全文获取类型
收费全文 | 5952篇 |
免费 | 489篇 |
国内免费 | 603篇 |
专业分类
化学 | 812篇 |
晶体学 | 20篇 |
力学 | 728篇 |
综合类 | 87篇 |
数学 | 4126篇 |
物理学 | 1271篇 |
出版年
2023年 | 73篇 |
2022年 | 135篇 |
2021年 | 125篇 |
2020年 | 191篇 |
2019年 | 185篇 |
2018年 | 161篇 |
2017年 | 191篇 |
2016年 | 182篇 |
2015年 | 134篇 |
2014年 | 227篇 |
2013年 | 444篇 |
2012年 | 276篇 |
2011年 | 359篇 |
2010年 | 309篇 |
2009年 | 491篇 |
2008年 | 454篇 |
2007年 | 415篇 |
2006年 | 369篇 |
2005年 | 336篇 |
2004年 | 246篇 |
2003年 | 236篇 |
2002年 | 210篇 |
2001年 | 158篇 |
2000年 | 163篇 |
1999年 | 139篇 |
1998年 | 166篇 |
1997年 | 114篇 |
1996年 | 97篇 |
1995年 | 56篇 |
1994年 | 51篇 |
1993年 | 39篇 |
1992年 | 43篇 |
1991年 | 33篇 |
1990年 | 24篇 |
1989年 | 20篇 |
1988年 | 22篇 |
1987年 | 19篇 |
1986年 | 18篇 |
1985年 | 15篇 |
1984年 | 18篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 13篇 |
1980年 | 15篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1975年 | 6篇 |
1974年 | 6篇 |
1973年 | 3篇 |
1972年 | 3篇 |
排序方式: 共有7044条查询结果,搜索用时 15 毫秒
31.
Abayomi Samuel Oke Oluwafemi Isaac Bada Ganiyu Rasaq Victoria Adodo 《Mathematical Methods in the Applied Sciences》2022,45(1):137-149
Coronavirus pandemic (COVID-19) hit the world in December 2019, and only less than 5% of the 15 million cases were recorded in Africa. A major call for concern was the significant rise from 2% in May 2020 to 4.67% by the end of July 15, 2020. This drastic increase calls for quick intervention in the transmission and control strategy of COVID-19 in Africa. A mathematical model to theoretically investigate the consequence of ignoring asymptomatic cases on COVID-19 spread in Africa is proposed in this study. A qualitative analysis of the model is carried out with and without re-infection, and the reproduction number is obtained under re-infection. The results indicate that increasing case detection to detect asymptomatically infected individuals will be very effective in containing and reducing the burden of COVID-19 in Africa. In addition, the fact that it has not been confirmed whether a recovered individual can be re-infected or not, then enforcing a living condition where recovered individuals are not allowed to mix with the susceptible or exposed individuals will help in containing the spread of COVID-19. 相似文献
32.
Ajit A. Diwan 《Discrete Mathematics》2019,342(4):1060-1062
Let be a perfect matching in a graph. A subset of is said to be a forcing set of , if is the only perfect matching in the graph that contains . The minimum size of a forcing set of is called the forcing number of . Pachter and Kim (1998) conjectured that the forcing number of every perfect matching in the -dimensional hypercube is , for all . This was revised by Riddle (2002), who conjectured that it is at least , and proved it for all even . We show that the revised conjecture holds for all . The proof is based on simple linear algebra. 相似文献
33.
Multistrain diseases, which are infected through individual contacts, pose severe public health threat nowadays. In this paper, we build competitive and mutative two‐strain edge‐based compartmental models using probability generation function (PGF) and pair approximation (PA). Both of them are ordinary differential equations. Their basic reproduction numbers and final size formulas are explicitly derived. We show that the formula gives a unique positive final epidemic size when the reproduction number is larger than unity. We further consider competitive and mutative multistrain diseases spreading models and compute their basic reproduction numbers. We perform numerical simulations that show some dynamical properties of the competitive and mutative two‐strain models. 相似文献
34.
The edge‐percolation and vertex‐percolation random graph models start with an arbitrary graph G, and randomly delete edges or vertices of G with some fixed probability. We study the computational complexity of problems whose inputs are obtained by applying percolation to worst‐case instances. Specifically, we show that a number of classical ‐hard problems on graphs remain essentially as hard on percolated instances as they are in the worst‐case (assuming ). We also prove hardness results for other ‐hard problems such as Constraint Satisfaction Problems and Subset‐Sum, with suitable definitions of random deletions. Along the way, we establish that for any given graph G the independence number and the chromatic number are robust to percolation in the following sense. Given a graph G, let be the graph obtained by randomly deleting edges of G with some probability . We show that if is small, then remains small with probability at least 0.99. Similarly, we show that if is large, then remains large with probability at least 0.99. We believe these results are of independent interest. 相似文献
35.
最近,孙华定义了一类新的精细化Eulerian多项式,即$$A_n(p,q)=\sum_{\pi\in \mathfrak{S}_n}p^{{\rm odes}(\pi)}q^{{\rm edes}(\pi)},\ \ n\ge 1,$$ 其中$S_n$表示$\{1,2,\ldots,n\}$上全体$n$阶排列的集合, odes$(\pi)$与edes$(\pi)$分别表示$S_n$中排列$\pi$的奇数位与偶数位上降位数的个数.本文利用经典的Eulerian多项式$A_n(q)$ 与Catalan 序列的生成函数$C(q)$,得到精细化Eulerian 多项式$A_n(p,q)$的指数型生成函数及$A_n(p,q)$的显示表达式.在一些特殊情形,本文建立了$A_n(p,q)$与$A_n(0,q)$或$A_n(p,0)$之间的联系,并利用Eulerian数表示多项式$A_n(0,q)$的系数.特别地,这些联系揭示了Euler数$E_n$与Eulerian数$A_{n,k}$之间的一种新的关系. 相似文献
36.
37.
John Asplund Gregory Clark Garner Cochran va Czabarka Arran Hamm Gwen Spencer Lszl Szkely Libby Taylor Zhiyu Wang 《组合设计杂志》2019,27(10):586-597
The crossing number of a graph is the smallest number of edge crossings over all drawings of in the plane. For any , the ‐planar crossing number of , is defined as the minimum of over all graphs with . Pach et al [Comput. Geom.: Theory Appl. 68 (2018), pp. 2–6] showed that for every , we have and that this bound does not remain true if we replace the constant by any number smaller than . We improve the upper bound to as . For the class of bipartite graphs, we show that the best constant is exactly for every . The results extend to the rectilinear variant of the ‐planar crossing number. 相似文献
38.
39.
40.
借助数轴透析“一种金属与溶液中多种盐反应”和“多种金属与溶液中一种盐反应”中溶质与滤渣的组成,直观地将反应进程和结果展现其中,建立思维模型,化难为易,有利于学生掌握与应用。 相似文献