全文获取类型
收费全文 | 1573篇 |
免费 | 104篇 |
国内免费 | 89篇 |
专业分类
化学 | 154篇 |
力学 | 24篇 |
综合类 | 48篇 |
数学 | 1287篇 |
物理学 | 253篇 |
出版年
2024年 | 3篇 |
2023年 | 16篇 |
2022年 | 50篇 |
2021年 | 35篇 |
2020年 | 45篇 |
2019年 | 51篇 |
2018年 | 41篇 |
2017年 | 54篇 |
2016年 | 51篇 |
2015年 | 39篇 |
2014年 | 76篇 |
2013年 | 137篇 |
2012年 | 56篇 |
2011年 | 74篇 |
2010年 | 96篇 |
2009年 | 93篇 |
2008年 | 128篇 |
2007年 | 107篇 |
2006年 | 94篇 |
2005年 | 48篇 |
2004年 | 63篇 |
2003年 | 59篇 |
2002年 | 48篇 |
2001年 | 47篇 |
2000年 | 40篇 |
1999年 | 34篇 |
1998年 | 34篇 |
1997年 | 16篇 |
1996年 | 23篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 7篇 |
1992年 | 6篇 |
1991年 | 9篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1987年 | 3篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 7篇 |
1983年 | 1篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1971年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有1766条查询结果,搜索用时 15 毫秒
51.
G-protein coupled receptors (GPCRs) play a key role in different biological processes, such as regulation of growth, death and metabolism of cells. They are major therapeutic targets of numerous prescribed drugs. However, the ligand specificity of many receptors is unknown and there is little structural information available. Bioinformatics may offer one approach to bridge the gap between sequence data and functional knowledge of a receptor. In this paper, we use a bagging classification tree algorithm to predict the type of the receptor based on its amino acid composition. The prediction is performed for GPCR at the sub-family and sub-sub-family level. In a cross-validation test, we achieved an overall predictive accuracy of 91.1% for GPCR sub-family classification, and 82.4% for sub-sub-family classification. These results demonstrate the applicability of this relative simple method and its potential for improving prediction accuracy. 相似文献
52.
V. I. Arnold 《Functional Analysis and Its Applications》2003,37(3):177-190
We study the graph each of whose edges connects an element of a given ring with the square of itself. For a finite commutative group (e.g., for the multiplicative group of coprime residue classes modulo a positive integer), we describe this graph explicitly: each of its connected components is an oriented attracting cycle equipped with identical
-vertex rooted trees of special form whose roots reside on the cycle. We also compute the graphs of permutation groups on not too many elements and of the subgroups of even permutations; the connected components of these graphs are also uniformly equipped cycles. 相似文献
53.
Rein van der Hout 《Journal of Differential Equations》2003,192(1):188-201
Let and be the unit disk and the unit sphere, and let be a radially symmetric harmonic map heat flow, whose singularities coincide with downward energy jumps. Then its finite time singularities are simple in the sense that precisely one harmonic sphere separates at a time. 相似文献
54.
In this note, we consider a finite set X and maps
W from the set $ \mathcal{S}_{2|2} (X) $ of all 2, 2-
splits of X into $ \mathbb{R}_{\geq 0} $. We show that such a map
W is induced, in a canonical way, by a binary
X-tree for which a positive length $ \mathcal{l} (e) $ is
associated to every inner edge e if and only if (i) exactly
two of the three numbers W(ab|cd),W(ac|bd), and
W(ad|cb) vanish, for any four distinct elements
a, b, c, d in X,
(ii) $ a \neq d \quad\mathrm{and}\quad W (ab|xc) + W(ax|cd) = W(ab|cd) $ holds for all
a, b, c, d, x
in X with
#{a, b, c, x} = #{b, c, d, x} = 4
and $ W(ab|cx),W(ax|cd) $ > 0, and (iii) $ W (ab|uv) \geq \quad \mathrm{min} (W(ab|uw), W(ab|vw)) $
holds for any five distinct elements a, b, u, v, w in
X. Possible generalizations
regarding arbitrary $ \mathbb{R} $-trees and applications regarding tree-reconstruction algorithms
are indicated.AMS Subject Classification: 05C05, 92D15, 92B05. 相似文献
55.
