Chủ Nhật, 11 tháng 6, 2017

Hydrological consequences of landscape fragmentation in mountainous northern Vietnam: Buffering of Hortonian overland flow
Title:

Hydrological consequences of landscape fragmentation in mountainous northern Vietnam: Buffering of Hortonian overland flow
Authors: Ziegler, Alan D
Tran T. Liem
Tran Duc Vien
Keywords: land-cover conversion
deforestation
KINEROS2
swidden agriculture
tropical watershed hydrology
SE Asia
runoff generation
filter strips
Issue Date: 2007
Publisher: ELSEVIER SCIENCE BV, PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS
Citation: ISIKNOWLEDGE
Abstract: We use a hydrology-based fragmentation index to explore the influence of land-cover distribution on the generation and buffering of Hortonian overland flow (HOF) in two disturbed upland basins in northern Vietnam (Tan Minh). Both the current degree of fragmentation in Tan Minh and the current spatial arrangement of buffers (relative to HOF source areas) provide only limited opportunities for infiltrating surface runoff from upslope source areas, in part because of the high connectivity of swidden fields on long hillslopes. The intentional placement of buffers below HOF sources and the reduction of the down-slope lengths of swidden fields could reduce the occurrence of HOF on individual hillslopes. Reduction of the total watershed total depth of HOF would require maintaining a sufficient area of buffering land covers; and this may necessitate the use of longer fallow periods. These measures are, however, counter to the land-practice trends witnessed in the last several decades (i.e., no buffers, cultivation of long slopes, and increasingly shorter fallow periods). The two most likely scenarios of future land-cover change in Tan Minh-one representing increased fragmentation, the other decreased-both lead to an increase in HOF because of reduced buffering potential. The unlikely scenario of abandonment of agriculture and subsequent regeneration of forest, leads to both less fragmentation and less HOF. The study highlights the hydrological impacts associated with fragmentation at Tan Minh, which is the product of decades of local and regional forcing factors that have dictated the degree and timing of timber removal and swiddening at the site. (c) 2007 Elsevier B.V. All rights reserved.
Description: JOURNAL OF HYDROLOGY Volume: 337 Issue: 1-2 Pages: 52-67 Published: APR 15 2007 ; TNS07629
URI: http://www.sciencedirect.com/science/article/pii/S0022169407000236
http://repository.vnu.edu.vn/handle/VNU_123/31368
ISSN: 0022-1694
Appears in Collections:Bài báo của ĐHQGHN trong Web of Science

Thứ Năm, 8 tháng 6, 2017

Distribution of persistent organic pollutants and polycyclic aromatic hydrocarbons in sediment samples from Vietnam
 Title:

Distribution of persistent organic pollutants and polycyclic aromatic hydrocarbons in sediment samples from Vietnam
Authors: Nguyen Thi Phuong Thao
Pham Hung Viet
Tran Thi Ngoc Lan
Keywords: persistent organic pollutant
polycyclic aromatic hydrocarbon
Vietnam
sediment sample
high resolution gas chromatography/high resolution mass spectrometry
Issue Date: 2007
Publisher: PHARMACEUTICAL SOC JAPAN, 2-12-15 SHIBUYA, SHIBUYA-KU, TOKYO, 150-0002, JAPAN
Citation: ISIKNOWLEDGE
Abstract: The presence of eight kinds of persistent organic pollutants (POPs) such as DDT and its metabolites (DDTs), hexachlorocyclohexanes (HCHs), chlordane compounds (CHLs), drin compounds (Drins), heptachlor, hexachlorobenzene (HCB), heptachlor-epoxide, polychlorinated biphenyls (PCBs) and sixty-four polycyclic aromatic hydrocarbon compounds (PAHs) was identified using high resolution gas chromatography/high resolution mass spectrometry (HRGC/HRMS) to investigate their distribution in surface sediment from Hanoi, Hue, and Ho Chi Minh in Vietnam. A survey of sediment samples from Osaka was conducted for comparison. The concentrations of Sigma DDTs, Sigma CHLs, Sigma PCBs and Sigma PAHs in Vietnam were 0.19-140, N.D.-9.0, 0.11-110, and 30-5500 ng/g-dry, respectively. Concentrations of these compounds in urban areas were higher than those in other areas. In addition, the Sigma DDT concentrations in Vietnamese urban areas were higher than those in Osaka. These results suggest that most DDTs would be used as insecticides for the purpose of health services rather than as agricultural chemicals. PAH pollution in urban areas and suburbs is caused mainly by runoff of petrol, whereas in rural areas, the combustion of fossil fuels and biomass is the major pollutant source.
Description: TNS07623 ; JOURNAL OF HEALTH SCIENCE Volume: 53 Issue: 3 Pages: 291-301 Published: JUN 2007
URI: http://repository.vnu.edu.vn/handle/VNU_123/31214
ISSN: 1344-9702
1347-5207
Appears in Collections:Bài báo của ĐHQGHN trong Web of Science

