代的来临,海量信息不断地涌入因特网,人们想要获得自己所需要的信息,越来越离不开搜索引擎。但目前的搜索引擎往往无法满足人们搜索信息的根本要求,常常有人抱怨,我所搜索的信息都是我不需要的。针对现有搜索引擎的不足,把信息推送技术引入到搜索引擎中,建立了一种基于信息推送的搜索引擎模型,解决了现有搜索引擎在搜索过程中存在的资源迷向问题,真正实现了信息找人的搜索方式。 相似文献
172.
173.
In this article, an inter-antenna inter-subblock shifting and inversion (IASSI) scheme is proposed to reduce the peak-to-average power ratio (PAPR) in multi-input multi- output orthogonal frequency division multiplexing (MIMO- OFDM) systems. It exploits multiple antennas and subblocks to provide additional degrees of freedom to benefit the system. To reduce the implementation complexity of the proposed scheme, two simple suboptimal schemes are further presented based on the minimum current maximum criterion; one adopts sequential search and the other employs random binary grouping. The simulation results exhibit the effectiveness of these proposed schemes. 相似文献
174.
数据在采集和传输过程中由于多种原因会造成矩阵残缺,因此在数据分析之前需要对残缺矩阵进行修复。常见的KNN修复方法 k值选取不合理,且需在整个矩阵中搜索近邻,影响算法的修复效果。在其基础上提出了一种k值自适应的局部KNN矩阵修复方法,合理考虑了k值的选取和近邻项的搜索范围。实验证明了该方法能有效提高矩阵修复的正确率,且算法的时效性有所提高。 相似文献
175.
176.
This study introduces a rollon–rolloff waste collection vehicle routing problem involving large containers that accumulate huge amounts of garbage at construction sites and shopping districts. In this problem, tractors move one container at a time between customer locations, a depot, disposal facilities, and container storage yards. The complicated constraints discussed in this study arise from having multiple disposal facilities, multiple container storage yards, seven service types of customer demands, different time windows for customer demands and facilities, various types and sizes of containers, and the lunch break of tractor drivers. In addition, real-world issues, such as changing service types, multiple demands at a customer’s location, and tractors with different work schedules, are dealt with. This study proposes a large neighborhood search based iterative heuristic approach consisting of several algorithms for the problem. The effectiveness of the proposed methods is demonstrated by computational experiments using benchmark data, some instances of which are derived from real-world problems. 相似文献
177.
Various phenomenon-mimicking algorithms, such as genetic algorithm, simulated annealing, tabu search, ant colony optimization, and particle swarm optimization, have their own algorithm parameters. These parameters need to be skillfully assigned in order to obtain good results. It is burdensome, especially to novice users, to assign these parameters. The same is true for the harmony search algorithm which was inspired by music performance. Thus, this study proposes a novel technique to eliminate tedious and experience-requiring parameter assigning efforts. The new parameter-setting-free (PSF) technique which this study suggests contains one additional matrix which contains an operation type (random selection, memory consideration, or pitch adjustment) for every variable in harmony memory. Three examples illustrate that the PSF technique can find good solutions robustly. 相似文献
178.
In this paper the optimization of additively decomposed discrete functions is investigated. For these functions genetic algorithms have exhibited a poor performance. First the schema theory of genetic algorithms is reformulated in probability theory terms. A schema defines the structure of a marginal distribution. Then the conceptual algorithm BEDA is introduced. BEDA uses a Boltzmann distribution to generate search points. From BEDA a new algorithm, FDA, is derived. FDA uses a factorization of the distribution. The factorization captures the structure of the given function. The factorization problem is closely connected to the theory of conditional independence graphs. For the test functions considered, the performance of FDA—in number of generations till convergence—is similar to that of a genetic algorithm for the OneMax function. This result is theoretically explained. 相似文献
179.
This paper describes a new multiobjective interactive memetic algorithm applied to dynamic location problems. The memetic
algorithm integrates genetic procedures and local search. It is able to solve capacitated and uncapacitated multi-objective
single or multi-level dynamic location problems. These problems are characterized by explicitly considering the possibility
of a facility being open, closed and reopen more than once during the planning horizon. It is possible to distinguish the
opening and reopening periods, assigning them different coefficient values in the objective functions. The algorithm is part
of an interactive procedure that asks the decision maker to define interesting search areas by establishing limits to the
objective function values or by indicating reference points. The procedure will be applied to some illustrative location problems. 相似文献
180.
John A. Ford Yasushi Narushima Hiroshi Yabe 《Computational Optimization and Applications》2008,40(2):191-216
Conjugate gradient methods are appealing for large scale nonlinear optimization problems, because they avoid the storage of
matrices. Recently, seeking fast convergence of these methods, Dai and Liao (Appl. Math. Optim. 43:87–101, 2001) proposed a conjugate gradient method based on the secant condition of quasi-Newton methods, and later Yabe and Takano (Comput.
Optim. Appl. 28:203–225, 2004) proposed another conjugate gradient method based on the modified secant condition. In this paper, we make use of a multi-step
secant condition given by Ford and Moghrabi (Optim. Methods Softw. 2:357–370, 1993; J. Comput. Appl. Math. 50:305–323, 1994) and propose two new conjugate gradient methods based on this condition. The methods are shown to be globally convergent
under certain assumptions. Numerical results are reported. 相似文献