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191.
Let G be a finite simple graph. Let S⊆V(G), its closed interval I[S] is the set of all vertices lying on shortest paths between any pair of vertices of S. The set S is convex if I[S]=S. In this work we define the concept of a convex partition of graphs. If there exists a partition of V(G) into p convex sets we say that G is p-convex. We prove that it is NP-complete to decide whether a graph G is p-convex for a fixed integer p≥2. We show that every connected chordal graph is p-convex, for 1≤p≤n. We also establish conditions on n and k to decide if the k-th power of a cycle Cn is p-convex. Finally, we develop a linear-time algorithm to decide if a cograph is p-convex. 相似文献
192.
We consider the sandwich problem, a generalization of the recognition problem introduced by Golumbic et al. (1995) [15], with respect to classes of graphs defined by excluding induced subgraphs. We prove that the sandwich problem corresponding to excluding a chordless cycle of fixed length k is NP-complete. We prove that the sandwich problem corresponding to excluding Kr?e for fixed r is polynomial. We prove that the sandwich problem corresponding to 3PC(⋅,⋅)-free graphs is NP-complete. These complexity results are related to the classification of a long-standing open problem: the sandwich problem corresponding to perfect graphs. 相似文献
193.
F. L. Macedo R. A. Candeia L. L. M. Sales M. B. Dantas A. G. Souza M. M. Conceição 《Journal of Thermal Analysis and Calorimetry》2011,106(2):531-534
Searching for other alternative sources, which are not part of the food chain, and which are able to supply the biofuel market
is a promising option. In this context, it has been searched to investigate the oiticica oil, approaching its availability
to the biodiesel synthesis, as well as its thermal stability. Few works retreat parameters such as: the optimization of the
biodiesel synthesis, its physical–chemical properties, and thermal parameters etc. The characterization results revealed
that the oil showed very high kinematic viscosity, and acidity value around 13 mg KOH/g, requiring a pre-treatment. To reduce
the acid in the oil, it has been done the esterification of oil, which was studied in different molar ratios oiticica oil/ethanol
(1:9) and 2.0% catalyst, in order to get the best reduction the index of acidity. The lowest level of acidity of the oil obtained
after the esterification was 4.4 mg KOH/g. The reaction rate for the synthesis of biodiesel, compared to the initial mass
of oiticica oil ester was 85%. This income can be overcome by pursuing an even smaller reduction of acid value of biodiesel
oiticica. The acid value of biodiesel was 1.8 mg KOH/g. The results have revealed that the oiticica oil and biodiesel are
stable at 224 and 179 °C, respectively. 相似文献
194.
N. A. Santos R. Rosenhaim M. B. Dantas T. C. Bicudo E. H. S. Cavalcanti A. K. Barro I. M. G. Santos A. G. Souza 《Journal of Thermal Analysis and Calorimetry》2011,103(2):501-506
The synthesis of pigments from the system Ce1−x
O2–M
x
O (M = Cu, Co) was achieved via a polymeric precursors method, Pechini method. Differential scanning calorimetry (DSC) and
thermogravimetry (TG) techniques were used to accurately characterize the distinct thermal events occurring during synthesis.
The TG and DSC results revealed a series of decomposition temperatures due to different exothermal events, which were identified
as H2O elimination, organic compounds degradation, and phase formation. X-Ray diffraction patterns show the presence of pure cubic
CeO2 phase for the samples with low Cu and Co loading. A decrease of the specific surface area with increasing copper and cobalt
content was observed. The UV–visible diffuse reflectance technique was employed to study the optical properties in the 200–800 nm
range. Colorimetric coordinates L*, a*, b* were calculated for the pigment powders. The powders presented a variety of colors from yellow for pure CeO2, to brown for the ones loaded with copper and gray for the ones with cobalt. 相似文献
195.
196.
Valdeir A. Silva José W. Silva Paulo C. de Morais Noelio O. Dantas 《Brazilian Journal of Physics》2018,48(2):119-125
Bayesian inference was used as a new approach to calculate of rare earth (RE) ion spectroscopic parameters within the Judd?Ofelt theory using the Li2O-B2O3-Al2O3 glass system doped with Nd2O3 and TiO2. This system was synthesized by the fusion method, and the physical properties of the as-synthesized material were investigated. Optical absorption, photoluminescence, micro-Raman, mass density, refractive index, and radiative lifetime calcuations were performed. We investigated the effects of crystal field changes on Nd3+-ions caused due to co-doping with increasing TiO2 content. We observed that co-doping with TiO2 altered the radiative transition rates A(J,J′), favored symmetry enhancement around the Nd3+-ions, and promoted the onset of vibrational modes, contributed to the attenuation of O-H bonds, and substantially increased the spectroscopic quality, χ. 相似文献
197.
N.O. Dantas L. Damigo Fanyao Qu R.S. Silva P.P.C. Sartoratto K.L. Miranda E.C. Vilela F. Pelegrini P.C. Morais 《Journal of Non》2008,354(42-44):4727-4729
Chemical precipitation of metal-ions from aqueous solution has been successfully used to produce Zn1?xMnxO nanocrystals, in the form of nano-powder. X-ray diffraction (XRD) measurements reveal that the as-prepared samples are single-phase materials and their lattice constant changes with the variation of Mn-concentration, which indicates the incorporation of Mn2+ into the hosting ZnO. These findings are corroborated by the observation of the well defined six hyperfine lines of Mn2+ ion in the electron paramagnetic resonance (EPR) spectra of the samples with a low Mn-concentration, and of a broad EPR line, which manifests the onset of Mn–Mn exchange interaction, in the samples with an elevated value of x. 相似文献
198.
In this paper, we prove that a free nilpotent group of finite rank is transitive self-similar. In contrast, we prove that a free metabelian group of rank \(r \ge 2\) is not transitive self-similar.
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