Thứ Tư, 7 tháng 6, 2017

Managing Vietnam's maritime boundary disputes
Title:

Managing Vietnam's maritime boundary disputes
Authors: Thao, Nguyen Hong
Amer, Ramses
Keywords: boundary
Gulf of Thailand
Gulf of tonkin
maritime disputes
spratly archipelago
Vietnam
Issue Date: 2007
Publisher: TAYLOR & FRANCIS INC, 325 CHESTNUT ST, SUITE 800, PHILADELPHIA, PA 19106 USA
Citation: ISIKNOWLEDGE
Abstract: This article examines the progress made in managing Vietnam's maritime boundary disputes and analyzes the challenges that lie ahead relating to unsettled disputes. The continuity and change in Vietnam's approach to dispute a settlement and the difficulties in managing the unresolved maritime disputes are assessed. Vietnam has made considerable progress in managing its maritime disputes; however, continued efforts are needed to address the unresolved disputes.
Description: TNS07622 ; OCEAN DEVELOPMENT AND INTERNATIONAL LAW Volume: 38 Issue: 3 Pages: 305-324 Published: JUL-SEP 2007
URI: http://repository.vnu.edu.vn/handle/VNU_123/31210
ISSN: 0090-8320
Appears in Collections:Bài báo của ĐHQGHN trong Web of Science

Thứ Ba, 6 tháng 6, 2017

On the exponential stability of dynamic equations on time scales
 Title:

On the exponential stability of dynamic equations on time scales
Authors: Nguyen Huu Du
Le Huy Tien
Keywords: exponential stability
uniformly exponential stability
time scales
perron theorem
linear dynamic equation
Issue Date: 2007
Publisher: ACADEMIC PRESS INC ELSEVIER SCIENCE, 525 B ST, STE 1900, SAN DIEGO, CA 92101-4495 USA
Citation: ISIKNOWLEDGE
Abstract: In this paper, we deal with some theorems on the exponential stability of trivial solution of time-varying non-regressive dynamic equation on time scales with bounded graininess. In particular, well-known Perron's theorem is generalized on time scales. Under rather restrictive condition, that is, integral boundedness of coefficient operators, we obtain a characterization of the uniformly exponential stability. (c) 2006 Elsevier Inc. All rights reserved.
Description: TNS07619 ; JOURNAL OF MATHEMATICAL ANALYSIS AND APPLICATIONS Volume: 331 Issue: 2 Pages: 1159-1174 Published: JUL 15 2007
URI: http://repository.vnu.edu.vn/handle/VNU_123/31172
ISSN: 0022-247X
Appears in Collections:Bài báo của ĐHQGHN trong Web of Science

Thứ Hai, 5 tháng 6, 2017

An approach for obtaining approximate formulas for the Rayleigh wave velocity

 Authors:

