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81.
The universal cover T
G
of a connected graph G is the unique (possibly infinite) tree covering G, i.e., that allows a locally bijective homomorphism from T
G
to G. It is well-known that if a graph G covers a graph H, then their universal covers are isomorphic, and that the latter can be tested in polynomial time by checking if G and H share the same degree refinement matrix. We extend this result to locally injective and locally surjective homomorphisms
by following a very different approach. Using linear programming techniques we design two polynomial time algorithms that
check if there exists a locally injective or a locally surjective homomorphism, respectively, from a universal cover T
G
to a universal cover T
H
(both given by their degree matrices). This way we obtain two heuristics for testing the corresponding locally constrained
graph homomorphisms. Our algorithm can also be used for testing (subgraph) isomorphism between universal covers, and for checking
if there exists a locally injective or locally surjective homomorphism (role assignment) from a given tree to an arbitrary
graph H. 相似文献
82.
Effect of numerical integration on meshless methods 总被引:1,自引:0,他引:1
Ivo Babuka Uday Banerjee John E. Osborn Qinghui Zhang 《Computer Methods in Applied Mechanics and Engineering》2009,198(37-40):2886-2897
In this paper, we present the effect of numerical integration on meshless methods with shape functions that reproduce polynomials of degree k1. The meshless method was used on a second order Neumann problem and we derived an estimate for the energy norm of the error between the exact solution and the approximate solution from the meshless method under the presence of numerical integration. This estimate was obtained under the assumption that the numerical integration scheme satisfied a form of Green’s formula. We also indicated how to obtain numerical integration schemes satisfying this property. 相似文献
83.
应用对应分析研究抗缺氧中藏药中微量元素Co、Cr、Cu、Fe、Mg、Mn、Ni and Zn的分布特征。该方法可用于直接处理抗缺氧中藏药样品元素分析的数据,以研究地奥心血康、红景天胶囊、安神散、五味石榴散、章松八味沉香散、十六味杜鹃散、二十五味余甘子丸、二十五味竹黄胶囊、十五味龙胆丸、秘诀清凉散、七十味珍珠丸、八味沉香散、六味丁香散、七味葡萄散等中藏药的品质与它们的微量元素组成和样品来源的关系。将样品点和微量元素变量反映到同一因子轴的图形上,并且分类,能够揭露出样品来源与变量之间的内在联系,中藏药中人体必需的微量元素Fe、Mg、Co、Cu、Mn和Ni等含量比较高,与药物疗效密切相关,为研制新型的具有微量元素优势的药物提供科学的依据。 相似文献
84.
Poly(vinyl chloride) (PVC) was irradiated by electron beam in vacuum at 20 KGy to produce living free radicals, and then reacted with acrylic acid (AA) in solution to obtain the PVC‐g‐AA copolymers. The copolymers were characterized by Fourier transform infrared spectroscopy. Porous membranes were prepared from copolymers by the phase inversion technique. The morphology of PVC‐g‐AA membranes was studied by field emission scanning electron microscopy. The mean pore size and pore size distribution were determined by a mercury porosimeter. The mean pore size was 0.19 μm, and the bulk porosity was 56.02%. The apparent static water contact angle was 89.0°. The water drop penetration rate was 2.35 times to the original membrane. The maximum stress was 4.10 MPa. Filtration experiments were carried out to evaluate the fouling resistance of the PVC‐g‐AA membrane. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 相似文献
85.
Volumetric mass transfer coefficient, power input, and gas holdup are key parameters in the design of mechanically agitated gas‐liquid contactors. Although the majority of industrial batches are of higher viscosity, reliable transport characteristics correlations for viscous batches are lacking in literature. These correlations are often based on the power input as the scale of energy dissipation. In order to develop reliable power input correlations, its measurements were carried out in a pilot‐plant vessel using multiple impellers of various types and diameters. Power input correlation shapes providing the best match with the comprehensive database are also expected to predict most precisely the impeller power in industrial‐scale vessels. 相似文献
86.
Seok Ku Jeon Chan Seung Park Sang Done Kim Byung Ho Song Joseph M. Norbeck 《Korean Journal of Chemical Engineering》2008,25(6):1279-1285
Steam-methane reforming (SMR) reaction was studied using a tubular reactor packed with NiO/γ-Al2O3 catalyst to obtain synthesis gases with H2/CO ratios optimal for the production of synthetic diesel fuel from steam-hydrogasification of carbonaceous materials. Pure
CH4 and CH4-CO2 mixtures were used as reactants in the presence of steam. SMR runs were conducted at various operation parameters. Increasing
temperature from 873 to 1,023 K decreased H2/CO ratio from 20 to 12. H2/CO ratio decreased from 16 to 12 with pressure decreasing from 12.8 to 1.7 bars. H2/CO ratio also decreased from about 11 to 7 with steam/CH4 ratio of feed decreasing from 5 to 2, the lowest limit to avoid severe coking. With pure CH4 as the feed, H2/CO ratio of synthesis gas could not be lowered to the optimal range of 4–5 by adjusting the operation parameters; however,
the limitation in optimizing the H2/CO ratio for synthetic diesel fuel production could be removed by introducing CO2 to CH4 feed to make CH4-CO2 mixtures. This effect can be primarily attributed to the contributions by CO2 reforming of CH4 as well as reverse water-gas shift reaction, which led to lower H2/CO ratio for the synthesis gas. A simulation technique, ASPEN Plus, was applied to verify the consistency between experimental
data and simulation results. The model satisfactorily simulated changes of H2/CO ratio versus the operation parameters as well as the effect of CO2 addition to CH4 feed. 相似文献
87.
