全文获取类型
收费全文 | 460篇 |
免费 | 33篇 |
国内免费 | 22篇 |
专业分类
化学 | 107篇 |
力学 | 8篇 |
综合类 | 2篇 |
数学 | 360篇 |
物理学 | 38篇 |
出版年
2024年 | 3篇 |
2023年 | 7篇 |
2022年 | 24篇 |
2021年 | 23篇 |
2020年 | 26篇 |
2019年 | 23篇 |
2018年 | 8篇 |
2017年 | 18篇 |
2016年 | 16篇 |
2015年 | 7篇 |
2014年 | 16篇 |
2013年 | 33篇 |
2012年 | 19篇 |
2011年 | 18篇 |
2010年 | 12篇 |
2009年 | 19篇 |
2008年 | 24篇 |
2007年 | 14篇 |
2006年 | 6篇 |
2005年 | 10篇 |
2004年 | 13篇 |
2003年 | 11篇 |
2002年 | 15篇 |
2001年 | 7篇 |
2000年 | 5篇 |
1999年 | 9篇 |
1998年 | 3篇 |
1997年 | 8篇 |
1996年 | 9篇 |
1995年 | 2篇 |
1994年 | 7篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 13篇 |
1985年 | 9篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1974年 | 4篇 |
排序方式: 共有515条查询结果,搜索用时 0 毫秒
121.
Anion exchange membrane fuel cells (AEMFCs) are the most promising low-temperature fuel cells and have received extensive attention. Compared to PEMFCs, the cost per unit of power can be significantly reduced for AEMFCs because, in theory, they allow the usage of non-precious metal catalysts and low-cost cell components. Owing to the development of advanced materials and performance improvement strategies, AEMFCs have achieved new records in both initial performance and durability. However, the high performance currently achieved is contingent on certain conditions, e. g., high Pt loading, large gas flowrates, and operation in pure O2, which are far from practical applications. Therefore, the transition to commercially relevant performance and durability is the next goal of AEMFCs. This paper reviews the performance data of H2-fueled AEMFCs since 2010 and summarizes possible performance optimization schemes, which can provide useful insights for developing next-generation AEMFCs. 相似文献
122.
In this paper, we propose a new formula of the real-time minimum safety headway based on the relative velocity of consecutive trains and present a dynamic model of high-speed passenger train movements in the rail line based on the proposed formula of the minimum safety headway. Moreover, we provide the control strategies of the high-speed passenger train operations based on the proposed formula of the real-time minimum safety headway and the dynamic model of highspeed passenger train movements. The simulation results demonstrate that the proposed control strategies of the passenger train operations can greatly reduce the delay propagation in the high-speed rail line when a random delay occurs. 相似文献
123.
M. E. Givens C. A. Zmudzinski R. P. Bryan J. J. Coleman 《Fiber and Integrated Optics》2013,32(4):343-352
Graded barrier quantum well heterostructure (GBQWH) broad area lasers have been shown to be capable of high power pulsed and cw operation. In this article, we consider several operational characteristics and design issues associated with broad area graded barrier quantum well heterostructure lasers grown by metalorganic chemical vapor deposition. In particular, the effect of junction heating on emission wavelength for cw device operation and the effects of various buffer layer structures on the material properties and device characteristics of GBQWH structures are addressed. Typical results for high power operation of uncoated broad area laser diodes are also outlined. 相似文献
124.
P. Ciarlet Jr 《Numerical Algorithms》1994,7(2):295-324
Hereafter, we describe and analyze, from both a theoretical and a numerical point of view, an iterative method for efficiently solving symmetric elliptic problems with possibly discontinuous coefficients. In the following, we use the Preconditioned Conjugate Gradient method to solve the symmetric positive definite linear systems which arise from the finite element discretization of the problems. We focus our interest on sparse and efficient preconditioners. In order to define the preconditioners, we perform two steps: first we reorder the unknowns and then we carry out a (modified) incomplete factorization of the original matrix. We study numerically and theoretically two preconditioners, the second preconditioner corresponding to the one investigated by Brand and Heinemann [2]. We prove convergence results about the Poisson equation with either Dirichlet or periodic boundary conditions. For a meshsizeh, Brand proved that the condition number of the preconditioned system is bounded byO(h
–1/2) for Dirichlet boundary conditions. By slightly modifying the preconditioning process, we prove that the condition number is bounded byO(h
–1/3). 相似文献
125.
