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91.
We investigate the structure of a large precision matrix in Gaussian graphical models by decomposing it into a low rank component and a remainder part with sparse precision matrix.Based on the decomposition,we propose to estimate the large precision matrix by inverting a principal orthogonal decomposition(IPOD).The IPOD approach has appealing practical interpretations in conditional graphical models given the low rank component,and it connects to Gaussian graphical models with latent variables.Specifically,we show that the low rank component in the decomposition of the large precision matrix can be viewed as the contribution from the latent variables in a Gaussian graphical model.Compared with existing approaches for latent variable graphical models,the IPOD is conveniently feasible in practice where only inverting a low-dimensional matrix is required.To identify the number of latent variables,which is an objective of its own interest,we investigate and justify an approach by examining the ratios of adjacent eigenvalues of the sample covariance matrix?Theoretical properties,numerical examples,and a real data application demonstrate the merits of the IPOD approach in its convenience,performance,and interpretability. 相似文献
92.
《Numerical Methods for Partial Differential Equations》2018,34(3):959-981
In this article, integrated radial basis functions (IRBFs) are used for Hermite interpolation in the solution of differential equations, resulting in a new meshless symmetric RBF method. Both global and local approximation‐based schemes are derived. For the latter, the focus is on the construction of compact approximation stencils, where a sparse system matrix and a high‐order accuracy can be achieved together. Cartesian‐grid‐based stencils are possible for problems defined on nonrectangular domains. Furthermore, the effects of the RBF width on the solution accuracy for a given grid size are fully explored with a reasonable computational cost. The proposed schemes are numerically verified in some elliptic boundary‐value problems governed by the Poisson and convection‐diffusion equations. High levels of the solution accuracy are obtained using relatively coarse discretisations. 相似文献
93.
The new variable-step, variable-order, ODE solver, HBT(p) of order p, presented in this paper, combines a three-stage Runge-Kutta method of order 3 with a Taylor series method of order p-2 to solve initial value problems , where y:R→Rd and f:R×Rd→Rd. The order conditions satisfied by HBT(p) are formulated and they lead to Vandermonde-type linear algebraic systems whose solutions are the coefficients in the formulae for HBT(p). A detailed formulation of variable-step HBT(p) in both fixed-order and variable-order modes is presented. The new method and the Taylor series method have similar regions of absolute stability. To obtain high-accuracy results at high order, this method has been implemented in multiple precision. 相似文献
94.
在比较不同探月任务取得的月表三维影像数据的基础上,选择中国嫦娥一号全月分幅数字高程模型(DEM)数据作为构建月表地形模型的数据源,并利用ArcGIS、Cass和AutoCAD等软件的功能及其之间的连接关系,研究了基于月球探测数据构建月表三维模型的技术和方法。以月表撞击坑Lichtenberg为例,建立了撞击坑的三维地形模型,并对其精度和影响精度的因素进行了分析。分析结果表明,对于500m分辨率的原始数据,模型误差较小,产生误差的原因主要包括生成等高线的密度、采点间距等因素。 相似文献
95.
96.
L. S. Pan H. P. Lee C. Lu 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,50(1):27-33
We report the synthesis and measurement of an
ultra-precise and extremely stable optical frequency in the
telecommunications window around 1543 nm. Using a fibre-based femtosecond
frequency comb we have phase-stabilised a fibre laser at 194 THz to an
optical frequency standard at 344 THz, thus transferring the properties of
the optical frequency standard to another spectral region. Relative to the
optical frequency standard, the synthesised frequency at 194 THz is
determined to within 1 mHz and its fractional frequency instability is
measured to be less than 2×10-15 at 1 s, reaching 5×
10-18 after 8000 s. We also measured the synthesised frequency against
a caesium fountain clock: here the frequency comparison itself contributes
less than 4 mHz (2×10-17) to the uncertainty. Our results
confirm the suitability of fibre based frequency comb technology for
precision measurements and frequency synthesis, and enable long-distance
comparison of optical clocks by using optical fibres to transmit the
frequency information. 相似文献
97.
The past decades have witnessed the emergence of low-dimensional carbon-based nanostructures owing to their unique properties and various subsequent applications. It is of fundamental importance to explore ways to achieve atomically precise fabrication of these interesting structures. The newly developed on-surface synthesis approach provides an efficient strategy for this challenging issue, demonstrating the potential of atomically precise preparation of low-dimensional nanostructures. Up to now, the formation of various surface nanostructures, especially carbon-based ones, such as graphene nanoribbons (GNRs), kinds of organic (organometallic) chains and films, have been achieved via on-surface synthesis strategy, in which in-depth understanding of the reaction mechanism has also been explored. This review article will provide a general overview on the formation of one-dimensional carbon-based nanostructures via on-surface synthesis method. In this review, only a part of the on-surface chemical reactions (specifically, C−X (X=Cl, Br, I) and C−H activation reactions) under ultra-high vacuum conditions will be covered. 相似文献
98.
Sophia Boden Franziska Reise Jessica Kania Thisbe K. Lindhorst Laura Hartmann 《Macromolecular bioscience》2019,19(4)
This study investigates the influence of an increasingly hydrophobic backbone of multivalent glycomimetics based on sequence‐defined oligo(amidoamines) on their resulting affinity toward bacterial lectins. Glycomacromolecules are obtained by stepwise assembly of tailor‐made building blocks on solid support, using both hydrophobic aliphatic and aromatic building blocks to enable a gradual change in hydrophobicity of the backbone. Their binding behavior toward model lectin Concanavalin A (ConA) is evaluated using isothermal titration calorimetry (ITC) and surface plasmon resonance (SPR) showing higher affinities for glycomacromolecules with higher content of hydrophobic and aromatic moieties in the backbone. Finally, glycomacromolecules are tested in a bacterial adhesion inhibition study against Escherichia coli where more hydrophobic backbones yield higher inhibitory potentials most likely due to additional secondary interactions with hydrophobic regions of the protein receptor as well as a change in conformation exposing carbohydrate ligands for increased binding. Overall, the results highlight the influence and thereby importance of the polymer backbone itself on the resulting properties of polymeric biomimetics. 相似文献
99.
100.