Pham Chi Vinh
Peter G. Malischewsky
Keywords: rayleigh waves
rayleigh wave velocity
rayleigh wave speed
approach of least squares
the best approximation
approximate formula
approximate expression
Issue Date: 2007
Publisher: ELSEVIER SCIENCE BV, PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS
Citation: ISIKNOWLEDGE
Abstract: In this paper, we introduce an approach for finding analytical approximate formulas for the Rayleigh wave velocity for isotropic elastic solids and anisotropic elastic media as well. The approach is based on the least-square principle. To demonstrate its application, we applied it in order to obtain an explanation for Bergmann's approximation, the earliest known approximation of the Rayleigh wave velocity for isotropic elastic solids, and used it to establish a new approximation. By employing this approach, the best approximate polynomials of the second order of the cubic power and the quartic power in the interval [0, 1] were found. By using the best approximate polynomial of the second order of the cubic power, we derived an approximate formula for the Rayleigh wave speed in isotropic elastic solids which is slightly better than the one given recently by Rahman and Michelitsch by employing Lanczos's approximation. Also by using this second order polynomial, analytical approximate expressions for orthotropic, incompressible and compressible elastic solids were found. For incompressible case, it is shown that the approximation is comparable with Rahman and Michelitsch's approximation, while for the compressible case, it is shown that our approximate formulas are more accurate than Mozhaev's ones. Remarkably, by using the best approximate polynomials of the second order of the cubic power and the quartic power in the interval [0, 1], we derived an approximate formula of the Rayleigh wave velocity in incompressible monoclinic materials, where the explicit exact formulas of the Rayleigh wave velocity so far are not available. (c) 2007 Elsevier B.V. All rights reserved.
Description: TNS07618 ; WAVE MOTION Volume: 44 Issue: 7-8 Pages: 549-562 Published: AUG 2007
URI: http://repository.vnu.edu.vn/handle/VNU_123/31160
ISSN: 0165-2125
Appears in Collections:Bài báo của ĐHQGHN trong Web of Science
Anharmonic effective potential, correlation effects, and EXAFS cumulants calculated from a morse interaction potential for fcc metals
 Anharmonic effective pair potentials and effective local force constants have been studied for fee metals, assuming an interaction Morse potential and taking into account the influence of nearest neighbours of absorber and backscatterer atoms. Analytical expressions for the first three extended X-ray absorption fine structure (EXAFS) cumulants, as well as for the atomic mean square displacements, have been derived as a function of the Morse parameters. Numerical results for copper and nickel are compared with experimental data. A good agreement is found for the second cumulant. Non-negligible discrepancies are instead found for the first and third cumulants, which are tentatively attributed to the central nature of the Morse potential, which neglects many-body effects. URI: http://repository.vnu.edu.vn/handle/VNU_123/31148
Mitochondrial control region and population genetic patterns of Nycticebus bengalensis and N-Pygmaeus
Abstract:

Bengal slow lorises (Nycticebus bengalensis) and pygmy slow lorises (Nycticebus pygmaeus) are nocturnal which creates difficulties to study them in the field. There is a scarcity of data on them and their population genetics are poorly understood. We sequenced and analyzed a partial fragment in the first hypervariable region of the mitochondrial control region or D-loop HVRI of 21 Nycticebus bengalensis and 119 N. pygmaeus from the boundary between China and Vietnam where they are sympatric. Though the sample size for Nycticebus pygmaeus is much larger, the polymorphism level is much lower than that of N. bengalensis, possibly because of (1) external gene flow from other habitats of N. bengalensis, (2) gene ingression from Sunda slow lorises (N. coucang coucang) to N. bengalensis, (3) a skewed birth sex ratio in N. pygmaeus, and (4) a possible low survival rate of infant N. pygmaeus. Based on genetic comparisons to Nycticebus bengalensis, we propose that N. pygmaeus in southern China and northern Vietnam might have migrated from middle or southern Vietnam recently.
Description: TNS07616 ; INTERNATIONAL JOURNAL OF PRIMATOLOGY Volume: 28 Issue: 4 Pages: 791-799 Published: AUG 2007
URI: http://repository.vnu.edu.vn/handle/VNU_123/31136
Giant exchange bias in MnPd/Co bilayers
 A systematic study of exchange bias in MnPd/Co bilayers has been carried out, where the dependences of exchange bias, unidirectional anisotropy constant and coercivity on the thicknesses of MnPd and Co layers were investigated. A huge unidirectional anisotropy constant, J(K) = 2.5 erg/cm(2) was observed, which is in reasonable agreement with the theoretical prediction based on the model by Meiklejohn and Bean. The angular dependences of exchange bias field and coercivity have also been examined showing that both exchange bias and coercivity follow 1/ cos alpha rule. (c) 2007 Elsevier B.V. All rights reserved. http://repository.vnu.edu.vn/handle/VNU_123/31126
Manganese perovskites for room temperature magnetic refrigeration applications
 We found the large magnetocaloric effect (MCE) in La0.6Ca0.3Pb0.1MnO3 (sample No. 1), La0.7Ca0.2Pb0.1MnO3 (sample No. 2), and La0.7Ca0.1Pb0.2MnO3 (sample No. 3) perovskites, which were prepared by a conventional ceramic method. For a magnetic field change of 13.5 kOe, the magnetic entropy change (Delta S-M) reached values of 2.55, 2.53 and 3.72 J/kg K for samples Nos. 1, 2 and 3, respectively. Interestingly, the large Delta S-M was found to occur around 300 K for all samples investigated, which allows magnetic refrigeration at room temperature. These perovskites have the large magnetic entropy changes induced by low magnetic field change, which is beneficial for the household application of active magnetic refrigerant (AMR) materials. (C) 2007 Elsevier B.V. All rights reserved. http://repository.vnu.edu.vn/handle/VNU_123/31120

