全文获取类型
收费全文 | 127011篇 |
免费 | 11811篇 |
国内免费 | 8091篇 |
专业分类
化学 | 40479篇 |
晶体学 | 2093篇 |
力学 | 8253篇 |
综合类 | 1542篇 |
数学 | 39345篇 |
物理学 | 55201篇 |
出版年
2023年 | 573篇 |
2022年 | 912篇 |
2021年 | 1891篇 |
2020年 | 1801篇 |
2019年 | 2130篇 |
2018年 | 1942篇 |
2017年 | 2013篇 |
2016年 | 2160篇 |
2015年 | 1959篇 |
2014年 | 3317篇 |
2013年 | 5289篇 |
2012年 | 3640篇 |
2011年 | 4468篇 |
2010年 | 4761篇 |
2009年 | 9694篇 |
2008年 | 10772篇 |
2007年 | 9433篇 |
2006年 | 8234篇 |
2005年 | 6059篇 |
2004年 | 5633篇 |
2003年 | 5662篇 |
2002年 | 6098篇 |
2001年 | 5005篇 |
2000年 | 4650篇 |
1999年 | 4311篇 |
1998年 | 3662篇 |
1997年 | 2795篇 |
1996年 | 2531篇 |
1995年 | 2926篇 |
1994年 | 2752篇 |
1993年 | 2276篇 |
1992年 | 1745篇 |
1991年 | 1285篇 |
1990年 | 1066篇 |
1989年 | 886篇 |
1988年 | 884篇 |
1987年 | 669篇 |
1986年 | 481篇 |
1985年 | 1335篇 |
1984年 | 919篇 |
1983年 | 654篇 |
1982年 | 968篇 |
1981年 | 1120篇 |
1980年 | 986篇 |
1979年 | 881篇 |
1978年 | 807篇 |
1977年 | 681篇 |
1976年 | 646篇 |
1974年 | 383篇 |
1973年 | 520篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
利用hirota双线性法,得到(3+1)维孤子方程、(3+1)维KP-Boussinesq方程、(2+1)维修正Caudrey-Dodd-Gibbon-Kotera-S awada方程、Hirota-Satsuma浅水波方程的精确解,并做出一部分解的图形,进一步研究解的结构和性质. 相似文献
2.
Dariusz Marek Adam Domaski Joanna Domaska Jakub Szygua Tadeusz Czachrski Jerzy Klamka 《Entropy (Basel, Switzerland)》2021,23(5)
In this article, a way to employ the diffusion approximation to model interplay between TCP and UDP flows is presented. In order to control traffic congestion, an environment of IP routers applying AQM (Active Queue Management) algorithms has been introduced. Furthermore, the impact of the fractional controller and its parameters on the transport protocols is investigated. The controller has been elaborated in accordance with the control theory. The TCP and UDP flows are transmitted simultaneously and are mutually independent. Only the TCP is controlled by the AQM algorithm. Our diffusion model allows a single TCP or UDP flow to start or end at any time, which distinguishes it from those previously described in the literature. 相似文献
3.
《Discrete Mathematics》2021,344(12):112601
4.
5.
科学评价大学生科研创新能力对我国科研水平的提高具有重要意义.采用机器学习模型来预测大学生科研能力可以起到良好的效果,提出一种GAXGBoost模型来实现对大学生的科研能力预测.此模型是以Xgboost算法为基础,然后充分利用遗传算法的全局搜索能力自动搜索Xgboost最优超参数,避免了人为经验调参不准确的缺陷,最后采用精英选择策略以此确保每一轮都是最佳的进化结果.通过分析表明,所采用的GAXGBoost模型在大学生科研能力预测的结果中具有很高的精度,将此模型与Logistic Regression、Random Forest、SVM等模型进行对比,GAXGBoost模型的预测精度最高. 相似文献
6.
Presenting the devices invented by Toepler, Holtz, Wimshurst, and Wommelsdorf, the history of the influence machines, the progress made, and the remaining deficits are discussed. The theoretical considerations published in the literature lack generalizable statements on the efficiency. Based on R. W. Pohl's model (1927), an elementary analysis is made, with the result that the upper limit of the theoretically achievable efficiency of influence machines is only 50%. 相似文献
7.
The row iterative method is popular in solving the large‐scale ill‐posed problems due to its simplicity and efficiency. In this work we consider the randomized row iterative (RRI) method to tackle this issue. First, we present the semiconvergence analysis of RRI method for the overdetermined and inconsistent system, and derive upper bounds for the noise error propagation in the iteration vectors. To achieve a least squares solution, we then propose an extended version of the RRI (ERRI) method, which in fact can converge in expectation to the solution of the overdetermined or underdetermined, consistent or inconsistent systems. Finally, some numerical examples are given to demonstrate the convergence behaviors of the RRI and ERRI methods for these types of linear system. 相似文献
8.
《中国物理 B》2021,30(5):56501-056501
Thermal expansion control is always an obstructive factor and challenging in high precision engineering field. Here,the negative thermal expansion of Nb F_3 and Nb OF_2 was predicted by first-principles calculation with density functional theory and the quasi-harmonic approximation(QHA). We studied the total charge density, thermal vibration, and lattice dynamic to investigate the thermal expansion mechanism. We found that the presence of O induced the relatively strong covalent bond in Nb OF_2, thus weakening the transverse vibration of F and O in Nb OF_2, compared with the case of Nb F_3.In this study, we proposed a way to tailor negative thermal expansion of metal fluorides by introducing the oxygen atoms.The present work not only predicts two NTE compounds, but also provides an insight on thermal expansion control by designing chemical bond type. 相似文献
9.
Due to their intrinsic link with nonlinear Fokker-Planck equations and many other applications, distribution dependent stochastic differential equations (DDSDEs) have been intensively investigated. In this paper, we summarize some recent progresses in the study of DDSDEs, which include the correspondence of weak solutions and nonlinear Fokker-Planck equations, the well-posedness, regularity estimates, exponential ergodicity, long time large deviations, and comparison theorems. 相似文献
10.
We propose a conjecture on the relative twist formula of l-adic sheaves, which can be viewed as a generalization of Kato—Saito's conjecture. We verify this conjecture under some transversal assumptions. We also define a relative cohomological characteristic class and prove that its formation is compatible with proper push-forward. A conjectural relation is also given between the relative twist formula and the relative cohomological characteristic class. 相似文献