L.F. Escudero 《European Journal of Operational Research》1984,18(2):259-274
We present an algorithm for very large-scale linearly constrained nonlinear programming (LCNP) based on a Limited-Storage Quasi-newton method. In large-scale programming solving the reduced Newton equation at each iteration can be expensive and may not be justified when far from a local solution; besides, the amount of storage required by the reduced Hessian matrix, and even the computing time for its Quasi-Newton approximation, may be prohibitive. An alternative based on the reduced Truncated-Newton methodology, that has proved to be satisfactory for large-scale problems, is not recommended for very large-scale problems since it requires an additional gradient evaluation and the solving of two systems of linear equations per each minor iteration. We recommend a 2-step BFGS approximation of the inverse of the reduced Hessian matrix that does not require to store any matrix since the product matrix-vector is the vector to be approximated; it uses the reduced gradient and information from two previous iterations and the so-termed restart iteration. A diagonal direct BFGS preconditioning is used. 相似文献
126.
M. H. Breitner H. J. Pesch W. Grimm 《Journal of Optimization Theory and Applications》1993,78(3):443-463
In Part 1 of this paper (Ref. 1), necessary conditions for optimal open-loop strategies in differential games of pursuit-evasion type have been developed for problems which involve state variable inequality constraints and nonsmooth data. These necessary conditions lead to multipoint boundary-value problems with jump conditions. These problems can be solved very efficiently and accurately by the well-known multiple-shooting method. By this approach, optimal open-loop strategies and their associated saddle-point trajectories can be computed for the entire capture zone of the game. This also includes the computation of optimal open-loop strategies and saddle-point trajectories on the barrier of the pursuit-evasion game. The open-loop strategies provide an open-loop representation of the optimal feedback strategies. Numerical results are obtained for a special air combat scenario between one medium-range air-to-air missile and one high-performance aircraft in a vertical plane. A dynamic pressure limit for the aircraft imposes a state variable inequality constraint of the first order. Special emphasis is laid on realistic approximations of the lift, drag, and thrust of both vehicles and the atmospheric data. In particular, saddle-point trajectories on the barrier are computed and discussed. Submanifolds of the barrier which separate the initial values of the capture zone from those of the escape zone are computed for two representative launch positions of the missible. By this way, the firing range of the pursuing missile is determined and visualized.This paper is dedicated to the memory of Professor John V. Breakwell.The authors would like to express their sincere and grateful appreciation to Professors R. Bulirsch and K. H. Well for their encouraging interest in this work. 相似文献
127.
Jörn Sass 《Acta Appl Math》2007,97(1-3):221-238
We consider a market model where stock returns satisfy a stochastic differential equation with an unobservable, stochastic
drift process. The investor’s objective is to maximize expected utility of terminal wealth, but investment decisions are based
on the knowledge of the stock prices only. The performance of the resulting highly risky strategies can be improved considerably
by imposing convex constraints covering e.g. short selling restrictions. Using filtering methods we transform the model to
a model with full information. We provide a verification result and show how results on optimization under convex constraints
can be used directly for a continuous time Markov chain model for the drift. In special cases we derive representations of
the optimal trading strategies, including a stochastic volatility model.
Supported by the Austrian Science Fund, FWF grant P17947-N12. 相似文献
128.
Optimal dividends in the dual model 总被引:2,自引:0,他引:2
Benjamin Avanzi Hans U. Gerber Elias S.W. Shiu 《Insurance: Mathematics and Economics》2007,41(1):111-123
The optimal dividend problem proposed by de Finetti [de Finetti, B., 1957. Su un’impostazione alternativa della teoria collettiva del rischio. In: Transactions of the XVth International Congress of Actuaries, vol. 2. pp. 433-443] is to find the dividend-payment strategy that maximizes the expected discounted value of dividends which are paid to the shareholders until the company is ruined or bankrupt. In this paper, it is assumed that the surplus or shareholders’ equity is a Lévy process which is skip-free downwards; such a model might be appropriate for a company that specializes in inventions and discoveries. In this model, the optimal strategy is a barrier strategy. Hence the problem is to determine b∗, the optimal level of the dividend barrier. A key tool is the method of Laplace transforms. A variety of numerical examples are provided. It is also shown that if the initial surplus is b∗, the expectation of the discounted dividends until ruin is the present value of a perpetuity with the payment rate being the drift of the surplus process. 相似文献
129.
V. I. Arnold 《Functional Analysis and Its Applications》2002,36(2):83-92
The time mean of a smooth objective function along a phase trajectory of a controlled dynamical system is maximized. The simplest singularities of the dependence of the optimal mean value on the parameter in generic one-parameter families of controlled systems of this kind are listed. It turns out that the most common generic stable singularity is the discontinuity of the first or second derivative of the optimal mean value with respect to the parameter. 相似文献
130.
A differential game with two pursuers and one evader 总被引:1,自引:0,他引:1
This paper is concerned with a coplanar pursuit-evasion problem in which a faster evaderE with constant speedw>1 must pass between two pursuersP
1,P
2 having unit speed, the payoff being the distance of closest approach to either one of the pursuers. The control variables are the directions of the velocities ofP
1,P
2, andE. The path equations are integrated, and a closed-form solution is obtained in terms of elliptic functions of the first and second kind. A closed-loop solution is given graphically in several diagrams, for different values ofw. 相似文献