全文获取类型
收费全文 | 123617篇 |
免费 | 9562篇 |
国内免费 | 11141篇 |
专业分类
化学 | 52374篇 |
晶体学 | 991篇 |
力学 | 9691篇 |
综合类 | 1676篇 |
数学 | 33510篇 |
物理学 | 46078篇 |
出版年
2024年 | 172篇 |
2023年 | 1030篇 |
2022年 | 2355篇 |
2021年 | 2471篇 |
2020年 | 2535篇 |
2019年 | 2796篇 |
2018年 | 2471篇 |
2017年 | 2882篇 |
2016年 | 3467篇 |
2015年 | 2994篇 |
2014年 | 4362篇 |
2013年 | 8526篇 |
2012年 | 5256篇 |
2011年 | 5919篇 |
2010年 | 4874篇 |
2009年 | 6958篇 |
2008年 | 7479篇 |
2007年 | 8162篇 |
2006年 | 7225篇 |
2005年 | 6181篇 |
2004年 | 5536篇 |
2003年 | 5667篇 |
2002年 | 5614篇 |
2001年 | 4432篇 |
2000年 | 4290篇 |
1999年 | 3828篇 |
1998年 | 3541篇 |
1997年 | 2716篇 |
1996年 | 2519篇 |
1995年 | 2265篇 |
1994年 | 2125篇 |
1993年 | 1729篇 |
1992年 | 1765篇 |
1991年 | 1263篇 |
1990年 | 1075篇 |
1989年 | 919篇 |
1988年 | 840篇 |
1987年 | 679篇 |
1986年 | 585篇 |
1985年 | 622篇 |
1984年 | 624篇 |
1983年 | 322篇 |
1982年 | 487篇 |
1981年 | 540篇 |
1980年 | 403篇 |
1979年 | 395篇 |
1978年 | 313篇 |
1977年 | 300篇 |
1976年 | 226篇 |
1973年 | 166篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
1.
2.
3.
《Operations Research Letters》2022,50(2):137-144
In mine planning problems, cutoff grade optimization defines a threshold at every time period such that material above this value is processed, and the rest is considered waste. In orebodies with multiple minerals, which occur in practice, the natural extension is to consider a cutoff surface. We show that in two dimensions the optimal solution is a line, and in n dimensions it is a hyperplane. 相似文献
4.
This paper infers from a generalized Picone identity the uniqueness of the stable positive solution for a class of semilinear equations of superlinear indefinite type, as well as the uniqueness and global attractivity of the coexistence state in two generalized diffusive prototypes of the symbiotic and competing species models of Lotka–Volterra. The optimality of these uniqueness theorems reveals the tremendous strength of the Picone identity. 相似文献
5.
M. Amar D. Andreucci R. Gianni C. Timofte 《Journal of Mathematical Analysis and Applications》2021,493(2):124533
We prove a well-posedness result for two pseudo-parabolic problems, which can be seen as two models for the same electrical conduction phenomenon in heterogeneous media, neglecting the magnetic field. One of the problems is the concentration limit of the other one, when the thickness of the dielectric inclusions goes to zero. The concentrated problem involves a transmission condition through interfaces, which is mediated by a suitable Laplace-Beltrami type equation. 相似文献
6.
《Annales de l'Institut Henri Poincaré (C) Analyse Non Linéaire》2021,38(5):1337-1371
In this paper we consider minimizers of the functional where is a bounded open set and where are the first k eigenvalues on Ω of an operator in divergence form with Dirichlet boundary condition and with Hölder continuous coefficients. We prove that the optimal sets have finite perimeter and that their free boundary is composed of a regular part, which is locally the graph of a -regular function, and a singular part, which is empty if , discrete if and of Hausdorff dimension at most if , for some . 相似文献
7.
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
8.
9.
Bifurcation analysis of visual angle model with anticipated time and stabilizing driving behavior
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Xueyi Guan 《中国物理 B》2022,31(7):70507-070507
In the light of the visual angle model (VAM), an improved car-following model considering driver's visual angle, anticipated time and stabilizing driving behavior is proposed so as to investigate how the driver's behavior factors affect the stability of the traffic flow. Based on the model, linear stability analysis is performed together with bifurcation analysis, whose corresponding stability condition is highly fit to the results of the linear analysis. Furthermore, the time-dependent Ginzburg-Landau (TDGL) equation and the modified Korteweg-de Vries (mKdV) equation are derived by nonlinear analysis, and we obtain the relationship of the two equations through the comparison. Finally, parameter calibration and numerical simulation are conducted to verify the validity of the theoretical analysis, whose results are highly consistent with the theoretical analysis. 相似文献
10.
Chemical bonding in representative astrophysically relevant neutral,cation, and anion HCnH chains
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Ioan Baldea 《中国物理 B》2022,31(12):123101-123101
Most existing studies assign a polyynic and cumulenic character of chemical bonding in carbon-based chains relying on values of the bond lengths. Building on our recent work, in this paper we add further evidence on the limitations of such an analysis and demonstrate the significant insight gained via natural bond analysis. Presently reported results include atomic charges, natural bond order and valence indices obtained from ab initio computations for representative members of the astrophysically relevant neutral and charged HC2k/2k+1H chain family. They unravel a series of counter-intuitive aspects and/or help naive intuition in properly understanding microscopic processes, e.g., electron removal from or electron attachment to a neutral chain. Demonstrating that the Wiberg indices adequately quantify the chemical bonding structure of the HC2k/2k+1H chains—while the often heavily advertised Mayer indices do not—represents an important message conveyed by the present study. 相似文献