Query Optimization in Distributed Relational Databases

The query optimizer is the DBMS (data base management system) component whose task is to find an optimal execution plan for a given input query. Typically, optimization is performed using dynamic programming. However, in distributed execution environments, this approach becomes intractable, due to the increase in the search space incurred by distribution. We propose the use of the tabu search metaheuristic for distributed query optimization. A hashing-based data structure is used to keep track of the search memory, simplifying significantly the implementation of tabu search. To validate this proposal, we implemented the tabu search strategy in the scope of an existing optimizer, which runs several search strategies. We focus our attention on the more difficult problems in terms of the query execution space, in which the solution space includes bushy execution plans and Cartesian products, which are not dealt with very often in the literature. Using a real-life application, we show the effectiveness of tabu search when compared to other strategies.     

Physical characterization of commercial polyolefinic thermoplastic elastomers

In this work, a systematic study of physical characterization on a series of commercial polyolefinic thermoplastic elastomers (TPEs), is reported. Formulations from different manufacturers, having a wide range of Shore hardness values (from A45 to D51), were examined using simple, inexpensive and standard laboratory methods. From this analysis, the TPE chemical composition and its relationship with hardness and tensile set—the key parameters that define the TPE performance in most of the applications—could be established.

It was found that the strategy followed by the manufacturers to design TPEs is very similar. The EPDMs used for the different formulations look similar in ethylene content and thermal properties. Therefore, the TPE bulk modulus (or hardness) is mainly controlled by the PP content. Nice elastomeric behavior was observed only in grades with a dominant proportion of EPDM, in agreement with the deformation mechanism generally accepted for this type of materials. Grades with higher hardness values—and a dominant proportion of PP—showed a mechanical response corresponding to a toughened thermoplastic, even when these grades are marketed by the producers as “thermoplastic elastomers”. Differently from conventional crosslinked elastomers, where hardness and ability to recover from highly deformed states can be simultaneously controlled by changing the degree of crosslinking, the results of this work indicate that it is very difficult to increase TPE hardness without sacrificing elastomeric properties.     

A Complementarity-based Partitioning and Disjunctive Cut Algorithm for Mathematical Programming Problems with Equilibrium Constraints

In this paper a branch-and-bound algorithm is proposed for finding a global minimum to a Mathematical Programming Problem with Complementarity (or Equilibrium) Constraints (MPECs), which incorporates disjunctive cuts for computing lower bounds and employs a Complementarity Active-Set Algorithm for computing upper bounds. Computational results for solving MPECs associated with Bilivel Problems, NP-hard Linear Complementarity Problems, and Hinge Fitting Problems are presented to highlight the efficacy of the procedure in determining a global minimum for different classes of MPECs.     

Maximin location: Discretization not always works

In this note we show by means of a simple example that, if the maximin problem with (nonlinear) concave increasing utility functions is solved by inspecting the extreme points of the (generalized) Voronoi diagram (as usually proposed), one may have to inspect an infinite number of candidate points. The research of the second and third authors is partially supported by Grant PB96-1416-C02-02 of Ministerio de Educación y Cultura, Spain     

Solid-phase UV spectrophotometric method for determination of ciprofloxacin

A simple and inexpensive method for the determination of ciprofloxacin has been developed using solid-phase spectrophotometry. The intrinsic absorbance of ciprofloxacin fixed on a dextran-type cation-exchange resin, Sephadex SP C-25, was measured directly at 277 and 380 nm after packing the gel beads in a 1-mm cell. Using a sample volume of 10 ml, the calibration graph was linear over the range 0.05-0.3 μg ml⁻¹ with a R.S.D. of 1.11% (n=8). The sensitivity obtained is 40 times higher than that of the corresponding solution method. The method was applied to the determination of ciprofloxacin in pharmaceutical preparations and was validated by standard addition.     

Spherical-type hypersurfaces in a Riemannian manifold

We extend some rigidity results of Aleksandrov and Ros on compact hypersurfaces inR ⁿ to more general ambient spaces with the aid of the notion of almost conformal vector fields. These latter, at least locally, always exist and allow us to find interesting integral formulas fitting our purposes.     

Study of Hybrid Silica-Polyethyleneglycol Xerogels by Eu3+ Luminescence Spectroscopy

Good optical quality Eu3+-doped silica-polyethyleneglycol hybrids were prepared by the sol-gel process. Thermomechanical analysis showed an increase of the glass transition temperature, due to the stiffness of the polymeric network, as the amount of Eu3+ increased. Europium luminescent properties were used to study structural evolution during the sol-gel transition. For lower doping concentrations dried gels present statistical distributions of Eu3+, typical of an amorphous environment, while for higher concentrations a crystalline-like environment of Eu3+ was observed. A broad emission band was observed in the visible part of the electromagnetic spectrum and assigned to the intrinsic emission from the hybrid polymeric network.     

Mechanisms of direct and TiO2-photocatalysed UV degradation of phenylurea herbicides.

Phenylurea herbicides undergo low-yield (phi(PI) <15 %) monophotonic photoionisation upon 193-nm laser flash excitation. The so-formed radical cations (phenylurea.+) are highly acidic (-1.5 < pKa <0.5) and deprotonate readily to yield the corresponding neutral radical (phenylurea.). Pulse radiolysis experiments allowed limitation of the reduction potential of phenylurea.+ within 2.22 V versus the normal hydrogen electrode (NHE) < E degrees (phenylurea.+/phenylurea) < 2.43 V versus NHE. The main photoproducts of UVC (lambda=193 nm) photodegradation of phenylureas correspond to a photo-Fries rearrangement. One-electron reduction with e-(aq) yields the corresponding radical anions (phenylurea.-), for which 4.3< pKa < 5.33. The rate constants for reaction with e-(aq) show that in photocatalysis the generation of phenylurea.- and O2.- on the surface of the photocatalyst may be competitive. High reactivity toward e-(aq) is predicted from linear free-energy relationships (LFER) for phenylureas bearing electron-withdrawing groups. Reaction with HO. takes place mainly via addition to the aromatic ring and/or H. abstraction from a saturated carbon atom (98 %), rather than one-electron oxidation (2 %). High reactivity toward oxidation by HO. is predicted from LFER for phenylureas bearing electron-donating groups. Adsorption studies for TiO2 in its polymorphic forms of rutile and anatase, as well as with the commercial mixture Degussa P-25, show photocatalysis is independent of the specific area of the catalyst. A variety of compounds are generated during the photocatalytic degradation of Diuron, while only two hydroxychloro derivatives are observed upon prolonged direct 365 nm irradiation. The photocatalytic degradation proceeds mainly by oxidation of the Me group of the side chain, hydroxylation of the aromatic ring, and dechlorination. The photoproducts of photocatalytic degradation differ from one polymorphic form of TiO2 to another.