全文获取类型
收费全文 | 4278篇 |
免费 | 333篇 |
国内免费 | 357篇 |
专业分类
化学 | 167篇 |
力学 | 174篇 |
综合类 | 107篇 |
数学 | 3971篇 |
物理学 | 549篇 |
出版年
2024年 | 3篇 |
2023年 | 67篇 |
2022年 | 75篇 |
2021年 | 132篇 |
2020年 | 135篇 |
2019年 | 112篇 |
2018年 | 121篇 |
2017年 | 116篇 |
2016年 | 137篇 |
2015年 | 82篇 |
2014年 | 160篇 |
2013年 | 344篇 |
2012年 | 156篇 |
2011年 | 227篇 |
2010年 | 195篇 |
2009年 | 278篇 |
2008年 | 272篇 |
2007年 | 292篇 |
2006年 | 272篇 |
2005年 | 228篇 |
2004年 | 177篇 |
2003年 | 172篇 |
2002年 | 181篇 |
2001年 | 144篇 |
2000年 | 155篇 |
1999年 | 132篇 |
1998年 | 107篇 |
1997年 | 92篇 |
1996年 | 74篇 |
1995年 | 53篇 |
1994年 | 55篇 |
1993年 | 34篇 |
1992年 | 26篇 |
1991年 | 17篇 |
1990年 | 17篇 |
1989年 | 5篇 |
1988年 | 13篇 |
1987年 | 10篇 |
1986年 | 24篇 |
1985年 | 27篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1981年 | 5篇 |
1979年 | 2篇 |
1978年 | 4篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有4968条查询结果,搜索用时 46 毫秒
1.
Dynamical analysis,circuit realization,and application in pseudorandom number generators of a fractional-order laser chaotic system 下载免费PDF全文
Chenguang Ma 《中国物理 B》2021,30(12):120504-120504
A new five-dimensional fractional-order laser chaotic system (FOLCS) is constructed by incorporating complex variables and fractional calculus into a Lorentz-Haken-type laser system. Dynamical behavior of the system, circuit realization and application in pseudorandom number generators are studied. Many types of multi-stable states are discovered in the system. Interestingly, there are two types of state transition phenomena in the system, one is the chaotic state degenerates to a periodical state, and the other is the intermittent chaotic oscillation. In addition, the complexity of the system when two parameters change simultaneously is measured by the spectral entropy algorithm. Moreover, a digital circuit is design and the chaotic oscillation behaviors of the system are verified on this circuit. Finally, a pseudo-random sequence generator is designed using the FOLCS, and the statistical characteristics of the generated pseudo-random sequence are tested with the NIST-800-22. This study enriches the research on the dynamics and applications of FOLCS. 相似文献
3.
4.
The satisfiability (SAT) problem is a core problem in computer science. Existing studies have shown that most industrial SAT instances can be effectively solved by modern SAT solvers while random SAT instances cannot. It is believed that the structural characteristics of different SAT formula classes are the reasons behind this difference. In this paper, we study the structural properties of propositional formulas in conjunctive normal form (CNF) by the principle of structural entropy of formulas. First, we used structural entropy to measure the complex structure of a formula and found that the difficulty solving the formula is related to the structural entropy of the formula. The smaller the compressing information of a formula, the more difficult it is to solve the formula. Secondly, we proposed a -approximation strategy to approximate the structural entropy of large formulas. The experimental results showed that the proposed strategy can effectively approximate the structural entropy of the original formula and that the approximation ratio is more than 92%. Finally, we analyzed the structural properties of a formula in the solution process and found that a local search solver tends to select variables in different communities to perform the next round of searches during a search and that the structural entropy of a variable affects the probability of the variable being flipped. By using these conclusions, we also proposed an initial candidate solution generation strategy for a local search for SAT, and the experimental results showed that this strategy effectively improves the performance of the solvers CCAsat and Sparrow2011 when incorporated into these two solvers. 相似文献
5.
本文基于新的Kronecker型替换,给出两个由黑盒表示的稀疏多项式的新确定性插值算法.令f∈R[x1,……,xn]是一个稀疏黑盒多项式,其次数上界为D.当R是C或者是有限域时,相对于已有算法,新算法具有更好的计算复杂度或者关于D的复杂度更低.特别地,对于一般黑盒模型,D是复杂度中的主要因素,而在所有的确定性算法中,本文的第二个算法的复杂度关于D是最低的. 相似文献
6.
We generalize Nagel’s formula for the Szegö kernel and use it to compute the Szegö kernel on a class of non-compact CR manifolds whose tangent space decomposes into one complex direction and several totally real directions. We also discuss the control metric on these manifolds and relate it to the size of the Szegö kernel. 相似文献
7.
8.
We give a sheaf theoretic interpretation of Potts models with external magnetic field, in terms of constructible sheaves and their Euler characteristics. We show that the polynomial countability question for the hypersurfaces defined by the vanishing of the partition function is affected by changes in the magnetic field: elementary examples suffice to see non-polynomially countable cases that become polynomially countable after a perturbation of the magnetic field. The same recursive formula for the Grothendieck classes, under edge-doubling operations, holds as in the case without magnetic field, but the closed formulae for specific examples like banana graphs differ in the presence of magnetic field. We give examples of computation of the Euler characteristic with compact support, for the set of real zeros, and find a similar exponential growth with the size of the graph. This can be viewed as a measure of topological and algorithmic complexity. We also consider the computational complexity question for evaluations of the polynomial, and show both tractable and NP-hard examples, using dynamic programming. 相似文献
9.
Fabio Fagnani Sandro Zampieri 《Transactions of the American Mathematical Society》1997,349(5):1993-2006
In this paper we study symbolic dynamics over alphabets which are modules over a principal ideal domain, considering topological shifts which are also submodules. Our main result is the classification, up to algebraic and topological conjugacy, of the torsion-free, transitive, finite memory shifts.
10.