Chủ Nhật, 4 tháng 6, 2017

Longitudinal Hall effect in Terfecohan thin films with perpendicular magnetic anisotropy

 Longitudinal extraordinary Hall Effect (LEHE) of magnetic Tb( Fe0.55Co0.45)(1.5) (known as Terfecohan) thin films with perpendicular magnetic anisotropy has been investigated as a function of both the intensity of applied magnetic fields and the angle alpha between the applied field and film normal directions. The Hall voltage loops exhibit a parallelogram shape, which is almost similar to those of the perpendicular magnetization. The high-field Hall voltage susceptibility is positive at alpha = 0. Its value decreases with increasing alpha and changes in sign at alpha(m) 20 degrees, which is considered as the easy magnetizable direction of the film. This finding is comparable with those obtained from the magnetization, magnetic force microscopy (MFM) and conversion electron Mossbauer spectra (CEMS) measurements. The obtained LEHE behaviors are rather promising for applications such as magnetic recording heads and magnetic field detectors, where a large output signal is required at low magnetic fields. (c) 2007 Elsevier B.V. All rights reserved.. Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/31096
Magnetization and magnetostriction process in spring-magnet TbFeCo/Fe multilayers with variable TbFeCo thickness

 Different natures of the magnetization reversal are studied by means of magnetization and magnetostriction measurements for magnetostrictive "spring-magnet'' multilayers of TbFeCo/Fe with a. fixed Fe layer thickness of 10 nm and variable TbFeCo layer thickness of 12, 16 and 20 nm. In such multilayered systems, magnetization reversal occurs at different coercive fields for each layer related to formation of the domain wall at interfaces. The results show that at low temperatures, transition from the ferromagnetic saturation state (with domain walls) to the ferrimagnetic transient saturation state (without domain walls) takes place by the reversal of the magnetic moments in the high-magnetization Fe layers. This process is governed by large magnetic anisotropy of the TbFeCo layer. Increasing temperature, this anisotropy decreases and transition results from the reversal of magnetic moments in the small magnetization TbFeCo layers. Formation of the domain wall at interfaces is clearly evidenced by the negative contribution to the parallel magnetostriction. (c) 2007 Elsevier B.V. All rights reserved. Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/31064
Study of the determination of inorganic arsenic species by CE with capacitively coupled contactless conductivity detection
 The determination of arsenic(III) and arsenic(V), as inorganic arsenite and arsenate, was, investigated by CE with capacitively coupled contactless conductivity detection (CE-C-4 D). It was found necessary to determine the two inorganic arsenic species separately employing two different electrolyte systems. Electrolyte solutions consisting of 50 mM CAPS/2 MM L-arginine (Arg) (pH 9.0) and of 45 mM acetic acid (pH 3.2) were used for arsenic(III) and arsenic(V) determinations, respectively. Detection limits of 0.29 and 0.15 mu M were achieved for As(III) and As(V), respectively by using large-volume injection to maximize the sensitivity. The analysis of contaminated well water samples from Vietnam is demonstrated.
Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/31059
New fully portable instrument for the versatile determination of cations and anions by capillary electrophoresis with contactless conductivity detection
