On aggregation operators of transitive similarity and dissimilarity relations

Similarity and dissimilarity are widely used concepts. One of the most studied matters is their combination or aggregation. However, transitivity property is often ignored when aggregating despite being a highly important property, studied by many authors but from different points of view. We collect here some results in preserving transitivity when aggregating, intending to clarify the relationship between aggregation and transitivity and making it useful to design aggregation operators that keep transitivity property. Some examples of the utility of the results are also shown.Peer ReviewedPostprint (published version

A survey of recent estimates of price elasticities of demand for transport

This paper reviews 70 estimates of the price elasticity of demand for many different transport modes and market situations. The paper presents figures separately for passenger and freight transport and include estimates of both own-price and mode choice elasticities. It also presents some elasticity estimates on demand for gasoline, together with selected cross-price elasticities. In addition, it includes a brief exposition on the different concepts of elasticity - compensated, uncompensated, price, cross-price and mode choice - and discusses the relations between them. This paper shows that, since transportation is a derived demand, it tends to be inelastic. Although the review is confined to estimates of price elasticities, it notes that quality variables are often more important than price, particularly in the air, motor freight, and container markets. Finally, most of the estimates relate to developed countries, reflecting the availabilty of data, research resources, and domicile of the researchers. The elasticity estimates are nevertheless thought to be relevant to developing countries as well. But since intermodal competition is generally less intense in developing countries, this tends to make transport demand more inelastic, although the lower income levels in such countries may partly offset this effect.Environmental Economics&Policies,Economic Theory&Research,Access to Markets,Markets and Market Access,Consumption

Macrophyte - mollusc relationship in Lake Kariba

Five species of submerged vegetation Lagarosiphon ilicifolius, Najas pectinata, Vallisneria aethiopica, Ceratophyllum demersum and Potamogeton octandrus; 7 species of gastropods Melanoides tuberculata, Bellamya capillata, Biomphalaria pfeifferi, Bullinus tropicus, Cleopatra sp, and Lymnaea natalensis and 4 species of bivalves Corbicula africana, Caelatura mossambicensis, Mutela dubia and Aspatharia wahlbergii are correlated with environmental variables particularly slope and transparency, in Lake Kariba. A stepwise regression analysis further revealed interdependence between (Cleopatra sp., B. pfeifferi, L. natalensis, B. capillata, and V. aethiopica as well as between as between C. mossambicensis and L. ilicifolius and N. pectinata. The dependence of B. pfeifferi, L. natalensis, B. capillata, Cleopatra sp. on V. aethiopica and C. mossambicensis on L. ilicifolius and N. pectinata implies that a change in the biomass of the vegetation species may affect distribution and biomass of the faunal species

Factoring nonnegative matrices with linear programs

This paper describes a new approach, based on linear programming, for computing nonnegative matrix factorizations (NMFs). The key idea is a data-driven model for the factorization where the most salient features in the data are used to express the remaining features. More precisely, given a data matrix X, the algorithm identifies a matrix C such that X approximately equals CX and some linear constraints. The constraints are chosen to ensure that the matrix C selects features; these features can then be used to find a low-rank NMF of X. A theoretical analysis demonstrates that this approach has guarantees similar to those of the recent NMF algorithm of Arora et al. (2012). In contrast with this earlier work, the proposed method extends to more general noise models and leads to efficient, scalable algorithms. Experiments with synthetic and real datasets provide evidence that the new approach is also superior in practice. An optimized C++ implementation can factor a multigigabyte matrix in a matter of minutes.Comment: 17 pages, 10 figures. Modified theorem statement for robust recovery conditions. Revised proof techniques to make arguments more elementary. Results on robustness when rows are duplicated have been superseded by arxiv.org/1211.668

DRAWING THE PROFILE OF EFFICIENT FOOD INDUSTRIES-VERTICAL INTEGRATION, ECONOMIES OF SCALE, AND LOCATION ADVANTAGES IN THE DISTRIBUTION OF PRODUCTS: A CASE STUDY FROM THE GREEK FOOD INDUSTRY

A stochastic frontier production function that incorporates a model for technical inefficiency effects is used to investigate the industrial production of Greek food industries. Panel data comes from 29 Greek firms in 1988 through 1992. Parameters considered in the model for inefficiency effects include the degree of vertical integration, capital intensity, location, and time. A translog stochastic frontier function is estimated simultaneously with those variables in the model for inefficiency effects. The results indicate that technical efficiency among the firms ranges from 42 percent to 99 percent. More efficient firms are those with a higher degree of vertical integration that are located in rural areas and have sufficient investment in human capital to exploit the economies of scale obtained through investment in fixed capital. Most firms improve their performance over time, reducing the efficiency gap.Agribusiness,