全文获取类型
收费全文 | 23324篇 |
免费 | 2326篇 |
国内免费 | 1240篇 |
专业分类
化学 | 3084篇 |
晶体学 | 90篇 |
力学 | 2905篇 |
综合类 | 151篇 |
数学 | 14871篇 |
物理学 | 5789篇 |
出版年
2024年 | 51篇 |
2023年 | 269篇 |
2022年 | 324篇 |
2021年 | 455篇 |
2020年 | 700篇 |
2019年 | 656篇 |
2018年 | 634篇 |
2017年 | 650篇 |
2016年 | 662篇 |
2015年 | 584篇 |
2014年 | 961篇 |
2013年 | 2473篇 |
2012年 | 944篇 |
2011年 | 1313篇 |
2010年 | 1054篇 |
2009年 | 1447篇 |
2008年 | 1419篇 |
2007年 | 1393篇 |
2006年 | 1264篇 |
2005年 | 1029篇 |
2004年 | 979篇 |
2003年 | 914篇 |
2002年 | 847篇 |
2001年 | 716篇 |
2000年 | 712篇 |
1999年 | 640篇 |
1998年 | 533篇 |
1997年 | 524篇 |
1996年 | 374篇 |
1995年 | 290篇 |
1994年 | 274篇 |
1993年 | 216篇 |
1992年 | 170篇 |
1991年 | 170篇 |
1990年 | 115篇 |
1989年 | 90篇 |
1988年 | 83篇 |
1987年 | 81篇 |
1986年 | 71篇 |
1985年 | 100篇 |
1984年 | 127篇 |
1983年 | 70篇 |
1982年 | 89篇 |
1981年 | 82篇 |
1980年 | 72篇 |
1979年 | 68篇 |
1978年 | 63篇 |
1977年 | 43篇 |
1976年 | 30篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
991.
For the three-dimensional incompressible Navier–Stokes equations, we present a formulation featuring velocity, vorticity and helical density as independent variables. We find the helical density can be observed as a Lagrange multiplier corresponding to the divergence-free constraint on the vorticity variable, similar to the pressure in the case of the incompressibility condition for velocity. As one possible practical application of this new formulation, we consider a time-splitting numerical scheme based on an alternating procedure between vorticity–helical density and velocity–Bernoulli pressure systems of equations. Results of numerical experiments include a comparison with some well-known schemes based on pressure–velocity formulation and illustrate the competitiveness on the new scheme as well as the soundness of the new formulation. 相似文献
992.
In this paper, the dynamics of a system of two van der Pol equations with a finite delay are investigated. We show that there exist the stability switches and a sequence of Hopf bifurcations occur at the zero equilibrium when the delay varies. Using the theory of normal form and the center manifold theorem, the explicit expression for determining the direction of the Hopf bifurcations and the stability of the bifurcating periodic solutions are derived. 相似文献
993.
梁军 《数学的实践与认识》2016,(17):229-235
采用重心Lagrange插值配点法计算了二维Poisson方程.采用重心Lagrange插值法构造近似函数,由配点法离散Poisson方程及其边界条件.数值算例表明方法具有理论简单、计算精度高的特点. 相似文献
994.
Roumen Tsekov 《International Journal of Theoretical Physics》2009,48(9):2660-2664
The simplest density functional theory due to Thomas, Fermi, Dirac and Weizsäcker is employed to describe the non-equilibrium thermodynamic evolution of an electron gas. The temperature effect is introduced via the Fermi-Dirac entropy, while the irreversible dynamics is described by a non-linear diffusion equation. A dissipative Kohn-Sham equation is also proposed, which improves the Thomas-Fermi-Weizsäcker kinetic functional. 相似文献
995.
Cornelis van der Mee Pietro Contu Paolo Pintus 《Journal of Quantitative Spectroscopy & Radiative Transfer》2010,111(1):214-225
In this article we present a method to determine the band spectrum, band gaps, and discrete energy levels, of a one-dimensional photonic crystal with localized impurities. For one-dimensional crystals with piecewise constant refractive indices we develop an algorithm to recover the refractive index distribution from the period map. Finally, we derive the relationship between the period map and the scattering matrix containing the information on the localized modes. 相似文献
996.
