From aerogels to compact glasses: effect of the density on the dynamical structure factor

The dynamical structure factor, S(Q,E), of a model system, based on a percolator and using the harmonic approximation, has been calculated using the moments method especially in the smaller Q region. With this particular growth algorithm we calculate systems with densities as small as few percent of the complete lattice. The variation of the width of the Brillouin peak with density and its dependence on the exchanged wavevector are compared with the experimental data available for amorphous silica at different densities. The results show that the dynamical properties of sparse materials, such as aerogels and xerogels, can be reproduced considering only the dynamic of their network neglecting the contribution of the elemental units forming the network.     

A new linogram algorithm for computerized tomography

We propose a new linogram algorithm for the high quality Fourierreconstruction of digital N x N images from their Radon transform.The algorithm is based on univariate fast Fourier transformsfor nonequispaced data in the time domain and in the frequencydomain. The algorithm requires only O(N² logN) arithmetic operationsand preserves the good reconstruction quality of the filteredbackprojection.     

Particle trajectories in linear deep-water waves

Using phase plane analysis we show that within the framework of linear water wave theory the particle paths in a deep-water wave are not closed: there is a forward drift over a period, which decreases with greater depth.     

Pressure-broadening in the THz frequency region: The 1.113 THz line of water

It is well established that water plays a fundamental role in various atmospheric phenomena and that the accuracy of its collisional broadening parameters has a crucial influence on reduction of remote sensing data. Nevertheless, in this field the experimental data are still scarce and consequently the estimates reported in spectroscopic databases are not always reliable and/or accurate. In the view of filling this gap, the self-, N₂- and O₂-broadening parameters of the J=1_1,1←0_0,0 rotational transition of water (1.113 THz) have been determined at room temperature. The experimental investigation has also been supported by theoretical calculations.     

The Continuous Shearlet Transform in Arbitrary Space Dimensions

This paper is concerned with the generalization of the continuous shearlet transform to higher dimensions. Similar to the two-dimensional case, our approach is based on translations, anisotropic dilations and specific shear matrices. We show that the associated integral transform again originates from a square-integrable representation of a specific group, the full n-variate shearlet group. Moreover, we verify that by applying the coorbit theory, canonical scales of smoothness spaces and associated Banach frames can be derived. We also indicate how our transform can be used to characterize singularities in signals.     

A boundary value problem for fourth-order elastic beam equations

Multiplicity results for a fourth-order nonlinear boundary value problem are presented. The proof of the main result is based on the critical point theory.     

Sparse precision matrices for minimum variance portfolios

Computational Management Science - Financial crises are typically characterized by highly positively correlated asset returns due to the simultaneous distress on almost all securities, high...     

On minimum k-modal partitions of permutations

Partitioning a permutation into a minimum number of monotone subsequences is -hard. We extend this complexity result to minimum partitioning into k-modal subsequences; here unimodal is the special case k=1. Based on a network flow interpretation we formulate both, the monotone and the k-modal version, as mixed integer programs. This is the first proposal to obtain provably optimal partitions of permutations. LP rounding gives a 2-approximation for minimum monotone partitions and a (k+1)-approximation for minimum (upper) k-modal partitions. For the online problem, in which the permutation becomes known to an algorithm sequentially, we derive a logarithmic lower bound on the competitive ratio for minimum monotone partitions, and we analyze two (bin packing) online algorithms. These immediately apply to online cocoloring of permutation graphs.