 A new portable capillary electrophoresis instrument with capacitively coupled contactless conductivity detection was developed and optimized for the sensitive field measurements of ionic compounds in environmental samples. It is powered by batteries and the high voltage modules are capable of delivering up to 15 kV at either polarity for more than one working day. Inorganic cations and anions, including ions of heavy metals and arsenate, could be determined with detection limits in the range from about 0.2 to 1 mu M. The instrument was field tested in a remote region of Tasmania and nitrite and ammonium could be determined on-site at concentrations as low as 10 ppb in presence of other common inorganic ions at concentrations which were 2 to 3 orders of magnitude higher.
Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/31052
Continuous parallel-iterated RKN-type PC methods for nonstiff IVPs
 This paper investigates parallel predictor-corrector (PC) iteration schemes based on direct collocation Runge-Kutta-Nystrom (RKN) corrector methods with continuous output formulas for solving nonstiff initial-value problems (IVPs) for systems of special T second-order differential equations y ''(t) = f(t, y(t)). Consequently, the resulting parallel-iterated RKN-type PC methods are provided with continuous output formulas. The continuous numerical approximations are also used for predicting the stage values in the PC iteration processes. In this way, we obtain parallel PC methods with continuous output formulas and high-order predictors. Applications of the resulting parallel PC methods to a few widely-used test problems reveal that these new parallel PC methods are much more efficient when compared with the parallel-iterated RKN (PIRKN) methods and the sequential ODEX2 and DOPRIN codes from the literature. (c) 2006 IMACS. Published by Elsevier B.V. All rights reserved.
 Mời bạn đọc xem tại đây:  http://repository.vnu.edu.vn/handle/VNU_123/31026

Thứ Tư, 31 tháng 5, 2017

A novel efficient two-phase algorithm for training interpolation radial basis function networks
 nterpolation radial basis function (RBF) networks have been widely used in various applications. The output layer weights are usually determined by minimizing the sum-of-squares error or by directly solving interpolation equations. When the number of interpolation nodes is large, these methods are time consuming, difficult to control the balance between the convergence rate and the generality, and difficult to reach a high accuracy. In this paper, we propose a two-phase algorithm for training interpolation RBF networks with bell-shaped basis functions. In the first phase, the width parameters of basis functions are determined by taking into account the tradeoff between the error and the convergence rate. Then, the output layer weights are determined by finding the fixed point of a given contraction transformation. The running time of this new algorithm is relatively short and the balance between the convergence rate and the generality is easily controlled by adjusting the involved parameters, while the error is made as small as desired. Also, its running time can be further enhanced thanks to the possibility to parallelize the proposed algorithm. Finally, its efficiency is illustrated by simulations. (c) 2007 Elsevier B.V. All rights reserved. Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/31018
Chemical composition of the flower essential oil of Artabotrys hexapetalus (L. f.) bhandare of Vietnam
 The pleasant smelling flower essential oil of Artabotrys hexapetalus (L. f.) Bhandare (Annonaceae) was analyzed by GC and GC/MS. Twenty-six components of the oil including sesquiterpene hydrocarbons (33.3% of the oil) and oxygenated sesquiterpenoids (47.7%) were identified. P-caryophyllene (11.4%) and caryophyllene oxide (31.5%) were identified as the major components of the oil.
Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/31011
Chemical composition of the essential oil from the rhizomes of Alpinia henryi K. Schum. of Vietnam
 The essential oil from the fresh rhizomes of Alpinia henryi K. Schum. (Zingiberaceae) collected in Vietnam was analyzed by GC and GC/MS. Twenty-five compounds, representing 86.4% of the oil, were identified. The oil was characterized by a high content of 1,8-cineole (45.1%).

 Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/31005

Chủ Nhật, 28 tháng 5, 2017

Arsenic in groundwater of the Red River floodplain, Vietnam: Controlling geochemical processes and reactive transport modeling
 The mobilization of arsenic (As) to the groundwater was studied in a shallow Holocene aquifer on the Red River flood plain near Hanoi, Vietnam. The groundwater chemistry was investigated in a transect of 100 piezometers. Results show an anoxic aquifer featuring organic carbon decomposition with redox zonation dominated by the reduction of Fe-oxides and methanogenesis. Enhanced PCO2 pressure causes carbonate dissolution to take place but mainly in the soil and unsaturated zone. The concentration of As increases over depth to a concentration of up to 550 mu g/L. Most As is present as As(III) but some As(V) is always found. Arsenic correlates well with NH4, relating its release to organic matter decomposition and the source of As appears to be the Fe-oxides being reduced. Part of the produced Fe(II) is apparently reprecipitated as siderite containing less As. Results from sediment extraction indicate most As to be related to the Fe-oxide fractions. The measured amount of sorbed As is low. In agreement, speciation calculations for a Fe-oxide surface suggest As(III) to constitute only 3% of the surface sites while the remainder is occupied by carbonate and silica species. The evolution in water chemistry over depth is homogeneous and a reactive transport model was constructed to quantify the geochemical processes along the vertical groundwater flow component. A redox zonation model was constructed using the partial equilibrium approach with organic carbon degradation in the sediment as the only rate controlling parameter. Apart from the upper meter a constant degradation rate of 0.15 C mmol/L/yr could explain the redox zonation throughout the aquifer. Modeling also indicates that the Fe-oxide being reduced is of a stable type like goethite or hematite. Arsenic is contained in the Fe-oxides and is first released during their dissolution. Our model further suggests that part of the released As is adsorbed on the surface of the remaining Fe-oxides and in this way may be retarded. (c) 2007 Elsevier Ltd. All rights reserved.
 Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/30997
Systematic testing of an integrated systems model for coastal zone management using sensitivity and uncertainty analyses
 Systematic testing of integrated systems models is extremely important but its difficulty is widely underestimated. The inherent complexity of the integrated systems models, the philosophical debate about the model validity and validation, the uncertainty in model inputs, parameters and future context and the scarcity of field data complicate model validation. This calls for a validation framework and procedures which can identify the strengths and weaknesses of the model with the available data from observations, the literature and experts' opinions. This paper presents such a framework and the respective procedure. Three tests, namely, Parameter-Verification, Behaviour-Anomaly and Policy-Sensitivity are selected to test a Rapid assessment Model for Coastal-zone Management (RaMCo). The Morris sensitivity analysis, a simple expert elicitation technique and Monte Carlo uncertainty analysis are used to facilitate these three tests. The usefulness of the procedure is demonstrated for two examples. (C) 2006 Published by Elsevier Ltd.
Mời bạn xem đọc tại đây: http://repository.vnu.edu.vn/handle/VNU_123/30984
A new approach to testing an integrated water systems model using qualitative scenarios

 Integrated systems models have been developed over decades, aiming to support the decision-makers in the planning and managing of natural resources. The inherent model complexity, lack of knowledge about the linkages among model components, scarcity of field data, and uncertainty involved with internal and external factors of the real system call their practical usefulness into doubt. Validation tests designed for such models are just immature, and are argued to have some characteristics that differ from the ones used for validating other types of models. A new approach for testing integrated water systems models is proposed, and applied to test the RaMCo model. Expert knowledge is elicited in the form of qualitative scenarios and translated into quantitative projections using fuzzy set theory. Trend line comparison of the projections made by the RaMCO model and the qualitative projections based on expert knowledge revealed an insufficient number of land-use types adopted by the RaMCo model. This insufficiency makes the model inadequate to describe the consequences of the changes in socio-economic factors and policy options on the erosion from the catchment and the sediment yields at the inlet of a storage lake. (C) 2007 Elsevier Ltd. All rights reserved.
Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/30973
Chemical composition of the essential oil of Ocimum Basilicum cultivated in Mongolian Desert-Gobi
 Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/30964
Isolation and identification of antiplatelet aggregatory principles from the leaves of Piper lolot