Yuan-Qing Qiao Franklin D. Tall 《Proceedings of the American Mathematical Society》2003,131(12):3929-3936
In this paper we prove the equivalence between the existence of perfectly normal, non-metrizable, non-archimedean spaces and the existence of ``generalized Souslin lines", i.e., linearly ordered spaces in which every collection of disjoint open intervals is -discrete, but which do not have a -discrete dense set. The key ingredient is the observation that every first countable linearly ordered space has a dense non-archimedean subspace.
56.
声纳图像预处理是声纳图像目标识别与跟踪的前提;声纳图像对比度低,特性信息弱,为此,提出Contourlet域HMT模型(CT-HMT)的声纳图像去噪算法。Contourlet域中,不同方向间子带系数的相关性体现于DFB分解中,相邻尺度间父节点对应的4个子节点分布在2个可分离的方向子带上,父、子节点状态"持续性"采用Markov模型建模,尺度内Contourlet系数的"聚集性"采用混合高斯模型建模;最后,用贝叶斯准则估计无噪图像的Contourlet系数,实现声纳图像去噪。实验结果从视觉效果和定量分析两方面验证表明,本文算法能有效地抑制噪声,提取声纳图像的弱特征信息,较好地保全了图像的边缘和轮廓信息。 相似文献
57.
Eva E. Rufino-Palomares Amalia Prez-Jimnez Leticia García-Salguero Khalida Mokhtari Fernando J. Reyes-Zurita Juan Peragn-Snchez Jos A. Lupiez 《Molecules (Basel, Switzerland)》2022,27(7)
There is currently a worldwide consensus and recognition of the undoubted health benefits of the so-called Mediterranean diet, with its intake being associated with a lower risk of mortality. The most important characteristics of this type of diet are based on the consumption of significant amounts of fruit, vegetables, legumes, and nuts, which provide, in addition to some active ingredients, fiber and a proportion of vegetable protein, together with extra virgin olive oil (EVOO) as the main sources of vegetable fat. Fish and meat from poultry and other small farm animals are the main sources of protein. One of the main components, as already mentioned, is EVOO, which is rich in monounsaturated fatty acids and to a lesser extent in polyunsaturated fatty acids. The intake of this type of nutrient also provides an important set of phytochemicals whose health potential is widely spread and agreed upon. These phytochemicals include significant amounts of anthocyanins, stilbenes, flavonoids, phenolic acids, and terpenes of varying complexities. Therefore, the inclusion in the diet of this type of molecules, with a proven healthy effect, provides an unquestionable preventive and/or curative activity on an important group of pathologies related to cardiovascular, infectious, and cancerous diseases, as well as those related to the metabolic syndrome. The aim of this review is therefore to shed light on the nutraceutical role of two of the main phytochemicals present in Olea europaea fruit and leaf extracts, polyphenols, and triterpenes, on healthy animal growth. Their immunomodulatory, anti-infective, antioxidant, anti-aging, and anti-carcinogenic capabilities show them to be potential nutraceuticals, providing healthy growth. 相似文献
58.
We present an O(min(Kn,n2)) algorithm to solve the maximum integral multiflow and minimum multicut problems in rooted trees, where K is the number of commodities and n is the number of vertices. These problems are NP-hard in undirected trees but polynomial in directed trees. In the algorithm we propose, we first use a greedy procedure to build the multiflow then we use duality properties to obtain the multicut and prove the optimality. 相似文献
59.
60.
Sy-David Friedman Radek Honzik Šárka Stejskalová 《Annals of Pure and Applied Logic》2018,169(6):548-564
Starting from a Laver-indestructible supercompact κ and a weakly compact λ above κ, we show there is a forcing extension where κ is a strong limit singular cardinal with cofinality ω, , and the tree property holds at . Next we generalize this result to an arbitrary cardinal μ such that and . This result provides more information about possible relationships between the tree property and the continuum function. 相似文献