Multi-dimensional Boltzmann Sampling of Languages

This paper addresses the uniform random generation of words from a context-free language (over an alphabet of size $k$), while constraining every letter to a targeted frequency of occurrence. Our approach consists in a multidimensional extension of Boltzmann samplers \cite{Duchon2004}. We show that, under mostly \emph{strong-connectivity} hypotheses, our samplers return a word of size in $[(1-\varepsilon)n, (1+\varepsilon)n]$ and exact frequency in $\mathcal{O}(n^{1+k/2})$ expected time. Moreover, if we accept tolerance intervals of width in $\Omega(\sqrt{n})$ for the number of occurrences of each letters, our samplers perform an approximate-size generation of words in expected $\mathcal{O}(n)$ time. We illustrate these techniques on the generation of Tetris tessellations with uniform statistics in the different types of tetraminoes.Comment: 12p

Consumer Perception of Local and Organic Products: Substitution or Complementary Goods?

Many consumers are interested in local products because of the perceived benefits of freshness, stronger taste and higher quality. To consumers the origin attribute represents a strong purchasing criterion. With respect to organic produce, local food products may be perceived either as substitutes or as complementary. A qualitative approach to data collection (focus groups) and to data processing (content analysis) has been used to analyse Italian consumers’ perception with respect to local and organic food products. In the framework of the EU project QLIF (FP6-506358) a discussion guide to focus group interview was used in order to identify important purchase criteria, the willingness to pay, as well as the role of organic food products in purchasing criteria. Two animal – yogurt and eggs – and two non animal products – bread and tomatoes – were taken into account. Focus groups interviews indicate that Italian consumers place much importance on the local origin of food products, especially if fresh consumed. The origin with its implication of seasonality, territoriality and localness are among the major motivating and trust factors, however not always linked to organic food products. The lack of availability of local and organic food products together with retailing issues are taken into consideration. Differentiation throughout animal and non-animal products and between processed food products and commodities is analysed. Organic seems to suffer in global markets, localness may suggest a solution. The research provides insights on substitution and complementary marketing strategies

Distances on Rhombus Tilings

The rhombus tilings of a simply connected domain of the Euclidean plane are known to form a flip-connected space (a flip is the elementary operation on rhombus tilings which rotates 180{\deg} a hexagon made of three rhombi). Motivated by the study of a quasicrystal growth model, we are here interested in better understanding how "tight" rhombus tiling spaces are flip-connected. We introduce a lower bound (Hamming-distance) on the minimal number of flips to link two tilings (flip-distance), and we investigate whether it is sharp. The answer depends on the number n of different edge directions in the tiling: positive for n=3 (dimer tilings) or n=4 (octogonal tilings), but possibly negative for n=5 (decagonal tilings) or greater values of n. A standard proof is provided for the n=3 and n=4 cases, while the complexity of the n=5 case led to a computer-assisted proof (whose main result can however be easily checked by hand).Comment: 18 pages, 9 figures, submitted to Theoretical Computer Science (special issue of DGCI'09

Stochastic Flips on Two-letter Words

This paper introduces a simple Markov process inspired by the problem of quasicrystal growth. It acts over two-letter words by randomly performing \emph{flips}, a local transformation which exchanges two consecutive different letters. More precisely, only the flips which do not increase the number of pairs of consecutive identical letters are allowed. Fixed-points of such a process thus perfectly alternate different letters. We show that the expected number of flips to converge towards a fixed-point is bounded by $O(n^3)$ in the worst-case and by $O(n^{5/2}\ln{n})$ in the average-case, where $n$ denotes the length of the initial word.Comment: ANALCO'1

Organic farming values in Switzerland – results of a focus group study

Eight focus group discussions, conducted in 2004 and 2005, about ethical values among established and recently converted organic producers (mountain and lowland area) and other stakeholders in Switzerland are summarised. The aim was to contrib-ute to the elaboration of principles for the new EU regulation for organic food and farming as part of the EU Organic-Revision project. The participants’ values were contrasted with the four new principles of organic production of IFOAM (International Federation of Organic Agriculture Movements). Health and ecological sustainability were key issues. The producers saw secure livelihood, the maintenance of their family farm, authenticity, animal welfare and farming with nature as major issues. The sys-tem approach was important for other stakeholders. The major value conflicts were seen to arise from conventionalization and globalisation of the organic agriculture sector. Another central discussion point was the overregulation and inspection. Ethical values and principles should play a more important role in standard setting and trade