全文获取类型
收费全文 | 5850篇 |
免费 | 468篇 |
国内免费 | 598篇 |
专业分类
化学 | 750篇 |
晶体学 | 15篇 |
力学 | 737篇 |
综合类 | 87篇 |
数学 | 4170篇 |
物理学 | 1157篇 |
出版年
2024年 | 9篇 |
2023年 | 73篇 |
2022年 | 141篇 |
2021年 | 128篇 |
2020年 | 190篇 |
2019年 | 176篇 |
2018年 | 160篇 |
2017年 | 189篇 |
2016年 | 180篇 |
2015年 | 133篇 |
2014年 | 225篇 |
2013年 | 441篇 |
2012年 | 274篇 |
2011年 | 353篇 |
2010年 | 299篇 |
2009年 | 459篇 |
2008年 | 422篇 |
2007年 | 396篇 |
2006年 | 355篇 |
2005年 | 327篇 |
2004年 | 239篇 |
2003年 | 234篇 |
2002年 | 202篇 |
2001年 | 161篇 |
2000年 | 170篇 |
1999年 | 158篇 |
1998年 | 177篇 |
1997年 | 128篇 |
1996年 | 100篇 |
1995年 | 53篇 |
1994年 | 51篇 |
1993年 | 39篇 |
1992年 | 43篇 |
1991年 | 32篇 |
1990年 | 22篇 |
1989年 | 20篇 |
1988年 | 22篇 |
1987年 | 19篇 |
1986年 | 17篇 |
1985年 | 12篇 |
1984年 | 18篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1972年 | 3篇 |
排序方式: 共有6916条查询结果,搜索用时 62 毫秒
981.
982.
983.
Ramsés Djidjou-Demasse Gbenga J. Abiodun Abiodun M. Adeola Joel O. Botai 《Studies in Applied Mathematics》2020,144(4):389-411
In this paper, we develop and analyze a malaria model with seasonality of mosquito life-history traits: periodic-mosquitoes per capita birth rate, -mosquitoes death rate, -probability of mosquito to human disease transmission, -probability of human to mosquito disease transmission, and -mosquitoes biting rate. All these parameters are assumed to be time dependent leading to a nonautonomous differential equation system. We provide a global analysis of the model depending on two threshold parameters and (with ). When , then the disease-free stationary state is locally asymptotically stable. In the presence of the human disease-induced mortality, the global stability of the disease-free stationary state is guarantied when . On the contrary, if , the disease persists in the host population in the long term and the model admits at least one positive periodic solution. Moreover, by a numerical simulation, we show that a sub-critical (backward) bifurcation is possible at . Finally, the simulation results are in accordance with the seasonal variation of the reported cases of a malaria-epidemic region in Mpumalanga province in South Africa. 相似文献
984.
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
985.
Konstantinos Tsouvalas 《代数通讯》2018,46(8):3397-3412
We define Euler characteristics on classes of residually finite and virtually torsion free groups and we show that they satisfy certain formulas in the case of amalgamated free products and HNN extensions over finite subgroups. These formulas are obtained from a general result which applies to the rank gradient and the first L2?Betti number of a finitely generated group. 相似文献
986.
987.
By extending the Levy wavefunction constrained search to Fock Space,one can define a wavefunction constrained search for electron densities in systems having noninteger number of electrons.For pure-state v-representable densities,the results are equivalent to what one would obtain with the zero-temperature grand canonical ensemble.In other cases,the wavefunction constrained search in Fock space presents an upper bound to the grand canonical ensemble functional.One advantage of the Fock-space wavefunction constrained search functional over the zero-temperature grand-canonical ensemble constrained search functional is that certain specific excited states(i.e.,those that are not ground-statev-representable) are the stationary points of the Fock-space functional.However,a potential disadvantage of the Fock-space constrained search functional is that it is not convex. 相似文献
988.
《先进技术聚合物》2018,29(2):820-824
We prepared a ternary composite polymer electrolyte from poly(ethylene carbonate) (PEC), lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and non‐calcined silica nanofibers (SNFs) having 3 average diameters (300, 700, and 1000 nm). The SNF composite electrolytes were obtained as homogeneous, self‐standing membranes. The ionic conductivity of PEC/LiTFSI 100 mol% was increased by the addition of SNFs, and the thinner SNFs with average diameter 300 nm were most effective in improving the conductivity. The conductivity was of the order of 10−4 S/cm at 60°C. The lithium transference number of the SNF300 composite was greater than 0.7. Stress‐strain curves of the composites indicated significant increases in Young's modulus and maximum stress for the PEC electrolytes. The 5% weight‐loss temperature of the composites also improved with the addition of SNF. 相似文献
989.
P.C. Duineveld 《Applied Scientific Research》1997,58(1-4):409-439
The encounter of bubble pairs of O(1 mm) in both pure water and aqueous surfactant solutions was studied experimentally. In pure water, two equally sized bubbles were found to coalesce if the Weber number, W = V2 R/, based on the velocity of approach, V, was below a critical value, Wcr = 0.18, where and are the density and surface tension of the liquid respectively and R the equivalent radius of the bubbles. After coalescence bubbles perform volume and shape oscillations.When Wcr is exceeded, bubbles bounce. After bouncing, bubbles can either coalesce or separate without coalescing. This was found to depend on the Weber number, based on the rise velocity U, We = U2 R/. If this number was below a critical value, bubbles coalesced after bouncing. The relative motion of the bubbles was found to be damped out by acoustic damping due to surface oscillations rather then by viscosity.If We was above a critical value, which was close to that for path instability of a single bubble (We = 3.3), the bubbles separated after bouncing. This is probably caused by shedding of vortices which dominate the relative motion of the bubbles. This mechanism may cause bubbles in bubbly flows not aggregating in horizontal planes, as was found in calculations based on potential flow theory. For modelling bubbly flows it will therefore be essential to incorporate the influence of vorticity.When surfactants are added to the water it was found that bubbles are prevented to coalesce above a critical concentration, which is nearly identical to that of single rising bubbles. Above this critical concentration, bubbles behave as rigid spheres and trajectories cannot be predicted by potential flow theory. 相似文献
990.
The problem of estimating diffusion coefficients has been considered extensively in both discrete and continuous time. We consider here an approach based on counting occupation numbers of diffusing particles. The problem, and our approach, are motivated by statistical mechanics. 相似文献