全文获取类型
收费全文 | 18171篇 |
免费 | 2142篇 |
国内免费 | 748篇 |
专业分类
化学 | 6059篇 |
晶体学 | 47篇 |
力学 | 1315篇 |
综合类 | 105篇 |
数学 | 6958篇 |
物理学 | 6577篇 |
出版年
2024年 | 18篇 |
2023年 | 165篇 |
2022年 | 184篇 |
2021年 | 419篇 |
2020年 | 482篇 |
2019年 | 473篇 |
2018年 | 338篇 |
2017年 | 405篇 |
2016年 | 663篇 |
2015年 | 558篇 |
2014年 | 878篇 |
2013年 | 1418篇 |
2012年 | 896篇 |
2011年 | 962篇 |
2010年 | 816篇 |
2009年 | 1114篇 |
2008年 | 1277篇 |
2007年 | 1272篇 |
2006年 | 1039篇 |
2005年 | 811篇 |
2004年 | 751篇 |
2003年 | 748篇 |
2002年 | 705篇 |
2001年 | 566篇 |
2000年 | 553篇 |
1999年 | 486篇 |
1998年 | 469篇 |
1997年 | 336篇 |
1996年 | 267篇 |
1995年 | 235篇 |
1994年 | 222篇 |
1993年 | 203篇 |
1992年 | 168篇 |
1991年 | 116篇 |
1990年 | 98篇 |
1989年 | 102篇 |
1988年 | 121篇 |
1987年 | 81篇 |
1986年 | 73篇 |
1985年 | 113篇 |
1984年 | 93篇 |
1983年 | 28篇 |
1982年 | 56篇 |
1981年 | 49篇 |
1980年 | 53篇 |
1979年 | 39篇 |
1978年 | 33篇 |
1977年 | 39篇 |
1976年 | 32篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
111.
This paper develops a theory for the global solution of nonconvex optimization problems with parameter-embedded linear dynamic systems. A quite general problem formulation is introduced and a solution is shown to exists. A convexity theory for integrals is then developed to construct convex relaxations for utilization in a branch-and-bound framework to calculate a global minimum. Interval analysis is employed to generate bounds on the state variables implied by the bounds on the embedded parameters. These bounds, along with basic integration theory, are used to prove convergence of the branch-and-bound algorithm to the global minimum of the optimization problem. The implementation of the algorithm is then considered and several numerical case studies are examined thoroughly 相似文献
112.
AbbasA. Zaman 《Particle & Particle Systems Characterization》2003,20(5):342-350
Effects of a low molecular weight physically adsorbed polyethylene oxide (PEO) and the range of the electrostatic repulsion on the rheological behavior of silica dispersions (as a model system) has been investigated. Particular attention is given to the evolution of the rheological behavior with increasing the polymer concentration in the system and also effectiveness of the polymer as a dispersant under extreme conditions (high ionic strength). Results indicate that at small separation distances and low polymer coverage, the polymer chains are long enough to adsorb on the surface of two particles simultaneously causing bridging flocculation in the system and hence increasing the viscosity and linear viscoelastic functions of the dispersion. A significant increase was observed in the viscosity of the dispersion at salt concentrations high enough to eliminate electrostatics between the particles. Under these conditions,the viscosity of the system increased significantly when PEO was added to the dispersion showing that at high electrolyte concentrations, a neutral polymer such as PEO is not able to stabilize the system. 相似文献
113.
It is suggested that the question of existence of a jamming phase transition in a broad class of single-lane cellular-automaton traffic models may be studied using a correspondence to the asymmetric chipping model. In models where such correspondence is applicable, jamming phase transition does not take place. Rather, the system exhibits a smooth crossover between free-flow and jammed states, as the car density is increased. 相似文献
114.
Based on the generalized Huygens–Fresnel diffraction integral and the expansion of the hard aperture function into a finite sum of complex Gaussian functions, the approximate analytical expression of elegant Laguerre–Gaussian beams passing through a paraxial ABCD optical system with an annular aperture is derived. Meanwhile, the corresponding closed-forms for the unapertured, circular apertured or circular black screen cases are also given. The obtained results provide more convenience for studying their propagation and transformation than the usual way by using diffraction integral formula directly. Some numerical examples are given to illustrate the propagation properties of elegant Laguerre–Gaussian beams. 相似文献
115.
研究捕食者具有非单调功能反应和周期脉冲扰动的捕食者一食饵系统,利用脉冲微分方程的Floquet理论和比较定理,得到了系统灭绝和持续生存的充分条件.最后,通过数值模拟阐明系统在周期脉冲扰动下的复杂性. 相似文献
116.
117.
Some previous works show that symmetric fixed- and variable-stepsize linear multistep methods for second-order systems which do not have any parasitic root in their first characteristic polynomial give rise to a slow error growth with time when integrating reversible systems. In this paper, we give a technique to construct variable-stepsize symmetric methods from their fixed-stepsize counterparts, in such a way that the former have the same order as the latter. The order and symmetry of the integrators obtained is proved independently of the order of the underlying fixed-stepsize integrators. As this technique looks for efficiency, we concentrate on explicit linear multistep methods, which just make one function evaluation per step, and we offer some numerical comparisons with other one-step adaptive methods which also show a good long-term behaviour.
118.
119.
David M. Evans 《组合设计杂志》2004,12(6):459-465
For all ‘reasonable’ finite t, k, and s, we construct a t‐(?0, k, 1) design and a group of automorphisms which is transitive on blocks and has s orbits on points. In particular, there is a 2‐(?0, 4, 1) design with a block‐transitive group of automorphisms having two point orbits. This answers a question of P. J. Cameron and C. E. Praeger. The construction is presented in a purely combinatorial way, but is a by‐product of a new way of looking at a model‐theoretic construction of E. Hrushovski. © 2004 Wiley Periodicals, Inc. 相似文献
120.
Kyu Hwang Yeon Suk Seong Kim Chung In Um Thomas F. George 《International Journal of Theoretical Physics》2003,42(9):2043-2059
Quantum Hamiltonian systems corresponding to classical systems related by a general canonical transformation are considered. The differential equation to find the unitary operator, which corresponds to the canonical transformation and connects quantum states of the original and transformed systems, is obtained. The propagator associated with their wave functions is found by the unitary operator. Quantum systems related by a linear canonical point transformation are analyzed. The results are tested by finding the wave functions of the under-, critical-, and over-damped harmonic oscillator from the wave functions of the harmonic oscillator, free-particle system, and negative harmonic potential system, using the unitary operator to connect them, respectively. 相似文献