首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   20506篇
  免费   2141篇
  国内免费   1985篇
化学   2091篇
晶体学   24篇
力学   1965篇
综合类   486篇
数学   14225篇
物理学   5841篇
  2023年   164篇
  2022年   253篇
  2021年   342篇
  2020年   426篇
  2019年   443篇
  2018年   426篇
  2017年   545篇
  2016年   617篇
  2015年   477篇
  2014年   1041篇
  2013年   1324篇
  2012年   1060篇
  2011年   1298篇
  2010年   1132篇
  2009年   1433篇
  2008年   1425篇
  2007年   1516篇
  2006年   1332篇
  2005年   1274篇
  2004年   1020篇
  2003年   947篇
  2002年   803篇
  2001年   724篇
  2000年   656篇
  1999年   565篇
  1998年   497篇
  1997年   482篇
  1996年   360篇
  1995年   290篇
  1994年   238篇
  1993年   191篇
  1992年   155篇
  1991年   131篇
  1990年   138篇
  1989年   92篇
  1988年   87篇
  1987年   74篇
  1986年   64篇
  1985年   83篇
  1984年   65篇
  1983年   48篇
  1982年   68篇
  1981年   58篇
  1980年   42篇
  1979年   40篇
  1978年   36篇
  1977年   30篇
  1976年   31篇
  1973年   21篇
  1972年   12篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
1.
研究了具有死区输入的预设约束未知高阶严格反馈非线性系统的控制问题,提出了一种基于免疫函数的自抗扰预设漏斗约束自适应控制策略。首先,针对系统内部的未知问题,采用免疫函数与扩张状态观测器结合对系统内部未知项进行观测;其次,通过Lyapunov方法与漏斗控制相结合设计控制器,使得跟踪误差能够维持在预先设定的漏斗约束范围内;同时,利用双曲正切函数速率变化快这一特性设计自适应控制律,引入指令滤波器避免反步法中重复求导问题,分析证明了闭环系统所有信号的有界性。仿真实例表明了控制方法的有效性。  相似文献   
2.
Let (X, d, μ) be a metric measure space with non-negative Ricci curvature. This paper is concerned with the boundary behavior of harmonic function on the (open) upper half-space X×+. We derive that a function f of bounded mean oscillation (BMO) is the trace of harmonic function u(x,t ) on X×+,u(x,0 )=f( x), whenever u satisfies the following Carleson measure condition supxB,rB 0rBfB(x B, rB)|t u(x ,t)|2d μ (x)dttC< where =( x ,t) denotes the total gradient and B(xB,r B) denotes the (open) ball centered at xB with radius rB. Conversely, the above condition characterizes all the harmonic functions whose traces are in BMO space.  相似文献   
3.
郭敬  张玉杰 《应用光学》2022,43(5):879-885
目前的节能照明控制算法仍有陷入局部最优的问题。为了寻求全局最优解,提高室内照明的节能效果,设计一种遗传模拟退火算法对照明系统的控制参数进行优化求解。该算法通过在遗传操作后对优秀个体进行模拟退火处理,增强了算法的局部搜索能力。根据迭代的次数和种群的适应度对遗传概率进行自适应调节,使得算法在前期丰富种群多样性,避免算法“早熟”。提出基于人工神经网络的照度模型来计算室内照度分布,对照明舒适度进行评估,为构造优化算法的适应函数提供了依据。通过仿真实验,在本文介绍的照明场景应用遗传模拟退火算法,并与传统粒子群算法和遗传算法进行比较,其照明节能性能分别高出5.30%和13.61%。  相似文献   
4.
We propose an efficient scheme to generate a macroscopical quantum superposition state with a cavity optomechanical system, which is composed of a quantum Rabi-Stark model coupling to a mechanical oscillator. In a low-energy subspace of the Rabi-Stark model, the dressed states and then the effective Hamiltonian of the system are given. Due to the coupling of the mechanical oscillator and the atom-cavity system, if the initial state of the atom-cavity system is one of the dressed states, the mechanical oscillator will evolve into a corresponding coherent state. Thus, if the initial state of the atom-cavity system is a superposition of two dressed states, a coherent state superposition of the mechanical oscillator can be generated. The quantum coherence and their distinguishable properties of the two coherent states are exhibited by Wigner distribution. We show that the Stark term can enhance significantly the feasibility and quantum coherence of the generated macroscopic quantum superposition state of the oscillator.  相似文献   
5.
本文提出了一种新的带有时间幂次项的灰色GM(1,1,k,k2)模型,给出了其灰微分方程和白化微分方程基本形式。基于最小二乘法获得了该模型参数估计值,并推导了该模型时间响应函数。鉴于GM(1,1,k,k2)模型灰微分方程与白化微分方程之间存在跳跃关系,首先对灰微分方程的背景值进行了优化,并推导了优化后的背景值计算公式。为了克服初始值的影响,根据误差平方和最小,进一步优化了GM(1,1,k,k2)模型时间响应函数。最后,该优化后的GM(1,1,k,k2)模型被应用于软土地基沉降预测,获得了较好的模拟预测效果,说明模型是可行的。  相似文献   
6.
7.
In this paper, first, we prove the weighted Hermite–Hadamard–Mercer inequalities for convex functions, after we establish some new weighted inequalities connected with the right‐sides of weighted Hermite–Hadamard–Mercer type inequalities for differentiable functions whose derivatives in absolute value at certain powers are convex. The results presented here would provide extensions of those given in earlier works.  相似文献   
8.
A new fast algorithm based on the augmented immersed interface method and a fast Poisson solver is proposed to solve three dimensional elliptic interface problems with a piecewise constant but discontinuous coefficient. In the new approach, an augmented variable along the interface, often the jump in the normal derivative along the interface is introduced so that a fast Poisson solver can be utilized. Thus, the solution of the Poisson equation depends on the augmented variable which should be chosen such that the original flux jump condition is satisfied. The discretization of the flux jump condition is done by a weighted least squares interpolation using the solution at the grid points, the jump conditions, and the governing PDEs in a neighborhood of control points on the interface. The interpolation scheme is the key to the success of the augmented IIM particularly. In this paper, the key new idea is to select interpolation points along the normal direction in line with the flux jump condition. Numerical experiments show that the method maintains second order accuracy of the solution and can reduce the CPU time by 20-50%. The number of the GMRES iterations is independent of the mesh size.  相似文献   
9.
The problem of fixed-time stability of switched systems is studied. With the aid of the multiple Lyapunov function method, constraints on switching signals are derived under which global fixed-time stability of zero solutions of considered systems can be guaranteed. Sufficient conditions of fixed-time stability for Persidskii-type systems are obtained. The developed approaches are applied to the problem of the fixed-time deployment of mobile agents over a line segment under protocols with switched communication topology. Efficiency of the obtained results is demonstrated by a numerical simulation.  相似文献   
10.
This paper expands the notion of robust profit opportunities in financial markets to incorporate distributional ambiguity using Wasserstein distance as the ambiguity measure. Financial markets with risky and risk-free assets are considered. The infinite dimensional primal problems are formulated, leading to their simpler finite dimensional dual problems. A principal motivating question is how distributional ambiguity helps or hurts the robustness of the profit opportunity. Towards answering this question, some theory is developed and computational experiments are conducted. Finally some open questions and suggestions for future research are discussed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号