全文获取类型
收费全文 | 632篇 |
免费 | 30篇 |
国内免费 | 24篇 |
专业分类
化学 | 175篇 |
力学 | 9篇 |
综合类 | 31篇 |
数学 | 321篇 |
物理学 | 150篇 |
出版年
2024年 | 3篇 |
2023年 | 25篇 |
2022年 | 35篇 |
2021年 | 42篇 |
2020年 | 23篇 |
2019年 | 30篇 |
2018年 | 28篇 |
2017年 | 29篇 |
2016年 | 17篇 |
2015年 | 9篇 |
2014年 | 24篇 |
2013年 | 59篇 |
2012年 | 20篇 |
2011年 | 32篇 |
2010年 | 34篇 |
2009年 | 37篇 |
2008年 | 22篇 |
2007年 | 30篇 |
2006年 | 20篇 |
2005年 | 28篇 |
2004年 | 20篇 |
2003年 | 8篇 |
2002年 | 8篇 |
2001年 | 13篇 |
2000年 | 14篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 6篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1977年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有686条查询结果,搜索用时 31 毫秒
1.
William J. Reed 《Natural Resource Modeling》1989,3(4):463-480
It is assumed that the probability of destruction of a biological asset by natural hazards can be reduced through investment in protection. Specifically a model, in which the hazard rate depends on both the age of the asset and the accumulated invested protection capital, is assumed. The protection capital depreciates through time and its effectiveness in reducing the hazard rate is subject to diminishing returns. It is shown how the investment schedule to maximize the expected net present value of the asset can be determined using the methods of deterministic optimal control, with the survival probability regarded as a state variable. The optimal investment pattern involves “bang-bang-singular” control. A numerical scheme for determining jointly the optimal investment policy and the optimal harvest (or replacement) age is outlined and a numerical example involving forest fire protection is given. 相似文献
2.
The rational and sustainable exploitation of nature resources is one of the priority objectives of our society. In this article a method is proposed to evaluate and exploit the energetic resources contained in different forest formations. This method is based on the use of a combustion bomb calorimeter to determine the calorific values of the different samples studied. These results were complemented with chemical analysis of the samples and with environmental and geomorphological studies of the zones were samples were taken. The results obtained during the last 3 years for the residual biomass originated from the forest formations existing in Galicia ensure the reliability and usefulness of the method that can be extended to any forest formation all over the world. 相似文献
3.
O. Fülöp 《Discrete Mathematics》2005,294(3):285-290
We give a short proof for the Mixed Connectivity Certificate Theorem of Even, Itkis and Rajsbaum and provide an upper bound on the edge number of a certificate of local T-mixed connectivity up to k. 相似文献
4.
森林发展系统的一个非线性半离散模型 总被引:2,自引:2,他引:0
王定江 《数学的实践与认识》2003,33(2):86-90
本文建立了森林发展系统的一类非线性林龄面积结构的半离散模型 ,并讨论了半离散系统解的存在唯一性 ,给出了线性半离散系统稳定的一些充分条件 相似文献
5.
This paper studies the dynamical behavior of a class of total area dependent nonlinear age-structured forest evolution model. We give the problem of equal value for the forest system, and discuss the stable solution of system. We obtained the necessary and sufficient conditions for there exists the stable solution. 相似文献
6.
Maurits de Graaf 《Discrete Mathematics》2004,280(1-3):219-223
We show that each directed graph (with no parallel arcs) on n vertices, each with indegree and outdegree at least n/twhere t=2.888997… contains a directed circuit of length at most 3. 相似文献
7.
基于突变论的林火蔓延分析 总被引:8,自引:0,他引:8
在对森林火灾进行能量和质量守恒分析的基础上,把尖点突变模型应用于森林火灾蔓延的预测,使突变理论在 森林火灾的模型中从理论上有了初步的应用。文中计算了林火发生突变时的一些分歧点,分析了林火突变的特点和变化规 律。 相似文献
8.
We study the problem of directed polymers (DP) on a square lattice. The distribution of disorder is assumed to be independent but non-Gaussian. We show that for distributions with a power-law tailP() 1/||1+
, where>2, so that the mean and variance are well defined, the scaling exponentv of the DP model depends on in a continuous fashion. 相似文献
9.
Yi-Cheng Zhang 《Journal of statistical physics》1989,57(5-6):1123-1128
A replica trick is used to map the problem of directed polymers into a quantum mechanics problem ofn-body bound states. This bound-state problem is then treated using a self-consistent method of Hartree-Fock. Ford=2, the exact result of the DP exponents is reproduced; and for complex DP, the ground-state energyE
n n2 is found, confirming previous numerical results. 相似文献
10.