全文获取类型
收费全文 | 4637篇 |
免费 | 219篇 |
国内免费 | 87篇 |
专业分类
化学 | 3147篇 |
晶体学 | 26篇 |
力学 | 150篇 |
综合类 | 10篇 |
数学 | 556篇 |
物理学 | 1054篇 |
出版年
2023年 | 21篇 |
2022年 | 57篇 |
2021年 | 64篇 |
2020年 | 80篇 |
2019年 | 72篇 |
2018年 | 69篇 |
2017年 | 65篇 |
2016年 | 125篇 |
2015年 | 120篇 |
2014年 | 162篇 |
2013年 | 263篇 |
2012年 | 305篇 |
2011年 | 348篇 |
2010年 | 206篇 |
2009年 | 161篇 |
2008年 | 284篇 |
2007年 | 271篇 |
2006年 | 265篇 |
2005年 | 266篇 |
2004年 | 208篇 |
2003年 | 178篇 |
2002年 | 159篇 |
2001年 | 100篇 |
2000年 | 118篇 |
1999年 | 59篇 |
1998年 | 46篇 |
1997年 | 44篇 |
1996年 | 43篇 |
1995年 | 59篇 |
1994年 | 55篇 |
1993年 | 55篇 |
1992年 | 46篇 |
1991年 | 32篇 |
1990年 | 30篇 |
1989年 | 26篇 |
1988年 | 30篇 |
1987年 | 32篇 |
1986年 | 21篇 |
1985年 | 28篇 |
1984年 | 30篇 |
1982年 | 30篇 |
1981年 | 29篇 |
1980年 | 22篇 |
1979年 | 39篇 |
1978年 | 32篇 |
1977年 | 32篇 |
1976年 | 26篇 |
1975年 | 22篇 |
1974年 | 22篇 |
1973年 | 14篇 |
排序方式: 共有4943条查询结果,搜索用时 16 毫秒
161.
On a nonlinear multigrid algorithm with primal relaxation for the image total variation minimisation
Digital image restoration has drawn much attention in the recent years and a lot of research has been done on effective variational partial differential equation models and their theoretical studies. However there remains an urgent need to develop fast and robust iterative solvers, as the underlying problem sizes are large. This paper proposes a fast multigrid method using primal relaxations. The basic primal relaxation is known to get stuck at a ‘local’ non-stationary minimum of the solution, which is usually believed to be ‘non-smooth’. Our idea is to utilize coarse level corrections, overcoming the deadlock of a basic primal relaxation scheme. A further refinement is to allow non-regular coarse levels to correct the solution, which helps to improve the multilevel method. Numerical experiments on both 1D and 2D images are presented. 相似文献
162.
163.
Jennifer S.K. Chan Anthony Y.C. Kuk Carrie H.K. Yam 《Journal of multivariate analysis》2005,94(2):300-312
Geyer (J. Roy. Statist. Soc. 56 (1994) 291) proposed Monte Carlo method to approximate the whole likelihood function. His method is limited to choosing a proper reference point. We attempt to improve the method by assigning some prior information to the parameters and using the Gibbs output to evaluate the marginal likelihood and its derivatives through a Monte Carlo approximation. Vague priors are assigned to the parameters as well as the random effects within the Bayesian framework to represent a non-informative setting. Then the maximum likelihood estimates are obtained through the Newton Raphson method. Thus, out method serves as a bridge between Bayesian and classical approaches. The method is illustrated by analyzing the famous salamander mating data by generalized linear mixed models. 相似文献
164.
Raymond H. Chan Michael K. Ng Robert J. Plemmons 《Numerical Linear Algebra with Applications》1996,3(1):45-64
In this paper, we propose a method to generalize Strang's circulant preconditioner for arbitrary n-by-n matrices An. The th column of our circulant preconditioner Sn is equal to the th column of the given matrix An. Thus if An is a square Toeplitz matrix, then Sn is just the Strang circulant preconditioner. When Sn is not Hermitian, our circulant preconditioner can be defined as . This construction is similar to the forward-backward projection method used in constructing preconditioners for tomographic inversion problems in medical imaging. We show that if the matrix An has decaying coefficients away from the main diagonal, then is a good preconditioner for An. Comparisons of our preconditioner with other circulant-based preconditioners are carried out for some 1-D Toeplitz least squares problems: min ∥ b - Ax∥2. Preliminary numerical results show that our preconditioner performs quite well, in comparison to other circulant preconditioners. Promising test results are also reported for a 2-D deconvolution problem arising in ground-based atmospheric imaging. 相似文献
165.