Multiscale approach to the morphology,structure, and segmental dynamics of complex degradable aliphatic polyurethanes 下载免费PDF全文
Milena Špírková Luďka Machová Libor Kobera Jiří Brus Rafał Poręba Magdalena Serkis Alexander Zhigunov 《应用聚合物科学杂志》2015,132(10)
A multiscale approach spanning from the segmental (subnanometer) up to micrometer level was applied for detailed study of the self‐assembly of aliphatic block polyurethane (PU) elastomers. To understand the principles of the self‐organization of hard and soft segments in the complex multi‐component systems, several two‐component model PU samples, that is, the products of 1,6‐diisocyanatohexane (HDI) with three diols differing in the length and constitution were also prepared, characterized, and investigated: (i) polycarbonate‐based macrodiol (MD), (ii) biodegradable oligomeric diol (DL‐L; product of butane‐1,4‐diol and D,L‐lactide), and (iii) butane‐1,4‐diol (BD). The study (particularly 13C‐1H PILGRIM NMR spectra) reveals complex internal organization and interesting (application appealing) behavior of multi‐component PUs. Hard segments (HDI+BD products) feature self‐assembled and significantly folded chain conformations with interdomain spacing 15–22 nm (small‐angle X‐ray scattering analysis). The small domains are hierarchically assembled in various structural formations of µm size (spherulites) depending on PU composition, as detected by transmission electron microscopy and atomic force microscopy. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41590. 相似文献
88.
Cultivar differences of total anthocyanins and anthocyanidins in red and purple-fleshed potatoes and their relation to antioxidant activity 总被引:1,自引:0,他引:1
Jaromír Lachman Karel Hamouz Miloslav Šulc Matyáš Orsák Vladimír Pivec Alena Hejtmánková Petr Dvořák Jaroslav Čepl 《Food chemistry》2009
Total anthocyanins (TAC) and individual anthocyanidins (AN) after hydrolysis were measured in 15 red and purple-fleshed potato cultivars produced in five different locations in the Czech Republic and a new cultivar Blaue St. Galler from Switzerland. It was found that TAC, expressed as cyanidin content, varied between 0.7 mg 100 g−1 FW (cv. Blue Congo) and 74.3 mg 100 g−1 FW (cv. Blaue Ludiano). Major differences in cultivars were found for AN relative abundance. For cv. Highland Burgundy Red a high proportion of pelargonidin (98.7%) was characteristic, whereas cv. British Columbia Blue contained almost exclusively cyanidin. Cultivars Violette and Vitelotte showed a relatively high content of malvidin. Cultivar Shetland Black differed from others with its higher content of peonidin (on average 36.7%). High petunidin abundance in the cultivars Valfi, Blue Congo, Salad Blue, Blaue St. Galler, Blaue Hindel Bank, Blaue Ludiano, Blaue Schweden, Farbe Kartoffel and Salad Red was found. TAC and AN contents highly corresponded with antioxidant activity (AA) determined with the ABTS, FRAP and DPPH assays in vitro. High AA was shown by the cultivars Vitelotte, Violette, Blaue Ludiano, Hafija, and Highland Burgundy Red. Increased height above sea level, higher annual sum of precipitation, and lower annual average temperatures caused higher AA and TAC. A high degree of hydroxylation and/or methoxylation of individual anthocyanidins could contribute in conjunction with other phenolics to high AA (peonidin, delphinidin and malvidin in the cultivars Blue Congo, Highland Burgundy Red and Shetland Black). Consequently, new red and purple-fleshed cultivars with high TAC and highly methoxylated and/or hydroxylated AN could be a promising source of favourable antioxidants in human nutrition. 相似文献
89.
The effects of the different exposure conditions on the electrochemical behavior of copper were evaluated in a growth medium containing Shewanella oneidensis MR-1. Impedance spectra were recorded at the corrosion potential (Ecorr) in three different cells for one week of exposure followed by cyclic voltammetry. A second time constant was observed in the impedance spectra of copper that was partially immersed in the test cell, where the electrode was in contact with an air/liquid interface (cell B). These spectra resembled those usually observed for metals covered with a polymer coating. Complete immersion of copper in the electrolyte (no air/liquid interface) or deaeration of cell B resulted in one-time-constant spectra that are typical of those found for passive metals. Excellent corrosion protection was provided by MR-1 regardless of exposure condition. Ecorr increased with time for the partially immersed Cu electrode exposed to the aerated solution in cell B, while it decreased for the other two exposure conditions. Cathodic polarization curves recorded after exposure for 7 days showed two reduction peaks for copper tested in cell B, while no reduction peaks were observed for the other cases. Similar results were obtained using cyclic voltammetry. 相似文献
90.
Further evidence was delivered that certain impurities, which could be contained in MgO samples, might be responsible for observed variability in MgO basicity and catalytic performance in oxidative coupling of methane. The surface basicity/base strength distribution of a series of MgO samples containing or not containing Ca and Na impurities was determined by a temperature-programmed desorption of CO2. It was revealed that samples containing Ca and Na impurities have much more medium, strong and very strong basic sites. The surface basicity of MgO samples containing added alkali or alkaline earth compounds or water was characterized by a test reaction of transformation of 2-butanol. It was confirmed that the introduction of these compounds to a pure MgO enhanced both its basicity and activity in oxidative coupling of methane. 相似文献