全文获取类型
收费全文 | 15346篇 |
免费 | 1414篇 |
国内免费 | 491篇 |
专业分类
化学 | 4815篇 |
晶体学 | 17篇 |
力学 | 935篇 |
综合类 | 69篇 |
数学 | 6544篇 |
物理学 | 4871篇 |
出版年
2024年 | 22篇 |
2023年 | 148篇 |
2022年 | 269篇 |
2021年 | 359篇 |
2020年 | 407篇 |
2019年 | 413篇 |
2018年 | 285篇 |
2017年 | 337篇 |
2016年 | 561篇 |
2015年 | 473篇 |
2014年 | 716篇 |
2013年 | 1152篇 |
2012年 | 679篇 |
2011年 | 730篇 |
2010年 | 647篇 |
2009年 | 883篇 |
2008年 | 1043篇 |
2007年 | 1052篇 |
2006年 | 869篇 |
2005年 | 638篇 |
2004年 | 538篇 |
2003年 | 587篇 |
2002年 | 585篇 |
2001年 | 450篇 |
2000年 | 466篇 |
1999年 | 393篇 |
1998年 | 381篇 |
1997年 | 255篇 |
1996年 | 213篇 |
1995年 | 176篇 |
1994年 | 178篇 |
1993年 | 159篇 |
1992年 | 148篇 |
1991年 | 94篇 |
1990年 | 87篇 |
1989年 | 90篇 |
1988年 | 107篇 |
1987年 | 71篇 |
1986年 | 67篇 |
1985年 | 103篇 |
1984年 | 80篇 |
1983年 | 24篇 |
1982年 | 50篇 |
1981年 | 47篇 |
1980年 | 49篇 |
1979年 | 38篇 |
1978年 | 33篇 |
1977年 | 34篇 |
1976年 | 30篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
41.
42.
43.
Debabrata Biswas 《Pramana》1994,42(6):447-453
The length spectrum of periodic orbits in integrable hamiltonian systems can be expressed in terms of the set of winding numbers
{M
1,…,M
f} on thef-tori. Using the Poisson summation formula, one can thus express the density, Σδ(T−T
M), as a sum of a smooth average part and fluctuations about it. Working with homogeneous separable potentials, we explicitly
show that the fluctuations are due to quantal energies. Further, their statistical properties are universal and typical of
a Poisson process as in the corresponding quantal energy eigenvalues. It is interesting to note however that even though long
periodic orbits in chaotic billiards have similar statistical properties, the form of the fluctuations are indeed very different. 相似文献
44.
We present new results for the current as a function of transmission rate in the one-dimensional totally asymmetric simple exclusion process (TASEP) with a blockage that lowers the jump rate at one site from one tor<1. Exact finitevolume results serve to bound the allowed values for the current in the infinite system. This proves the existence of a nonequilibrium phase transition, corresponding to an immiscibility gap in the allowed values of the asymptotic densities which the infinite system can have in a stationary state. A series expansion inr, derived from the finite systems, is proven to be asymptotic for all sufficiently large systems. Padé approximants based on this series, which make specific assumptions about the nature of the singularity atr=1, match numerical data for the infinite system to 1 part in 104. 相似文献
45.
We consider the control of an infinite capacity shuttle which transports passengers between two terminals. The passengers
arrive at each terminal according to a compound Poisson process and the travel time from one terminal to the other is a random
variable following an arbitrary distribution. The following control limit policy is considered: dispatch the shuttle at terminali, at the instant that the total number of passengers waiting at terminali reaches or exceeds a predetermined control limitm
i
. The objective of this paper is to obtain the mean waiting time of an arbitrary passenger at each terminal for given control
valuesm
1 andm
2. We also discuss a search procedure to obtain the optimal control values which minimize the total expected cost per unit
time under a linear cost structure. 相似文献
46.