Ring-shaped lead zirconate titanate (PZT) piezoelectric vibrators were subjected to non-uniform mechanical stress applied by bolt clamping. The effect of mechanical stress on the effective electromechanical coupling factor (keff) and mechanical quality factor (Qm) of the thickness and wall thickness modes was studied by an equivalent electric circuit analysis. The initiation and propagation of cracks under mechanical stress were also discussed based on the resonance method and the indentation technique. keff for both the thickness and wall thickness modes decreased with increase in mechanical stress due to de-poling of the PZT. Qm of the thickness mode dropped sharply with increase in mechanical stress while Qm of the wall thickness mode remained almost unchange. Existence of microcracks in a PZT vibrator can be detected by the occurrence of spurious vibrations at the wall thickness mode in the electrical impedance vs. frequency spectra. 相似文献
166.
Wai Kiu Chan Joshua Daniels 《Proceedings of the American Mathematical Society》2005,133(11):3121-3131
Let be a power of an odd prime, and be the ring of polynomials over a finite field of elements. A quadratic form over is said to be regular if globally represents all polynomials that are represented by the genus of . In this paper, we study definite regular quadratic forms over . It is shown that for a fixed , there are only finitely many equivalence classes of regular definite primitive quadratic forms over , regardless of the number of variables. Characterizations of those which are universal are also given.
167.
168.
Timothy M. Chan 《Discrete and Computational Geometry》2012,47(4):661-690
We revisit one of the most fundamental classes of data structure problems in computational geometry: range searching. Matoušek
(Discrete Comput. Geom. 10:157–182, 1993) gave a partition tree method for d-dimensional simplex range searching achieving O(n) space and O(n
1−1/d
) query time. Although this method is generally believed to be optimal, it is complicated and requires O(n
1+ε
) preprocessing time for any fixed ε>0. An earlier method by Matoušek (Discrete Comput. Geom. 8:315–334, 1992) requires O(nlogn) preprocessing time but O(n
1−1/d
log
O(1)
n) query time. We give a new method that achieves simultaneously O(nlogn) preprocessing time, O(n) space, and O(n
1−1/d
) query time with high probability. Our method has several advantages:
• |
It is conceptually simpler than Matoušek’s O(n
1−1/d
)-time method. Our partition trees satisfy many ideal properties (e.g., constant degree, optimal crossing number at almost
all layers, and disjointness of the children’s cells at each node). 相似文献
169.
Timothy M. Chan 《Discrete and Computational Geometry》2012,48(1):1-18
A favorite open problem in combinatorial geometry is to determine the worst-case complexity of a level in an arrangement. Up to now, nontrivial upper bounds in three dimensions are known only for the linear cases of planes and triangles. We propose the first technique that can deal with more general surfaces in three dimensions. For example, in an arrangement of n ??pseudo-planes?? or ??pseudo-spherical patches?? (where the main criterion is that each triple of surfaces has at most two common intersections), we prove that there are at most O(n 2.997) vertices at any given level. 相似文献
170.
This Letter reports the novel use of poly(9‐vinylcarbazole) (PVK) as a dielectric interfacial layer for n‐type organic field‐effect transistors (n‐OFETs). With PVK, both the air stability and electron mobility of N,N′‐ditridecylperylene‐3,4,9,10‐tetracarboxylic diimide (PTCDI‐C13)‐based OFETs were improved. Among the PVKs with different weight‐average molecular weight (Mw), PVK with high Mw showed good performance. The high glass transition temperature of PVK enabled thermal post annealing of the active layer, which resulted in a high electron mobility of 0.61 cm2/Vs. This mobility was maintained at 90% and 59% after 4 days and 105 days in air, respectively. The PVK interfacial layer reduced the trapped charges in the PTCDI‐C13‐based n‐OFET for air‐exposure and caused a decrease in the threshold voltage shift.
|