全文获取类型
收费全文 | 3391篇 |
免费 | 132篇 |
国内免费 | 141篇 |
专业分类
化学 | 75篇 |
晶体学 | 2篇 |
力学 | 61篇 |
综合类 | 40篇 |
数学 | 3120篇 |
物理学 | 366篇 |
出版年
2023年 | 17篇 |
2022年 | 51篇 |
2021年 | 75篇 |
2020年 | 62篇 |
2019年 | 81篇 |
2018年 | 76篇 |
2017年 | 72篇 |
2016年 | 103篇 |
2015年 | 60篇 |
2014年 | 130篇 |
2013年 | 354篇 |
2012年 | 133篇 |
2011年 | 145篇 |
2010年 | 121篇 |
2009年 | 190篇 |
2008年 | 180篇 |
2007年 | 212篇 |
2006年 | 150篇 |
2005年 | 139篇 |
2004年 | 98篇 |
2003年 | 124篇 |
2002年 | 122篇 |
2001年 | 109篇 |
2000年 | 90篇 |
1999年 | 93篇 |
1998年 | 77篇 |
1997年 | 69篇 |
1996年 | 61篇 |
1995年 | 60篇 |
1994年 | 56篇 |
1993年 | 27篇 |
1992年 | 29篇 |
1991年 | 39篇 |
1990年 | 19篇 |
1989年 | 23篇 |
1988年 | 22篇 |
1987年 | 18篇 |
1986年 | 14篇 |
1985年 | 34篇 |
1984年 | 19篇 |
1983年 | 16篇 |
1982年 | 15篇 |
1981年 | 5篇 |
1980年 | 9篇 |
1979年 | 19篇 |
1978年 | 8篇 |
1977年 | 15篇 |
1976年 | 6篇 |
1975年 | 7篇 |
1973年 | 3篇 |
排序方式: 共有3664条查询结果,搜索用时 15 毫秒
71.
Pencil beam algorithms used in computerized electron beam dose planning are usually described using the small angle multiple scattering theory. Alternatively, the pencil beams can be generated by Monte Carlo simulation of electron transport. In a previous work, the 4th version of the Electron Gamma Shower (EGS) Monte Carlo code was used to obtain dose distributions from monoenergetic electron pencil beam, with incident energy between 1 MeV and 50 MeV, interacting at the surface of a large cylindrical homogeneous water phantom. In 2000, a new version of this Monte Carlo code has been made available by the National Research Council of Canada (NRC), which includes various improvements in its electron-transport algorithms. In the present work, we were interested to see if the new physics in this version produces pencil beam dose distributions very different from those calculated with oldest one. The purpose of this study is to quantify as well as to understand these differences. We have compared a series of pencil beam dose distributions scored in cylindrical geometry, for electron energies between 1 MeV and 50 MeV calculated with two versions of the Electron Gamma Shower Monte Carlo Code. Data calculated and compared include isodose distributions, radial dose distributions and fractions of energy deposition. Our results for radial dose distributions show agreement within 10% between doses calculated by the two codes for voxels closer to the pencil beam central axis, while the differences are up to 30% for longer distances. For fractions of energy deposition, the results of the EGS4 are in good agreement (within 2%) with those calculated by EGSnrc at shallow depths for all energies, whereas a slightly worse agreement (15%) is observed at deeper distances. These differences may be mainly attributed to the different multiple scattering for electron transport adopted in these two codes and the inclusion of spin effect, which produces an increase of the effective range of electrons. 相似文献
72.
This paper is devoted to numerical solutions for a class of jump-diffusions with regime switching. After briefly reviewing the notion of jump-diffusions with regime switching, finite-difference procedures are constructed. Under simple conditions, it is proved that the algorithm converges to the desired limit by means of a martingale problem formulation. Numerical experiments are carried out to demonstrate the performance of the algorithm. 相似文献
73.
We study Markov jump decision processes with both continuously and instantaneouslyacting decisions and with deterministic drift between jumps. Such decision processes were recentlyintroduced and studied from discrete time approximations point of view by Van der Duyn Schouten.Weobtain necessary and sufficient optimality conditions for these decision processes in terms of equations and inequalities of quasi-variational type. By means of the latter we find simple necessaryand sufficient conditions for the existence of stationary optimal policies in such processes with finite state and action spaces, both in the discounted and average per unit time reward cases. 相似文献
74.
The identification of disease-relevant genes represents a challenge in microarray-based disease diagnosis where the sample size is often limited. Among established methods, reversible jump Markov Chain Monte Carlo (RJMCMC) methods have proven to be quite promising for variable selection. However, the design and application of an RJMCMC algorithm requires, for example, special criteria for prior distributions. Also, the simulation from joint posterior distributions of models is computationally extensive, and may even be mathematically intractable. These disadvantages may limit the applications of RJMCMC algorithms. Therefore, the development of algorithms that possess the advantages of RJMCMC methods and are also efficient and easy to follow for selecting disease-associated genes is required. Here we report a RJMCMC-like method, called random frog that possesses the advantages of RJMCMC methods and is much easier to implement. Using the colon and the estrogen gene expression datasets, we show that random frog is effective in identifying discriminating genes. The top 2 ranked genes for colon and estrogen are Z50753, U00968, and Y10871_at, Z22536_at, respectively. (The source codes with GNU General Public License Version 2.0 are freely available to non-commercial users at: http://code.google.com/p/randomfrog/.) 相似文献
75.
We present a numerical algorithm for pricing derivatives on electricity prices. The algorithm is based on approximating the generator of the underlying price process on a lattice of prices, resulting in an approximation of the stochastic process by a continuous time Markov chain. We numerically study the rate of convergence of the algorithm for the case of the Merton jump-diffusion model and apply the algorithm to calculate prices and sensitivities of both European and Bermudan electricity derivatives when the underlying price follows a stochastic process which exhibits both fast mean-reversion and jumps of large magnitude. 相似文献
76.
《Optimization》2012,61(4-5):495-505
This paper investigates properties of the optimality equation and optimal policies in discrete time Markov decision processes with expected discounted total rewards under weak conditions that the model is well defined and the optimality equation is true. The optimal value function is characterized as a solution of the optimality equation and the structure of optimal policies is also given. 相似文献
77.
In this paper, we mainly studied the limit properties for the
countable nonhomogeneous Markov chains. We established some limit properties for the
functions of the countable nonhomogeneous Markov chains with variables under the
convergence in the sense, which extended the similar conclusions for the
functions with two variables. At last, as a corollary, we given the similar result in
the homogeneous Markov stock market. 相似文献
78.
This paper studies maximum likelihood estimation for a parameterised elliptic diffusion in a manifold. The focus is on asymptotic properties of maximum likelihood estimates obtained from continuous time observation. These are well known when the underlying manifold is a Euclidean space. However, no systematic study exists in the case of a general manifold. The starting point is to write down the likelihood function and equation. This is achieved using the tools of stochastic differential geometry. Consistency, asymptotic normality and asymptotic optimality of maximum likelihood estimates are then proved, under regularity assumptions. Numerical computation of maximum likelihood estimates is briefly discussed. 相似文献
79.
80.
Balázs Gerencsér 《Stochastic Processes and their Applications》2019,129(9):3570-3584
The paper concerns a particular example of the Gibbs sampler and its mixing efficiency. Coordinates of a point are rerandomized in the unit square to approach a stationary distribution with density proportional to for with some large parameter .Diaconis conjectured the mixing time of this process to be which we confirm in this paper. This improves on the currently known estimate. 相似文献