Averaging on Manifolds by Embedding Algorithm

We will propose a new algorithm for finding critical points of cost functions defined on a differential manifold. We will lift the initial cost function to a manifold that can be embedded in a Riemannian manifold (Euclidean space) and will construct a vector field defined on the ambient space whose restriction to the embedded manifold is the gradient vector field of the lifted cost function. The advantage of this method is that it allows us to do computations in Cartesian coordinates instead of using local coordinates and covariant derivatives on the initial manifold. We will exemplify the algorithm in the case of SO(3) averaging problems and will rediscover a few well known results that appear in literature.     

Combining Capon and APES for estimation of spectral lines

We propose combining the Capon and the APES spectral estimators for estimation of both the amplitude and the frequency of spectral lines. The so-obtained estimator does not suffer from Capon's biased amplitude estimates nor from APES' biased frequency estimates or resolution problem. Furthermore, the combined estimator is computationally simpler than APES and has about the same complexity as Capon. Numerical simulations are presented illustrating the increased performance.This work was supported in part by the Swedish Foundation for Strategic Research.     

Space–Time Block Coding for Channels with Intersymbol Interference

Stoica, P., and Lindskog, E., Space–Time Block Coding for Channels with Intersymbol Interference, Digital Signal Processing12 (2002) 616–627The downlink of many wireless communication systems can be a MISO channel. An important problem for a MISO channel is how to code across space and time to obtain the same ML receiver as for the corresponding SIMO channel. For flat fading channels, space–time block coding (STBC) is a recent breakthrough solution to this problem. In Lindskog and Paulraj (in Proceedings of ICC'2000, NewOrleans, LA, June 18–22, 2000), STBC has been generalized to channels with intersymbol interference (ISI) for the case of two transmit antennas and one receive antenna. In this paper we first revisit the generalized STBC scheme of Lindskog and Paulraj and show that it has the same appealing properties as the standard STBC for flat fading channels. Then we go on to present an extension of this scheme to ISI channels with any number of transmit and receive antennas.     

TSP: Mining top-k closed sequential patterns

Sequential pattern mining has been studied extensively in the data mining community. Most previous studies require the specification of a min_support threshold for mining a complete set of sequential patterns satisfying the threshold. However, in practice, it is difficult for users to provide an appropriate min_support threshold. To overcome this difficulty, we propose an alternative mining task: mining top-k frequent closed sequential patterns of length no less than min_, where k is the desired number of closed sequential patterns to be mined and min_ is the minimal length of each pattern. We mine the set of closed patterns because it is a compact representation of the complete set of frequent patterns. An efficient algorithm, called TSP, is developed for mining such patterns without min_support. Starting at (absolute) min_support=1, the algorithm makes use of the length constraint and the properties of top-k closed sequential patterns to perform dynamic support raising and projected database pruning. Our extensive performance study shows that TSP has high performance. In most cases, it outperforms the efficient closed sequential pattern-mining algorithm, CloSpan, even when the latter is running with the best tuned min_support threshold. Thus, we conclude that, for sequential pattern mining, mining top-k frequent closed sequential patterns without min_support is more preferable than the traditional min_support-based mining.     

Surface analysis and electrochemical behavior of Ti–20Zr alloy in simulated physiological fluids

An advanced Ti–20Zr alloy was obtained by double vacuum melting in a semi-levitation furnace with cold crucible. The alloy shows fully lamellar α + β microstructure. Cyclic potentiodynamic polarization curves revealed that the alloy passivated easier, more rapid than Ti, having a more stable passive film in Ringer solutions of different pH values, simulating severe functional conditions of an implant. In neutral and alkaline Ringer solutions, the alloy passive film improved its properties in time (1500 h) by the deposition of protective hydroxyapatite, as was demonstrated by XPS, SEM, EDX, Raman and FT-IR measurements. Alloy presented lower corrosion rates and higher polarization resistances (from linear polarization measurements) than those of Ti (tens of times) proving a more resistant passive film. Alloy open circuit potentials had more electropositive values in comparison with Ti and tended to nobler values in time, which denote better passive state and its enhancement in time, due to the new depositions from the physiological solutions. Nyquist and Bode spectra depicted a more protective passive film on the alloy surface than on Ti surface. The passive film is formed by two layers: an inner barrier layer and an outer porous layer. An electric equivalent circuit with two time constants was modeled.     

