Increasing the Weight of Minimum Spanning Trees

The problems of computing the maximum increase in the weight of the minimum spanning trees of a graph caused by the removal of a given number of edges, or by finite increases in the weights of the edges, are investigated. For the case of edge removals, the problem is shown to be NP-hard and an Ω(1/log k)-approximation algorithm is presented for it, where (input parameter) k > 1 is the number of edges to be removed. The second problem is studied, assuming that the increase in the weight of an edge has an associated cost proportional to the magnitude of the change. An O(n³m² log(n²/m)) time algorithm is presented to solve it.     

Improved Dynamic Text Indexing

In the dynamic text indexing problem, a text string has to be maintained under string insertions and deletions in order to answer on-line queries about arbitrary pattern occurrences. By means of some new techniques and data structures, we achieve improved worst-case bounds. We show that finding allpoccoccurrences of a pattern of lengthpin the current text of lengthntakesO(p + pocc + updlog p + log n) time, whereupdis the number of text uptakes performed so far; inserting or deleting a string of lengthsfrom the current text takesO(s log(s + n)) time.     

Aromatic behavior of three membered rings

The aromatic behavior of three membered ring compounds has been analyzed by mean of GAUSSIAN94 theoretical calculations at B3LYP//6-311++G(2d,p) level. Diamagnetic exaltation values obtained from magnetic susceptibilities as well as NICS show the expected aromatic character of the cyclopropyl cation, but the correspondent anion also shows aromatic behavior. This unexpected result is analyzed as well as the character of the other three membered ring derivatives.     

Billiards in Tubular Neighborhoods of Manifolds¶of Codimension 1

I prove that in (sufficiently small) tubular ρ$-neighborhoods of a given C ³ manifold of codimension 1, any two points can be connected by a billiard trajectory, and that in addition there exists such a trajectory having at most collision points, for a suitable H>0, provided the manifold is of class C ³. Received: 19 June 1998 / Accepted: 7 April 1999     

Stability of silver clusters in mordenites with different SiO2/Al2O3 molar ratio

The stability and decay of silver clusters characterized by absorption bands 320 and 285 nm incorporated in mordenites with different SiO₂/Al₂O₃ molar ratios were studied under ambient conditions. Significantly different rates of disappearance of these two bands were the basis for assigning them to different silver species. Oxidation converts the clusters peaking at 320 and 285 nm into other silver clusters characterized by absorption bands at 310 and 240 nm, respectively. The oxidation of silver clusters peaking at 285 nm is significantly faster than that at 320 nm. The stability of both original clusters decreases with acid site strength that, in its turn, depends on Si0₂/Al₂O₃ molar ratio. The minimum lifetime of clusters peaking at 320 and 285 nm equal to ca. 40 and 20 days, respectively, was observed for mordenite characterized by the strongest acid sites. For mordenites possessing weaker acid sites, the lifetime of these clusters is found to be more than 50 months. The oxidation of silver clusters is reversible, and re-reduced silver clusters were revealed to possess the same electron structure as original ones.     

Polyurethane Based Materials for the Production of Biomedical Materials

Polyurethanes (PUs) composed by hard and soft segments have been extensively used in the manufacturing of biocompatible prosthesis and medical devices. A broad variety of PUs can be obtained by modifying the balance between both segments. In the present work, different basically-flexible PUs have been prepared by employing different combinations of aliphatic hexamethylene diisocyanate, poly(ethylene glycol) (Mw 400 Da), poly(ϵ-caprolactone) diol (Mw 530 Da), and 1,4-butanediol. Thermal analysis of the synthesized PUs demonstrated high thermal stability and the assumption of glassy state well below room temperature, in agreement with their marked flexibility. Morphological characterization of PUs films indicated that films prepared by spin coating were smoother and more homogeneous than those obtained by casting. Biological assays performed by using 3T3/BALB-C mouse embryo fibroblast cell line confirmed the absence of toxicity and hence the biocompatibility of PU-films.     

Effective In Vitro Control of Two Phytopathogens of Agricultural Interest Using Cell-Free Extracts of Pseudomonas fluorescens and Chitosan

A biofungicide is a natural product that can be derived from various sources such as, among others, microorganisms, higher plants, animal products, phytochemicals, semiochemicals, and antagonist microorganisms. One of the most important approaches for the production of biofungicides is the combination of biocontrol agents. This study showed the inhibition growth of Alternaria alternata and Fusarium solani treated with cell-free extracts of P. fluorescens. Using thin-layer chromatography and plate assays it was also demonstrated that the cell-free extracts of P. fluorescens contained siderophores and derivates of 4-diacetylphloroglucinol and phenazine. Moreover, the combination of cell-free extracts of P. fluorescens and chitosan [50–1.5% (v/v)] had a synergistic effect since they notably inhibited the mycelial growth of A. altenata and F. solani. Various morphological alterations to the mycelia and conidia of the treated fungi as a result of this combination were also observed. The present study could be a starting point to control other fungal phytopathogens using different cell-free extracts and chitosan as biocontrol agents.     

On the kinetics of styrene emulsion polymerization above CMC. I. A mathematical model

A detailed mathematical model of the kinetics of styrene emulsion polymerization has been proposed. Its main features/assumptions are compartmentalization, micellar and homogeneous nucleation, particle formation by both initiator‐derived and desorbed radicals, dependence on the particle size of the rate coefficients, thermodynamic considerations, and aqueous phase kinetics. The model predicts that micellar nucleation dominates over homogeneous nucleation and that the evolution of the nucleation rate reaches a maximum, where desorbed radicals have an important contribution. Initiator‐derived radicals with only one monomeric unit have also a significant contribution on the rate of capture in particles. The results suggest that the correctness of the instantaneous termination approach depends not only on the size of the particle, but also on the type of entering radical (initiator‐derived or monomeric). © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2201–2218, 2000     

Theoretical study on Pd dimer and trimer interaction with the hydrogen molecule

The H₂ interaction with the Pd dimer and trimer were studied using multiconfigurational self-consistent field (MC-SCF) calculations with the relativistic effective core potential (RECP); the correlation energy correction was included in the extended multireference configuration interaction (MRCI), variational and perturbative to second order. Here, we considered the Pd₂ first six states: ³Σ⁺_u, ¹Σ⁺_g, ³Π_g, ³Δ_xy, ¹Σ⁺_u, and ³Σ⁺_g. For them, the four geometrical approaches included were the side-on H₂ toward Pd₂, for the hydrogen molecule in and out the Pd dimer plane; the perpendicular end-on H₂ toward Pd₂; and the perpendicular end-on Pd₂ to H₂. The Pd₂ ground state is ³Σ⁺_u, which only captures H₂ in the C_2v end-on approach, softly relaxing the H(SINGLE BOND)H bond. The closed-shell ¹Σ⁺_g captures the H₂ molecule in all the approaches considered: The side-on approach of this state presents deep wells and relaxes the H(SINGLE BOND)H bond, and the end-on approach captures H₂ with a relatively longer H(SINGLE BOND)H distance and also a deep well. The ³Π_g state was the only one which did not capture H₂. For the triangular Pd₃ clusters, H₂ was approached in the C_2v symmetry in and out of the Pd₃ plane. In the triangular case, H₂ was absorbed in both spin states, with deep wells and relaxing the H(SINGLE BOND)H distance. The linear Pd₃ singlet and triplet states capture outside of the Pd₃ and break the H(SINGLE BOND)H bond. © 1997 John Wiley & Sons, Inc.