K. Najim L. Pibouleau M. V. Le Lann 《Journal of Optimization Theory and Applications》1990,64(2):331-347
Optimization techniques are finding increasingly numerous applications in process design, in parallel to the increase of computer sophistication. The process synthesis problem can be stated as a largescale constrained optimization problem involving numerous local optima and presenting a nonlinear and nonconvex character. To solve this kind of problem, the classical optimization methods can lead to analytical and numerical difficulties. This paper describes the feasibility of an optimization technique based on learning systems which can take into consideration all the prior information concerning the process to be optimized and improve their behavior with time. This information generally occurs in a very complex analytical, empirical, or know-how form. Computer simulations related to chemical engineering problems (benzene chlorination, distillation sequence) and numerical examples are presented. The results illustrate both the performance and the implementation simplicity of this method.Nomenclature
c
i
penalty probability
-
cp
precision parameter on constraints
-
D
variation domain of the variablex
-
f(·)
objective function
-
g(·)
constraints
-
i,j
indexes
-
k
iteration number
-
N
number of actions
-
P
probability distribution vector
-
p
i
ith component of the vectorP as iterationk
-
r
number of reactors in the flowsheet
-
u(k)
discrete value or action chosen by the algorithm at iterationk
-
u
i
discrete value of the optimization variable in [u
min,u
max]
-
u
min
lowest value of the optimization variable
-
u
max
largest value of the optimization variable
-
Z
random number
-
x
variable for the criterion function
-
xp
precision parameter on criterion function
-
W(k)
performance index unit output at iterationk
-
0, 1
reinforcement scheme parameters
-
p
sum of the probability distribution vector components 相似文献
47.
A trajectory-following method for unconstrained optimization 总被引:2,自引:0,他引:2
A trajectory-following method with interesting properties is considered for solving unconstrained nonlinear programming problems. The trajectory is defined by a special system of ordinary differential equations. This system uses only the gradient of the objective function. Numerical examples are given.The work of the second author was supported by the DFG Schwerpunkt Anwendungs-bezogene Optimierung and Steuerung. 相似文献
48.
P. K. C. Wang 《Journal of Optimization Theory and Applications》1990,65(2):331-362
This paper considers the problem of the stabilization and control of distributed systems with time-dependent spatial domains. The evolution of the spatial domains with time is described by a finite-dimensional system of ordinary differential equations, while the distributed systems are described by first-order or second-order linear evolution equations defined on appropriate Hilbert spaces. First, results pertaining to the existence and uniqueness of solutions of the system equations are presented. Then, various optimal control and stabilization problems are considered. The paper concludes with some examples which illustrate the application of the main results.This work was supported by the Air Force Office of Scientific Research, Grant No. AFOSR 86-0132, by the National Science Foundation, Grant No. 87-18473, and by the Jet Propulsion Laboratory, Pasadena, California. 相似文献
49.
This article generalizes the one-agent growth theory with discounting to the case of several agents with recursive preferences. In a multi-consumption goods world, we show that, under some regularity conditions, any Pareto optimum can be viewed as a function of a trajectory of a dynamical system. The state space can be chosen to be the product of the space of capitals and the unit simplex. We define and study the properties of generalized value functions. 相似文献
50.
非线性抛物组非均匀网格差分解的唯一性和稳定性 总被引:3,自引:1,他引:3
1.引言 1.对一维非线性抛物组,在文献山中已构造一般非均匀网格差分格式,其中差分逼近的组合系数对不同的网格点和不同的网格层可以不同,并且运用不动点原理证明了差分解的存在性和收敛性.在非均匀网格差分格式中差分逼近的组合系数为常数的情形,文献[2]证明了具有有界二阶差商的离散向量解的存在性、唯一性和稳定性.本文将对文献[1]中构造的一般非均匀网格差分格式,证明所得到的差分解的唯一性和稳定性. 考虑如下非线性抛物组其中是未知的m-维向量函数是给定的矩阵函数,j(x,t,u,p)。是给定的m-维向量函数… 相似文献