Faster Parameterized Algorithms for <Emphasis Type="SmallCaps">Minimum Fill-in</Emphasis>

We present two parameterized algorithms for the Minimum Fill-in problem, also known as Chordal Completion: given an arbitrary graph G and integer k, can we add at most k edges to G to obtain a chordal graph? Our first algorithm has running time \(\mathcal {O}(k^{2}nm+3.0793^{k})\), and requires polynomial space. This improves the base of the exponential part of the best known parameterized algorithm time for this problem so far. We are able to improve this running time even further, at the cost of more space. Our second algorithm has running time \(\mathcal {O}(k^{2}nm+2.35965^{k})\) and requires \(\mathcal {O}^{\ast}(1.7549^{k})\) space. To achieve these results, we present a new lemma describing the edges that can safely be added to achieve a chordal completion with the minimum number of edges, regardless of k.     

The subspace Gaussian mixture model—A structured model for speech recognition

We describe a new approach to speech recognition, in which all Hidden Markov Model (HMM) states share the same Gaussian Mixture Model (GMM) structure with the same number of Gaussians in each state. The model is defined by vectors associated with each state with a dimension of, say, 50, together with a global mapping from this vector space to the space of parameters of the GMM. This model appears to give better results than a conventional model, and the extra structure offers many new opportunities for modeling innovations while maintaining compatibility with most standard techniques.     

A survey on location estimation techniques for events detected in Twitter

Detection of events using voluntarily generated content in microblogs has been the objective of numerous recent studies. One essential challenge tackled in these studies is estimating the locations of events. In this paper, we review the state-of-the-art location estimation techniques used in the localization of events detected in microblogs, particularly in Twitter, which is one of the most popular microblogging platforms worldwide. We analyze these techniques with respect to the targeted event type, granularity of estimated locations, location-related features selected as sources of spatial evidence, and the method used to make aggregate decisions based on the extracted evidence. We discuss the strengths and advantages of alternative solutions to various problems related to location estimation, as well as their preconditions and limitations. We examine the most widely used evaluation methods to analyze the accuracy of estimations and present the results reported in the literature. We also discuss our findings and highlight important research challenges that may need further attention.     

The Effect of Different Polishing Sequences on the Adhesion of Streptococcus mutans to Feldspathic Porcelain

The aim of this research was to determine whether extra-oral surface treatments on feldspathic porcelain surfaces influence initial adhesion of Streptococcus mutans. Ninety-six porcelain specimen discs were fabricated and divided into six equal groups according to surface treatment: fine-grit diamond polishing (Group 1); self-glazing (Group 2); overglazing (Group 3); overglazing followed by a finishing procedure and then overglazing (Group 4); Pearl Surface polishing (Group 5); and Diamond Twist SCL^TM polishing (Group 6). Surface roughness and hydrophobicity were assessed. An S. mutans suspension was incubated on each specimen group and evaluated. A one-way analysis of variance, post-hoc Tukey honestly significantly different test, Friedman test, and t-test were used for statistical analysis. Group 1 showed the highest surface roughness (p < 0.001) and bacterial adhesion (p < 0.05). Groups 5 and 6 specimen surfaces presented significantly higher contact angles (p < 0.05). Group 1 had the highest S. mutans adhesion, followed by Groups 3, 5, 6, 2, and 4 (p < 0.05). Reglazing after grinding may therefore decrease bacterial adhesion beneficially.     

Thermoelectric power generation using doped MWCNTs

In this paper, the performance of thermoelectric (TE) devices constructed with the TE elements packed with randomly oriented multiwalled carbon nanotubes (MWCNTs) has been presented. B-doped and N-doped MWCNTs were produced using different doping methods and their thermal and thermoelectric properties have been reported. The carrier type and carrier densities of the doped nanotubes were determined using Mott-Schottky analysis. Doping of the MWCNTs resulted in a simultaneous increase in the Seebeck coefficient and a decrease in the thermal conductivity leading to a significant improvement (up to 1900%) of the figure of merit (ZT) of the TE device. The effects of changing packing density, diameter, and length of the MWCNT-TE elements on the performance of the device were investigated. It was observed that the power generated by the TE device depended directly on the ZT of the device. A TE module was constructed by connecting several TE cells in series with alternating p-type and n-type MWCNT-TE elements and the scale-up of power generation as the number of TE cells was increased was examined. This study is expected to help researchers to evaluate and improve the performance of CNT-based TE devices over conventional devices.     

