全文获取类型
收费全文 | 5650篇 |
免费 | 1040篇 |
国内免费 | 432篇 |
专业分类
化学 | 553篇 |
晶体学 | 14篇 |
力学 | 584篇 |
综合类 | 155篇 |
数学 | 4250篇 |
物理学 | 1566篇 |
出版年
2024年 | 11篇 |
2023年 | 54篇 |
2022年 | 106篇 |
2021年 | 131篇 |
2020年 | 172篇 |
2019年 | 158篇 |
2018年 | 148篇 |
2017年 | 239篇 |
2016年 | 246篇 |
2015年 | 165篇 |
2014年 | 335篇 |
2013年 | 509篇 |
2012年 | 297篇 |
2011年 | 351篇 |
2010年 | 340篇 |
2009年 | 392篇 |
2008年 | 390篇 |
2007年 | 367篇 |
2006年 | 327篇 |
2005年 | 308篇 |
2004年 | 234篇 |
2003年 | 266篇 |
2002年 | 207篇 |
2001年 | 202篇 |
2000年 | 178篇 |
1999年 | 178篇 |
1998年 | 140篇 |
1997年 | 110篇 |
1996年 | 78篇 |
1995年 | 86篇 |
1994年 | 65篇 |
1993年 | 30篇 |
1992年 | 27篇 |
1991年 | 25篇 |
1990年 | 14篇 |
1989年 | 21篇 |
1988年 | 24篇 |
1987年 | 15篇 |
1986年 | 10篇 |
1985年 | 24篇 |
1984年 | 28篇 |
1983年 | 14篇 |
1982年 | 16篇 |
1981年 | 28篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 6篇 |
1977年 | 12篇 |
1976年 | 5篇 |
1959年 | 4篇 |
排序方式: 共有7122条查询结果,搜索用时 515 毫秒
71.
Shengqiang Liu Lansun ChenYoulin Jiang 《Journal of Mathematical Analysis and Applications》2002,271(1):124-138
In this paper, a two-species competitive model with stage structure is presented and studied. Results on the global extinction and permanence are given, which generalize the well-known three theorems for the two species competitive system and, moreover, they confirm the negative effect of stage structure on the permanence of populations as well as estimate the degree of such effect. Conclusions in this paper suggest that for a competitive community stage structure is also one of the important reasons that cause permanence and extinction. 相似文献
72.
In this paper, oscillatory properties of solutions of certain hyperbolic partial differential equations with multi-delays are investigated and a series of sufficient conditions for oscillations of the equations are established. The results fully indicate that the oscillations are caused by delays. 相似文献
73.
This paper studies the application of the continuous sensitivity equation method (CSEM) for the Navier–Stokes equations in the particular case of shape parameters. Boundary conditions for shape parameters involve flow derivatives at the boundary. Thus, accurate flow gradients are critical to the success of the CSEM. A new approach is presented to extract accurate flow derivatives at the boundary. High order Taylor series expansions are used on layered patches in conjunction with a constrained least‐squares procedure to evaluate accurate first and second derivatives of the flow variables at the boundary, required for Dirichlet and Neumann sensitivity boundary conditions. The flow and sensitivity fields are solved using an adaptive finite‐element method. The proposed methodology is first verified on a problem with a closed form solution obtained by the Method of Manufactured Solutions. The ability of the proposed method to provide accurate sensitivity fields for realistic problems is then demonstrated. The flow and sensitivity fields for a NACA 0012 airfoil are used for fast evaluation of the nearby flow over an airfoil of different thickness (NACA 0015). Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
74.
PERSISTENCE AND GLOBAL STABILITY FOR TWO-SPECIES NONAUTONOMOUS PREDATOR-PREY SYSTEM WITH DIFFUSION AND TIME DELAY 总被引:3,自引:0,他引:3
In this paper, a non-autonomous predator-prey model with diffusion andcontinuous time delay is studied, where the prey can diffuse between two pat-ches of a heterogeneous environment with barriers between patches, but for thepredator, the diffusion does not involve a barrier between patches, further itis assumed that all the parameters are time-dependent. It is shown that thesystem can be made persistent under some appropriate conditions. Moreover,sufficient conditions that guarantee the existence of a unique periodic solutionwhich is globally asymptotic stable are derived. 相似文献
75.
It is proved that if μ and v be finite measures on a measurable space (X,S) and v is absolutely continuous with respect to v, then it is holds that L(^*S,^*v) C L(^*S,^*v), while L(^*S,^*v) and L(^*S,^*v) are the Loeb algebras with respect to measure spaces (X,S,μ) and(X,S,μ). 相似文献
76.
We study the effect of arrival model uncertainties on the optimal routing in a system of parallel queues. For exponential service time distributions and Bernoulli routing, the optimal mean system delay generally depends on the interarrival time distribution. Any error in modeling the arriving process will cause a model-based optimal routing algorithm to produce a mean system delay higher than the true optimum. In this paper, we present an asymptotic analysis of the behavior of this error under heavy traffic conditions for a general renewal arrival process. An asymptotic analysis of the error in optimal mean delay due to uncertainties in the service time distribution for Poisson arrivals was reported in Ref. 6, where it was shown that, when the first moment of the service time distribution is known, this error in performance vanishes asymptotically as the traffic load approaches the system capacity. In contrast, this paper establishes the somewhat surprising result that, when only the first moment of the arrival distribution is known, the error in optimal mean delay due to uncertainties in the arrival model is unbounded as the traffic approaches the system capacity. However, when both first and second moments are known, the error vanishes asymptotically. Numerical examples corroborating the theoretical results are also presented.This work was supported by the National Science Foundation under Grants ECS-88-01912 and EID-92-12122 and by NASA under Contract NAG 2-595.The authors wish to thank an anonymous referee for pointing out Ref. 20, thus avoiding the need for an explicit proof of convexity of the cost function considered in the paper. 相似文献
77.
One of the main methods for solving stochastic programs is approximation by discretizing the probability distribution. However, discretization may lose differentiability of expectational functionals. The complexity of discrete approximation schemes also increases exponentially as the dimension of the random vector increases. On the other hand, stochastic methods can solve stochastic programs with larger dimensions but their convergence is in the sense of probability one. In this paper, we study the differentiability property of stochastic two-stage programs and discuss continuous approximation methods for stochastic programs. We present several ways to calculate and estimate this derivative. We then design several continuous approximation schemes and study their convergence behavior and implementation. The methods include several types of truncation approximation, lower dimensional approximation and limited basis approximation.His work is supported by Office of Naval Research Grant N0014-86-K-0628 and the National Science Foundation under Grant ECS-8815101 and DDM-9215921.His work is supported by the Australian Research Council. 相似文献
78.
李林 《Annals of Differential Equations》2004,20(2):118-124
This paper considers the scalar differential delay equation x(t) = -μx(t)-f(x(t-Τ(t)), t). By using the mapping method we obtain that the solutionswill be ultimately in some interval. 相似文献
79.
80.
U. Goginava 《分析论及其应用》2007,23(3):255-265
In this paper we prove that iff ∈ C([-π,π]2) and the function f is bounded partial p-variation for some p ∈ [1, ∞), then the double trigonometric Fourier series of a function f is uniformly (C;-α,-β) summable (α β< 1/p,α,β> 0) in the sense of Pringsheim. If α β≥ 1/p, then there exists a continuous function f0 of bounded partial double trigonometric Fourier series of fo diverge over cubes. 相似文献