In this paper we study a reaction–diffusion–advection predator–prey model in a river. The existence of predator-invasion traveling wave solutions and prey-spread traveling wave solutions in the upstream and downstream directions is established and the corresponding minimal wave speeds are obtained. While some crucial improvements in theoretical methods have been established, the proofs of the existence and nonexistence of such traveling waves are based on Schauder’s fixed-point theorem, LaSalle’s invariance principle and Laplace transform. Based on theoretical results, we investigate the effect of the hydrological and biological factors on minimal wave speeds and hence on the spread of the prey and the invasion of the predator in the river. The linear determinacy of the predator–prey Lotka–Volterra system is compared with nonlinear determinacy of the competitive Lotka–Volterra system to investigate the mechanics of linear and nonlinear determinacy. 相似文献
997.
The direct injection of CO2 into the deep ocean is one of the feasible ways for the mitigation of the global warming, although there is a concern about its environmental impact near the injection point. To minimize its biological impact, it is necessary to make CO2 disperse as quickly as possible, and it is said that injection with a pipe towed by a moving ship is effective for this purpose. Because the injection ship moves over a spatial scale of O(102km), a mesoscale model is necessary to analyse the dispersion of CO2. At the same time, since it is important to investigate high CO2 concentration near the injection point, a small‐scale model is also required. Therefore, in this study, a numerical model was developed to analyse CO2 dispersion in the deep ocean by using a fixed mesoscale and a moving small‐scale grid systems, the latter of which is nested and moves in the former along the trajectory of the moving ship. To overcome the artificial diffusion of mass concentration at the interface of the two different grid systems and to keep its spatial accuracy almost the same as that in the small‐scale, a particle Laplacian method was adopted and newly modified for anisotropic diffusion in the ocean. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
998.
ZHANG ZhaoXiang ZHANG XiaoWei & LI Feng Key Laboratory of Eco-Chemical Engineering Ministry of Education College of Chemistry Molecular Engineering Qingdao University of Science Technology Qingdao China School of Science Qingdao Technological University Qingdao China 《中国科学:化学》2010,(5)
A novel method has been developed by integration of multi-concentration and two-dimensional(2D) capillary electrophoresis(CE) for simultaneous enhancement of detection sensitivity and separation power in complex samples.Capillary zone electrophoresis(CZE) was used as the first dimension separation according to mobilities,from which the effluent fractions were further analyzed by micellar electrokinetic capillary chromatography(MEKC) acting as the second dimension.Cation-selective exhaustive injection(CSEI) ... 相似文献
999.
This study extends the upstream flux‐splitting finite‐volume (UFF) scheme to shallow water equations with source terms. Coupling the hydrostatic reconstruction method (HRM) with the UFF scheme achieves a resultant numerical scheme that adequately balances flux gradients and source terms. The proposed scheme is validated in three benchmark problems and applied to flood flows in the natural/irregular river with bridge pier obstructions. The results of the simulations are in satisfactory agreement with the available analytical solutions, experimental data and field measurements. Comparisons of the present results with those obtained by the surface gradient method (SGM) demonstrate the superior stability and higher accuracy of the HRM. The stability test results also show that the HRM requires less CPU time (up to 60%) than the SGM. The proposed well‐balanced UFF scheme is accurate, stable and efficient to solve flow problems involving irregular bed topography. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
1000.
The penalty finite element method for Navier–Stokes equations with nonlinear slip boundary conditions is investigated in this paper. Since this class of nonlinear slip boundary conditions include the subdifferential property, the weak variational formulation is a variational inequality problem of the second kind. Using the penalty finite element approximation, we obtain optimal error estimates between the exact solution and the finite element approximation solution. Finally, we show the numerical results which are in full agreement with the theoretical results. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献