Synthesis and characterization of thiophene functionalized polystyrene copolymers and their electrochemical properties

Thiophene functionalized polystyrene samples (TFPS) were synthesized by atom transfer radical polymerization (ATRP) of styrene, followed by Suzuki coupling with 3‐thiophene (Th) boronic acid. Conducting graft polymer of TFPS with thiophene was achieved at 1.5 V in tetrabutylammonium tetrafluoroborate/dichloromethane (TBAFB/DM) by electrochemical methods. Spectroelectrochemical analysis of the resulting copolymers [P(TFPS‐co‐Th)] reflected electronic transitions at 449, 721 and 880 nm, revealing π ? π* transition, polaron and bipolaron band formation, respectively. We also successfully established the utilization of dual type complementary colored polymer electrochromic devices using P(TFPS‐co‐Th)/poly(3,4‐ethylenedioxythiophene (PEDOT) in sandwich configuration. The switching ability, stability and optical memory of the electrochromic device were investigated by UV–visible spectrophotometry and cyclic voltammetry. Device switches between brown and blue color with a switching time of 1.3 s were prepared with optical contrast (%ΔT) of 25 %. Copyright © 2005 Society of Chemical Industry     

Synthesis of Open Zeolite Structures from Mixtures of Tetramethylammonium and Benzylmethylalkylammonium Cations: A Step Towards Driving Aluminium Location in the Framework

Three bulky organic cations, benzylmethylpyrrolidinium (BMP), (S,S)-2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium (BML) and benzylmethylhexamethyleniminium (BMH), were used in combination with the small cation tetramethylammonium (TMA) in the synthesis of zeolite materials. Two of them are distinguished by the number of carbon atoms forming the cyclic amine, which varies from 4 in BMP to 6 in BMH. In addition, a –CH₂–OH group has been introduced in the carbon atom belonging to the amine ring next to the nitrogen atom, to originate a chiral molecule, BML, the third SDA used in this work. It has been found that the combination of the small TMA with those three bulkier SDAs, promotes the crystallization of zeolite structures that contain cages and channels. BMP leads to the crystallization of the 10-ring zeolite ferrierite, where TMA is located inside the ferrierite cage. If the size of the N-cyclic amine moiety increases, the resulting cation cannot be accommodated in the relatively narrow 10-ring channels of ferrierite, and in these cases the zeolite MCM-22 crystallizes. This zeolite contains large cages whose cross section is delimited by a 12-ring, where both BMH and BML can be easily accommodated. In this system, cage-like, chlatrasil type structures often co-crystallize with the open structures, evidencing that an appropriate balance between TMA and the bulky SDA is required to achieve the crystallization of open-frameworks. When BMP is used, mixtures of structurally related ferrierite and cage-like RUB-10 are obtained if an excess of TMA is present in the synthesis gel, while MCM-22 crystallizes together with the structurally related cage-like MTN phase.     

COMPARISON OF MONTE CARLO TECHNIQUES TO PREDICT THE PROPAGATION OF A COLLIMATED BEAM IN PARTICIPATING MEDIA

Monte Carlo (MC) techniques are the most versatile approaches in solving the integrodifferential radiative transfer equation (RTE). They are based on numerical simulation of propagation of radiant energy within absorbing, emitting, and scattering media [1-4]. MC simulations can easily be applied to multidimensional, nonhomogenous, highly forward-scattering media with time-dependent boundary conditions, where other techniques are almost impossible to implement. On the down side, statistical errors associated with these techniques can be significant if the number of photons accounted for in the simulation is not sufficiently large, yet the computational penalty increases considerably with increasing number of photons. Here, we consider three different MC approaches in solving the RTE in a planeparallel, absorbing and isotropically scattering medium subjected to a collimated light source. The collimated light source is assumed to be an impulse function impinged instantaneously on the upper boundary of the medium. The strength of the light source is not important in the simulations, since all the computed quantities will be normalized accordingly. Time-dependent as well as steady-state cases are considered. Three FORTRAN codes were developed to predict radiative reflectance and transmittance. They are compared in terms of speed of convergence and statistical accuracy.     

Life study of a PBI-PEM fuel cell by current distribution measurement

PBI-based polymer electrolyte membrane (PEM) fuel cells have short durability comparing with Nafion ones. It is known that the presence of phosphoric acid accelerates catalyst sintering, and the polymer is susceptible to getting degraded via oxidative degradation. However, the influence of the current distribution on the degradation process has not been investigated. In this work, current distribution was measured continuously during a steady state life test that was performed in a 50-cm² PBI-PEM fuel cell. The membrane-electrode assembly was prepared using techniques that are supposed to ensure satisfactory reproducibility at global level. Two different stages, i.e., activation and degradation, were identified. They show disparities in both global and local performance. It was found that the current distribution map is very heterogeneous, with variations around ±50 % with respect to the average value during the whole lifetime of the cell. The non-uniformity of the current profile increases during the degradation, but it cannot be concluded that the current density distribution contributes to the short lifetime.