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By means of powder X-ray diffraction, powder neutron diffraction and transmission electron microscopy (TEM), we determined the crystal structures of a metal-ordered manganite YBaMn2O6 which undergoes successive phase transitions. A high-temperature metallic phase (Tc1=520 K<T) crystallizes in a triclinic P1 with the following unit cell: Z=2, a=5.4948(15) Å, b=5.4920(14) Å, c=7.7174(4) Å, α=89.804(20)°, β=90.173(20)°, γ=91.160(4)°. The MnO6 octahedral tilting is approximately written as a0b−c−, leading to a significant structural anisotropy within the ab plane. The structure for Tc2<T<Tc1 is a monoclinic P2 (Z=2, a=5.5181(4) Å, b=5.5142(4) Å, c=7.6443(3) Å, β=90.267(4)°) with an a−b−c− tilting. The structural features suggest a dx2−y2 orbital ordering (OO). Below Tc2=480 K, crystallographically inequivalent two octahedra show distinct volume difference, due to the Mn3+/Mn4+ charge ordering. The TEM study furthermore revealed a unique d3x2−r2/d3y2−r2 OO with a modified CE structure. It was found that the obtained crystal structures are strongly correlated to the unusual physical properties. In particular, the extremely high temperature at which charge degree of freedom freezes, Tc2, should be caused by the absence of the structural disorder and by heavily distorted MnO6 octahedra. 相似文献
74.
Lidia Angeleri Hügel Alberto Tonolo Jan Trlifaj 《Algebras and Representation Theory》2001,4(2):155-170
We relate the theory of envelopes and covers to tilting and cotilting theory, for (infinitely generated) modules over arbitrary rings. Our main result characterizes tilting torsion classes as the pretorsion classes providing special preenvelopes for all modules. A dual characterization is proved for cotilting torsion-free classes using the new notion of a cofinendo module. We also construct unique representing modules for these classes. 相似文献
75.
By direct calculation of rotation matrices of SO(3), we show how certainspecific sequence of eight consecutive rotations of digital angles can yielda tilting of a facet mirror. We also design a detailed program specificallyto tilt an array of mirrors from planar orientation to the required focusingorientation. We describe how to use the 8-step to realize the focusing ofthe mirror array. We have found, in our designed program, an importantfeature of row-sharing during the rotations for the columns and similarlythe column-sharing during the rotations for the row. This feature can save alot of operating time during the actual realization of the mechanicalmovements. 相似文献
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A general framework is introduced to estimate how much external information has been infused into a search algorithm, the so-called active information. This is rephrased as a test of fine-tuning, where tuning corresponds to the amount of pre-specified knowledge that the algorithm makes use of in order to reach a certain target. A function f quantifies specificity for each possible outcome x of a search, so that the target of the algorithm is a set of highly specified states, whereas fine-tuning occurs if it is much more likely for the algorithm to reach the target as intended than by chance. The distribution of a random outcome X of the algorithm involves a parameter that quantifies how much background information has been infused. A simple choice of this parameter is to use in order to exponentially tilt the distribution of the outcome of the search algorithm under the null distribution of no tuning, so that an exponential family of distributions is obtained. Such algorithms are obtained by iterating a Metropolis–Hastings type of Markov chain, which makes it possible to compute their active information under the equilibrium and non-equilibrium of the Markov chain, with or without stopping when the targeted set of fine-tuned states has been reached. Other choices of tuning parameters are discussed as well. Nonparametric and parametric estimators of active information and tests of fine-tuning are developed when repeated and independent outcomes of the algorithm are available. The theory is illustrated with examples from cosmology, student learning, reinforcement learning, a Moran type model of population genetics, and evolutionary programming. 相似文献
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可倾瓦径向滑动轴承支承的转子系统瞬态响应计算 总被引:3,自引:0,他引:3
研究了可倾瓦径向滑动轴承支承的转子系统瞬态响应的计算方法,提出了可倾瓦径向滑动轴承非线性滑膜力计算的数据库方法,将可倾瓦与轴颈的结构和运动参数归结到三个参数中,建立了起可倾瓦的非线性油膜力数据库,通过精确插值到每一运动参数组合下轴承提代的油力和轴瓦上所受的油膜力矩。 相似文献
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AbstractLet A be an n-Gorenstein ring. Employing the theory developed by Enochs on the existence of Gorenstein preenvelopes and precovers, we introduce the concept of Gorenstein tilting pair. Moreover, we give a simple characterization on Gorenstein tilting pair, which shows that Gorenstein cotilting and tilting modules are special examples of Gorenstein tilting pair. 相似文献
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