Characterization of corrosion phenomena of Zr–Ti–Cu–Al–Ni metallic glass by SEM and TEM

Corrosion phenomena are investigated for a Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ metallic glass immersed in hydrofluoric acid (HF) in open-circuit conditions and by means of electron microscopies (SEM and TEM). Several morphologies develop on the corroded surface and especially large and deep pits. TEM study demonstrates that Cu-rich nanocrystals of 5–10 nm are formed inside the corrosion pits (on their walls) during the corrosion process. These nanocrystals are not only by-products of the corrosion process but they very likely play a role in the development of the corrosion pitting morphology. They could have a dual role: (i) protecting the capped areas against dissolution and (ii) speeding the dissolution of neighboring uncapped areas by the creation of local galvanic cells.     

Phase-modulation scatterometry

Phase-modulation scatterometry is a metrology technique for determining, by means of a phase modulator as a key device, the parameters of gratings. The main source of error to be dealt with are the fluctuations of the phase-modulation amplitude. The grating zeroth-order reflectance modulated by the phase modulator is converted into a signal by the photodetector. The measurables are the direct term and the first two harmonics of the signal. For experimental data fitting, we used the ratio of the harmonics over the direct term because it significantly improves the accuracy. A sensitivity analysis was performed for two samples, one real and one theoretical, to find the measurement configuration that insures optimum determination precision for the grating parameters. For the real sample, comparison of the theoretical predictions for sensitivity with the actual values showed a good agreement. For both samples the sensitivity analysis indicated subnanometric precision for the critical dimension (grating linewidth).     

Parallelism in Gene Assembly

The process of gene assembly in ciliates, an ancient group of organisms, is one of the most complex instances of DNA manipulation known in any organisms. This process is fascinating from the computational point of view, with ciliates even using the linked lists data structure. Three molecular operations (ld, hi, and dlad) have been postulated for the gene assembly process. We initiate here the study of parallelism in this process, raising several natural questions, such as: when can a number of operations be applied in parallel to a gene pattern; or how many steps are needed to assemble (in parallel) a micronuclear gene. In particular, this gives rise to a new measure of complexity for the process of gene assembly in ciliates. “One of the oldest forms of life on Earth has been revealed as a natural born computer programmer.”     

Effect of Ni substitute in off-stoichiometric Bi(Pb)-Sr-Ca-Cu(Ni)-O superconductor. Excess conductivity,XRD analysis and thermal behaviour

Two phases (2223 and 2212) are identified in Bi_1.8Pb_0.3Sr₂Ca₂(Cu_1-xNi_x)_3.3O_y superconductor system, sintered at 847?°C for 322?h, in partial nitrogen atmosphere. The volume fraction of 2223-phase is strongly dependent on Ni doping: 78.37% for x?=?0.002, 70.29% for x?=?0.005% and 51.13% for x?=?0.015. The unit cells of 2223 and 2212 phases were indexed as tetragonal structures, having different lattice constants. Plots of resistance versus temperature (four points method) on cooling to 77?K, evidenced that the critical temperature for the transition to the superconductor phase, T_c, is linearly decreasing from 106.21 to 93.47?K when the Ni content is varying from x?=?0.002 to x?=?0.03. From log-log plots of the excess-conductivity we calculated the cross-over temperatures between 3D and 2D dimensionality as well as from 2D to SWF (short wavelength fluctuation) behaviour, the coupling factor and the coherence length for all the samples. Thermal analysis of the resulting samples (after the last sintering) was performed by heating each sample from room temperature (RT) to 1000?°C at a rate of 10?K?min^?1 in dynamic air atmosphere (150?cm³ min^?1). A clear dependence on Ni content is seen by TG and DSC, but a relative thermal equilibrium between the two phases, 2223 and 2212, in RT-869?°C range, is observed. Strong endothermic effects (melting accompanied by small decomposition processes) begin at around 869?°C for all Ni doped samples. The results for the specific heat capacities, calculated from DSC plots, are also presented. Contribution of the crystal lattice to the estimated specific heat capacity was in conformity with the Einstein model, the Einstein temperature values being dependent on Ni content.     

Conductor loss in hollow waveguides using surface integralformulation

The power-loss method along with a surface integral formulation is used to compute the attenuation constant in hollow waveguides of arbitrary cross-section. An E-field integral equation is developed for the surface electric currents which is transformed into a matrix equation using the method of moments. An iterative technique, i.e. Muller's method, is used to obtain the relation between the propagation constant and frequency. The attenuation constants have been calculated and formulated for various waveguides and are in good agreement with published data