Symbolic Algorithms for Language Equivalence and Kleene Algebra with Tests

We first propose algorithms for checking language equivalence of finite automata over a large alphabet. We use symbolic automata, where the transition function is compactly represented using a (multi-terminal) binary decision diagrams (BDD). The key idea consists in computing a bisimulation by exploring reachable pairs symbolically, so as to avoid redundancies. This idea can be combined with already existing optimisations, and we show in particular a nice integration with the disjoint sets forest data-structure from Hopcroft and Karp's standard algorithm. Then we consider Kleene algebra with tests (KAT), an algebraic theory that can be used for verification in various domains ranging from compiler optimisation to network programming analysis. This theory is decidable by reduction to language equivalence of automata on guarded strings, a particular kind of automata that have exponentially large alphabets. We propose several methods allowing to construct symbolic automata out of KAT expressions, based either on Brzozowski's derivatives or standard automata constructions. All in all, this results in efficient algorithms for deciding equivalence of KAT expressions

Light propagation control by finite-size effects in photonic crystals

We exhibit the strong influence on light propagation of the finite size in photonic band-gap material. We show that light emission can be controlled by the symmetry group of the boundary of the finite device. These results lead simply to important practical applications.Comment: 5 pages, 4 figures, Revte

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Internal convection in thermoelectric generator models

Coupling between heat and electrical currents is at the heart of thermoelectric processes. From a thermal viewpoint this may be seen as an additional thermal flux linked to the appearance of electrical current in a given thermoelectric system. Since this additional flux is associated to the global displacement of charge carriers in the system, it can be qualified as convective in opposition to the conductive part associated with both phonons transport and heat transport by electrons under open circuit condition, as, e.g., in the Wiedemann-Franz relation. In this article we demonstrate that considering the convective part of the thermal flux allows both new insight into the thermoelectric energy conversion and the derivation of the maximum power condition for generators with realistic thermal coupling.Comment: 8 pages, 3 figure

Nominal Logic Programming

Nominal logic is an extension of first-order logic which provides a simple foundation for formalizing and reasoning about abstract syntax modulo consistent renaming of bound names (that is, alpha-equivalence). This article investigates logic programming based on nominal logic. We describe some typical nominal logic programs, and develop the model-theoretic, proof-theoretic, and operational semantics of such programs. Besides being of interest for ensuring the correct behavior of implementations, these results provide a rigorous foundation for techniques for analysis and reasoning about nominal logic programs, as we illustrate via examples.Comment: 46 pages; 19 page appendix; 13 figures. Revised journal submission as of July 23, 200

Efficient light coupling into a photonic crystal waveguide with flatband slow mode

We design an efficient coupler to transmit light from a strip waveguide into the flatband slow mode of a photonic crystal waveguide with ring-shaped holes. The coupler is a section of a photonic crystal waveguide with a higher group velocity, obtained by different ring dimensions. We demonstrate coupling efficiency in excess of 95% over the 8 nm wavelength range where the photonic crystal waveguide exhibits a quasi constant group velocity vg = c/37. An analysis based on the small Fabry-P\'erot resonances in the simulated transmission spectra is introduced and used for studying the effect of the coupler length and for evaluating the coupling efficiency in different parts of the coupler. The mode conversion efficiency within the coupler is more than 99.7% over the wavelength range of interest. The parasitic reflectance in the coupler, which depends on the propagation constant mismatch between the slow mode and the coupler mode, is lower than 0.6% within this wavelength range.Comment: 11 pages, 7 figures, submitted to Photonics and Nanostructures - Fundamentals and Application

Constructing Independently Verifiable Privacy-Compliant Type Systems for Message Passing between Black-Box Components

Privacy by design (PbD) is the principle that privacy should be considered at every stage of the software engineering process. It is increasingly both viewed as best practice and required by law. It is therefore desirable to have formal methods that provide guarantees that certain privacy-relevant properties hold. We propose an approach that can be used to design a privacy-compliant architecture without needing to know the source code or internal structure of any individual component. We model an architecture as a set of agents or components that pass messages to each other. We present in this paper algorithms that take as input an architecture and a set of privacy constraints, and output an extension of the original architecture that satisfies the privacy constraints

Disentangling the processes driving plant assemblages in mountain grasslands across spatial scales and environmental gradients

1. Habitat filtering and limiting similarity are well-documented ecological assembly processes that hierarchically filter species across spatial scales, from a regional pool to local assemblages. However, information on the effects of fine-scale spatial partitioning of species, working as an additional mechanism of coexistence, on community patterns, is much scarcer. 2. In this study, we quantified the importance of fine-scale spatial partitioning, relative to habitat filtering and limiting similarity, in structuring grassland communities in the western Swiss Alps. To do so, 298 vegetation plots (2 m × 2 m ) each with five nested subplots (20 cm × 20 cm) were used for trait based assembly tests (i.e. comparisons with several alternative null expectations), examining the observed plot and subplot level α-diversity (indicating habitat filtering and limiting similarity) and the between-subplot β-diversity of traits (indicating fine-scale spatial partitioning). We further assessed variations in the detected signatures of these assembly processes along a set of environmental gradients. 3. We found habitat filtering to be the dominating assembly process at the plot level with diminished effect at the subplot level, while limiting similarity prevailed at the subplot level with weaker average effect at the plot level. Plot-level limiting similarity was positively correlated with fine-scale partitioning suggesting that the trait divergence may result from a combination of competitive exclusion between functionally similar species and environmental micro-heterogeneities. Overall, signatures of assembly processes only marginally changed along environmental gradients but the observed trends were more prominent at the plot than at the subplot scale. Synthesis: Our study emphasises the importance of considering multiple assembly processes and traits simultaneously across spatial scales and environmental gradients to understand the complex drivers of plant community composition

The use of indigenous knowledge in development: problems and challenges

The use of indigenous knowledge has been seen by many as an alternative way of promoting development in poor rural communities in many parts of the world. By reviewing much of the recent work on indigenous knowledge, the paper suggests that a number of problems and tensions has resulted in indigenous knowledge not being as useful as hoped for or supposed. These include problems emanating from a focus on the (arte)factual; binary tensions between western science and indigenous knowledge systems; the problem of differentiation and power relations; the romanticization of indigenous knowledge; and the all too frequent decontextualization of indigenous knowledge