全文获取类型
收费全文 | 20786篇 |
免费 | 910篇 |
国内免费 | 694篇 |
专业分类
系统科学 | 1537篇 |
丛书文集 | 537篇 |
教育与普及 | 690篇 |
理论与方法论 | 331篇 |
现状及发展 | 918篇 |
研究方法 | 59篇 |
综合类 | 18317篇 |
自然研究 | 1篇 |
出版年
2024年 | 110篇 |
2023年 | 205篇 |
2022年 | 317篇 |
2021年 | 350篇 |
2020年 | 177篇 |
2019年 | 81篇 |
2018年 | 824篇 |
2017年 | 859篇 |
2016年 | 615篇 |
2015年 | 432篇 |
2014年 | 545篇 |
2013年 | 472篇 |
2012年 | 782篇 |
2011年 | 1491篇 |
2010年 | 1375篇 |
2009年 | 1110篇 |
2008年 | 1272篇 |
2007年 | 1431篇 |
2006年 | 546篇 |
2005年 | 604篇 |
2004年 | 535篇 |
2003年 | 569篇 |
2002年 | 528篇 |
2001年 | 433篇 |
2000年 | 501篇 |
1999年 | 800篇 |
1998年 | 677篇 |
1997年 | 738篇 |
1996年 | 620篇 |
1995年 | 591篇 |
1994年 | 569篇 |
1993年 | 445篇 |
1992年 | 401篇 |
1991年 | 355篇 |
1990年 | 326篇 |
1989年 | 280篇 |
1988年 | 213篇 |
1987年 | 105篇 |
1986年 | 71篇 |
1985年 | 26篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Lanju XU 《系统科学与复杂性》2007,20(1):30-40
In the present paper, we focus on constructive spherical codes. By employing algebraic geometry codes, we give an explicit construction of spherical code sequences. By making use of the idea involved in the proof of the Gilbert-Varshamov bound in coding theory, we construct a spherical code sequence in exponential time which meets the best-known asymptotic bound by Shamsiev and Wyner. 相似文献
52.
53.
蒸汽机车锅炉水垢对机车的安全运行及工作效能有很大的影响,对蒸汽机车水垢的处理极其重要。蒸汽机车水垢是经过复杂的物理、化学变化而形成的。由于水垢的形成,造成锅炉的传热效率低,大量浪费燃料;降低锅炉出力危及锅炉安全运行;缩短使用寿命增大检修量,浪费工时、原料。为了缓解机车水垢的形成,可以采取尽量降低软化水硬度、确保洗修质量、合理安排排污时机和次数,提高排污效果等措施,并在实际运用中取得了一定的效果。 相似文献
54.
通过对西藏公务员工作满意度的调查,分析影响西藏公务员工作满意度的主要因素以及工作满意度体现在人口统计变量上的差异,并对此提出相应的对策建议。 相似文献
55.
56.
Rein Vihalemm 《Foundations of Science》2007,12(3):223-234
The philosophical analysis of chemistry has advanced at such a pace during the last dozen years that the existence of philosophy of chemistry as an autonomous discipline cannot be doubted any more. The present paper will attempt to analyse the experience of philosophy of chemistry at the, so to say, meta-level. Philosophers of chemistry have especially stressed that all sciences need not be similar to physics. They have tried to argue for chemistry as its own type of science and for a pluralistic understanding of science in general. However, when stressing the specific character of chemistry, philosophers do not always analyse the question ‘What is science?’ theoretically. It is obvious that a ‘monistic’ understanding of science should not be based simply on physics as the epitome of science, regarding it as a historical accident that physics has obtained this status. The author’s point is that the philosophical and methodological image of science should not be chosen arbitrarily; instead, it should be theoretically elaborated as an idealization (theoretical model) substantiated on the historical practice of science. It is argued that although physics has, in a sense, justifiably obtained the status of a paradigm of science, chemistry, which is not simply a physical science, but a discipline with a dual character, is also relevant for elaborating a theoretical model of science. The theoretical model of science is a good tool for examining various issues in philosophy of chemistry as well as in philosophy of science or science studies generally. 相似文献
57.
Global Optimization in Any Minkowski Metric: A Permutation-Translation Simulated Annealing Algorithm for Multidimensional Scaling 总被引:3,自引:1,他引:2
It is well known that considering a non-Euclidean Minkowski metric in Multidimensional Scaling, either for the distance model
or for the loss function, increases the computational problem of local minima considerably. In this paper, we propose an algorithm
in which both the loss function and the composition rule can be considered in any Minkowski metric, using a multivariate randomly
alternating Simulated Annealing procedure with permutation and translation phases. The algorithm has been implemented in Fortran
and tested over classical and simulated data matrices with sizes up to 200 objects. A study has been carried out with some
of the common loss functions to determine the most suitable values for the main parameters. The experimental results confirm
the theoretical expectation that Simulated Annealing is a suitable strategy to deal by itself with the optimization problems
in Multidimensional Scaling, in particular for City-Block, Euclidean and Infinity metrics. 相似文献
58.
S. K. MISHRA Shouyang WANG K. K. LAI 《系统科学与复杂性》2007,20(3):344-349
In this paper, we introduce a new class of generalized convex function, namely, a-pseudounivex function, by combining the concepts of pseudo-univex and α-invex functions. Further, we establish some relationships between vector variational-like inequality problems and vector optimization problems under the assumptions of α-pseudo-univex functions. Results obtained in this paper present a refinement and improvement of previously known results. 相似文献
59.
A theoretical model for mid-and low-latitude ionospheric electric fields in realistic geomagnetic fields 总被引:1,自引:1,他引:0
The geomagnetic fields, which play important roles in the ionospheric dynamo, can greatly affect the global distribution of ionospheric electric fields, currents and other ionospheric electrodynamics phenomena. In the study of ionospheric electrodynamics phenomena, such as the longitudinal variations of ionospheric electric fields, the non-dipolar component of the geomagnetic fields must be taken into account. In this paper, we deduce a theoretical electric field model for ionospheric dynamo at mid- and low-latitude which adopt a modified magnetic apex coordinates system. In the new electric field model, the geomagnetic fields can be calculated from either the IGRF model or the dipole field model, and the neutral winds and conductivities are calculated based on empirical models. Then the dynamo equation for the electric potential is finally solved in terms of the line-by-line iteration method, and the ionospheric electric fields and currents are derived from the calculated potential. Our model can reproduce the main features of the ionospheric electrodynamics processes, so it will be a useful tool for the investigation of the upper atmosphere and ionosphere. 相似文献
60.
根据实际工程应用的特点,改进了H_∞状态反馈控制的算法,并给出了一种选择H_∞权矩阵的方法;以此为基础设计及研究了一种能综合控制水轮发电机组的励磁和导叶开度,集励磁调节器、调速器功能于一体的综合控制系统.结果表明,该控制器在基本不影响系统响应性能的情况下,能明显地改善系统的鲁棒性. 相似文献