The methanolic extract of Piper lolot, having shown potent inhibitory activity on platelet aggregation induced by arachidonic acid (AA) and platelet activating factor (PAF), was subjected to activity-guided isolation to yield twelve new amide alkaloids, piperlotine A-L (1-12), along with twenty-nine known compounds. Their structures were elucidated on the basis of spectroscopic analysis. The isolated compounds were tested for their inhibitory activity on the rabbit platelet aggregation. The compounds piperlotine A (1), piperlotine C (3), piperlotine D (4), piperlotine E (5), 3-phenyl-1-(2,4,6-trihydroxyphenyl)propan-1-one (21), 3-(4-methoxyphenyl)-1-(2,4,6-trihydroxyphenyl)propan-1-one (22), 1-trans-cinnamoylpyrrolidine (24), sarmentine (26), pellitorine (27), methyl 3-phenylpropionate (32), and (10S)-10-hydroxypheophorbide a methyl ester (40) showed potent antiplatelet aggregation activity.
 Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/30952



An improved approximation of Bergmann's form for the Rayleigh wave velocity
 In the present paper an improved approximation for the Rayleigh wave velocity in isotropic elastic solids is obtained using the method of least squares. It is of Bergmann's form, i.e. the form of the ratio of two binomials. It is shown that this approximation is the best one of the Rayleigh wave velocity, in the sense of least squares, with respect to the class of functions whose elements are the ratio of two binomials. This approximation is much more accurate than Bergmann's one. Its maximum percentage error is 10 times smaller than that of Bergmann's. It is 7.6 times better than the one obtained recently by Royer and Clorennec [ D. Royer, D. Clorennec, An improved approximation for the Rayleigh wave equation, Ultrasonics 46 (2007) 23-24]. An approximation of Bergmann's form for the squared Rayleigh wave velocity is also derived and its maximum percentage error is 5 times smaller than that of Royer and Clorennec's approximation. Some polynomial approximations with very high accuracy are also obtained. (c) 2007 Elsevier B.V. All rights reserved.

Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/30941
The contribution of the exchange biased field direction in multilayer thin films to planar Hall resistance
Recently, planar Hall effect (PHE) has been widely pursued due to its application potential for biosensors. Planar Hall sensor is based on the anisotropy magnetoresistance and exhibits many advantages, such as large signal-to-noise ratio at low frequencies and high sensitivity at low applied field. The planar Hall resistance (PHR) curve in multilayer thin films with spinvalve structure has pre-eminent sensitivity when compared to single layer and bilayer thin films. In this work, we report a model for PHR calculation that includes the behaviour of single domain basic structure in the external magnetic field. Our results show a qualitative dependence between PHR curves and the angle (beta) between the exchange biased field direction and the easy axis of the free layer. As the beta increases the sensitivity of the PHR curves also increases. Further, it is shown that our calculation helps to determine the exchange biased field direction. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/30926
Influence of Nb substituted for Fe on the microstructure and magnetic properties of Fe-based nanocomposite alloy

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Thứ Bảy, 27 tháng 5, 2017

Planar Hall resistance sensor for biochip application

 in this work, we introduce a new type of sensor by using planar Hall effect in spin valve structure for biochip application due to advantage of increasing sensor sensitivity. A single Dynabeads (R) M-280 Streptavidin detection has been accomplished with the sensor pattern size of 3 X 3 mu m(2) that was fabricated from NiFe(6.0 nm)/Cu(3.5 nm)/NiFe(3.0 nm)/IrMn(10.0 nm) spin valve structure. Furthermore, it is also developed to integrated arrays by including 24 sensor patterns. In comparison with the other groups, our sensor performance is highlighted with the advantages of increased stability and high signal to noise; as such, the planar Hall effect sensor's behavior has proved a possibility for detection of the biomolecule. It is also feasible to provide a vehicle for studying other molecule interactions, particular single DNA molecule and for the detection of binding of the streptavidin functionalized magnetic beads to sensor bound biotin. Due to the simple fabrication scheme, this kind of Planar Hall effect based sensor can be easily integrated into other systems for applications. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Mời bạn đọc xem tại đây: http://repository.vnu.edu.vn/handle/VNU_123/30911
Cytochrome P3A4 inhibitors and other constituents of Fibraurea tinctoria

