Strong equivalence of logic programs under the infinite-valued semantics

We consider the notion of strong equivalence [V. Lifschitz, D. Pearce, A. Valverde, Strongly equivalent logic programs, ACM Transactions on Computational Logic 2 (4) (2001) 526-541] of normal propositional logic programs under the infinite-valued semantics [P. Rondogiannis, W.W. Wadge, Minimum model semantics for logic programs with negation-as-failure, ACM Transactions on Computational Logic 6 (2) (2005) 441-467] (which is a purely model-theoretic semantics that is compatible with the well-founded one). We demonstrate that two such programs are strongly equivalent under the infinite-valued semantics if and only if they are logically equivalent in the corresponding infinite-valued logic. In particular, we show that strong equivalence of normal propositional logic programs is decidable, and more specifically coNP-complete. Our results have a direct implication for the well-founded semantics since, as we demonstrate, if two programs are strongly equivalent under the infinite-valued semantics, then they are also strongly equivalent under the well-founded semantics.     

Optimal Influence Strategies in Social Networks

This paper suggests a modeling framework to investigate the optimal strategy followed by a monopolistic firm aiming to manipulate the process of opinion formation in a social network. The monopolist and a set of consumers communicate to form their beliefs about the underlying product quality. Since the firm’s associated optimization problem can be analytically solved only under specific assumptions, we rely on the sequential quadratic programming computational approach to characterize the equilibrium. When consumers’ initial beliefs are uniform, the firm’s optimal influence strategy always involves targeting the most influential consumer. For the case of non-uniform initial beliefs, the monopolist might target the less influential consumer if the latter’s initial opinion is low enough. The probability of investing more in the consumer with the lower influence increases with the distance between consumers’ initial beliefs and with the degree of trust attributed on consumers by the firm. The firm’s profit is minimized when consumers’ influences become equal, implying that the firm benefits from the presence of consumers with divergent strategic locations in the network. In the absence of a binding constraint on total investment, the monopolist’s incentives to manipulate the network decrease with consumers’ initial beliefs and might either increase or decrease with the trust put in consumers’ opinion by the firm. Finally, the firm’s strategic motivation to communicate persistently high beliefs during the opinion formation process is positively associated with the market size, with the available budget and with the direct influence of the most influential consumer on the other but negatively associated with consumers’ initial valuation of the good.     

Wireless Channel State-Aware and Adaptive Epidemic Dissemination in Ad Hoc Networks

Ad hoc networks are characterized by limited resources (e.g. energy, bandwidth). Efficient information dissemination while avoiding excessive energy cost can be achieved through the suitable design of a network. To this end we propose an information dissemination scheme which couples epidemic dissemination with adaptive modulation and coding (AMC). The proposed scheme tunes the message forwarding probability and the coding and modulation mode in order to achieve a balance between maximum coverage over the network and minimum energy expenditure. We achieve this based on the evaluation of suitably defined indicators related to the lower network layers and exploiting information on the current status of the wireless medium. Building on established previous AMC-related work, our simulation results indicate that our scheme brings significant improvement over non-adaptive approaches, comparable with other adaptive epidemic dissemination schemes. Our findings are quite promising for adaptive epidemic-based information dissemination schemes with a strong cross-layer component.     

Energy research and technology development data collection strategies: The case of Greece

The European Union (EU) from the beginning of 2007 has focused its emphasis on the development of a new policy that puts energy back at the heart of EU action. Indeed, it has very often been stated that the difficulty and complexity of achieving green energy targets in the EU will require strengthened measures to promote implementation of new energy technologies (NET), as well as measures to support the related energy Research and Technology Development (R&TD). Often forgotten is the fact, that most of all, a European-wide co-ordinated forum is needed to continuously develop and sophisticate the monitoring and methodology results, bringing together specialised statisticians, energy researchers and experts on energy socio-economics. Today a nebulous picture prevails on the existence of categorized data with regards to energy Research and Technology Development (R&TD) expenditure. In this context, aim of this paper is the presentation of energy R&TD data collection strategies, as well as the related findings for the Greek energy market.     

Solvothermal synthesis,nanostructural characterization and gas cryo-adsorption studies in a metal–organic framework (IRMOF-1) material

A nanoporous metal–organic framework material, exhibiting an IRMOF-1 type crystalline structure, was prepared by following a direct solvothermal synthesis approach, using zinc nitrate and terephthalic acid as precursors and dimethylformamide as solvent, combined with supercritical CO₂ activation and vacuum outgassing procedures. A series of advanced characterization methods were employed, including scanning electron microscopy, Fourier-transform infrared radiation spectroscopy and X-ray diffraction, in order to study the morphology, surface chemistry and structure of the IRMOF-1 material directly upon its synthesis. Porosity properties, such as Brunauer–Emmet–Teller (BET) specific area (～520 m²/g) and micropore volume (～0.2 cm³/g), were calculated for the activated sample based on N₂ gas sorption data collected at 77 K. The H₂ storage performance was preliminary assessed by low-pressure (0–1 bar) H₂ gas adsorption and desorption measurements at 77 K. The activated IRMOF-1 material of this study demonstrated a fully reversible H₂ sorption behavior combined with an adequate gravimetric H₂ uptake relative to its BET specific area, thus achieving a value of ～1 wt.% under close-to-atmospheric pressure conditions.     

Thermal treatment effect on structure, electrical conductivity and transient photoconductivity behavior of thiourea modified TiO2 sol-gel thin films

Thiourea modified nanocrystalline titanium dioxide (TiO₂) thin films were prepared by sol-gel route and were thermally treated at five different temperatures (400, 500, 600, 800 and 1000 °C). The films were studied using GIXRD, PIGE and UV-vis spectroscopy. It was observed that the anatase to rutile phase transformation of TiO₂ was inhibited by the thiourea modification. The transmittance of the modified films appeared reduced which was attributed both to the modification of TiO₂ with thiourea and the light scattering in the films. The dark conductivity and the transient photoconductivity of the modified TiO₂ sol-gel thin films were studied in vacuum and in air. The environment does not influence significantly the dark conductivity, because of the almost equivalent competition between oxygen and water adsorption. The photoconductivity reaches high values for all samples in both environments, with the sample treated at 500 °C to present the highest value. The larger values in vacuum can be attributed to the reduced amount of adsorbed oxygen at the surface, which acts as electron scavenger.     

FPGA-based architecture and implementation techniques of a low-complexity hybrid RAKE receiver for a DS-UWB communication system

In this paper, the architecture of a low-complexity Direct Sequence Ultra-Wideband (DS-UWB) receiver subsystem which incorporates a Channel Estimator (CE) and a novel hybrid Partial/Selective (HPS) maximal ratio combining (MRC) RAKE receiver is presented. Three different design techniques followed by FPGA implementation are investigated and compared and system performance results are provided. The proposed architectures combine the benefits of both partial and selective RAKE receiver algorithms and the obtained results demonstrate the trade-off between energy capture, performance and receiver complexity. All design approaches focus on a highly parallel, modular and optimized for high performance system which is necessary for demanding and low-cost applications of UWB communications.     

Dynamic modelling of a two-phase thermofluidic oscillator for efficient low grade heat utilization: Effect of fluid inertia

In this paper, a promising heat engine technology capable of utilizing low grade heat is examined. Based on a two-phase thermofluidic oscillator concept, the novelty and advantage of this particular system lie in its use of phase change and its lack of reliance on inertia to sustain oscillations, though it is recognized that inertia will always be present in any physical manifestation of the engine. The system is analysed using lumped linearized one-dimensional network models, both with and without inertia, based on thermoacoustic principles and extending these to account for phase change. The gain (temperature difference between source and sink heat exchangers) and frequency at which marginal stability (desirable continuous oscillations) can be achieved is calculated. The effects of the load resistance (fluid drag) and fluid inertia, as well as of the flow resistance due the feedback valve on the marginal stability gain, frequency and exergetic efficiency of the system are investigated. It is found that an increase in feedback resistance leads to a need for a higher gain for oscillatory behaviour to be achieved. In addition, even though an increase in either the resistance or inertia in the load, or the feedback resistance at low values of these variables has almost no effect on the required gain and the oscillation frequency of the system, an increase in these variables can lead at higher values to increased gains and reduced frequencies. A reduced feedback resistance and greater load inertia can also lead to considerably higher efficiencies, while increasing the load resistance allows for an increase in efficiency until a maximum is reached, after which the efficiency decreases again. The validity of certain approximations made previously is considered, and it is shown that these must be made with care. The results from this study can be used for the improved design and optimization of such oscillators, and similar systems.     

Optimization methodology of dynamic data structures based on genetic algorithms for multimedia embedded systems

Modern multimedia application exhibit high resource utilization. In order to efficiently run this kind of applications in embedded systems, the dynamic memory subsystem needs to be optimized. A key role in this optimization is played by the dynamic data structures that reside in every real-life application. This paper presents a novel and automated way to optimize dynamic data structures. The search space is pruned using genetic algorithms that converge to the best multilayered data structure implementation for the targeted applications.