 Four ne v furanoditerpenoids, fibrauretin A (1), fibrauretinoside A (2), epi-fibrauretinoside A (3), and epi-12-palmatoside G (4), and a new ecdysteroid glucoside, fibraurecdyside A (5), together with seven known compounds including two furanoditerpenoids (6 and 7), an ecdysteroid (8), and four quaternary protoberberine alkaloids (9-12) were isolated from the stems of Fibraurea tinctoria. The structures of 1-5 were established on the basis of spectroscopic evidence. Among these compounds, palmatine (9) and jatrorrhizine (10) showed inhibitory effects against cytochrome P450 3A4 (CYP3A4) with IC50 values of 0.9 and 2.1 mu M, respectively.

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Crotonkinins A and B and related diterpenoids from Croton tonkinensis as anti-inflammatory and antitumor agents

 Cytotoxicity-guided phytochemical investigation of a methanolic extract of Croton tonkinensis afforded two new kaurane diterpenoids (1, 2) and 10 known ent-kaurane-type diterpenoids (3-12). The structures of I and 2 were based on analysis of spectroscopic and mass spectral data. Compounds 3-12 were identified by comparison of their spectroscopic and physical data with those reported in the literature. Selected compounds from this plant were examined for cytotoxic and anti-inflammatory activities. Compounds 4 and 9 showed the highest cytotoxic activity against the tested tumor cell lines. Compounds 3, 4, 6, 8, 9, and 11 had IC50 values less than 5 mu M and were more potent than the nonspecific NOS inhibitor L-NAME in inhibiting LPS-induced NO production.
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Strict partitions and discrete dynamical systems

 We prove that the set of partitions with distinct parts of a given positive integer under dominance ordering can be considered as a configuration space of a discrete dynamical model with two transition rules and with the initial configuration being the singleton partition. This allows us to characterize its lattice structure, fixed point, and longest chains as well as their length, using Chip Firing Game theory. Finally, we study the recursive structure of infinite extension of the lattice of strict partitions. (c) 2007 Elsevier B.V. All rights reserved

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Optical transitions in polarized CdSe, CdSe/ZnSe, and CdSe/CdS/ZnS quantum dots dispersed in various polar solvents

 The optical transitions in ensembles of colloidal CdSe-based quantum dots (QDs) have been systematically studied as a function of the net QDs' polarity/polarization and of the solvent's polarity. While the general trend observed for all QD systems dispersed in different solvents is similar, the spectral shifts are more pronounced in core QDs than in core/shell structures. Our results can be rationalized by taking account of the electric field experienced by the QDs that results from their effective polarization in solvents of different polarities (quantum confined Stark effect) as well as from the effect of the external dielectric environment (solvatochromatic effect). (C) 2007 American Institute of Physics.

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Thứ Năm, 25 tháng 5, 2017

Tin-dioxide nanocrystals as Er3+ luminescence sensitizers: Formation of glass-ceramic thin films and their characterization

 Silica-tin dioxide thin films doped with Er3+ ions were fabricated and investigated. Different parameters such as heat-treatment temperatures, molar concentrations of SnO2 as well as Er3+ ions concentration were changed in order to obtain the best properties of presented thin films. Using several techniques, thin films were characterized and proved to be crack-free, water-free and smooth after a heat-treatment at 1200 degrees C Aiming to application in optics, the transparency of thin films was also evidenced by transmission spectra. Based on the photoluminescence measurements, the mechanism of energy transfer from SnO2 nanocrystals to Er3+ ions was examined and discussed. (C) 2016 Elsevier B.V. All rights reserved.
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Constituents of the Rhizomes of Boesenbergia pandurata and Their Antiausterity Activities against the PANC-1 Human Pancreatic Cancer Line

 Human pancreatic cancer cell lines have a remarkable tolerance to nutrition starvation, which enables them to survive under a tumor microenvironment. The search for agents that preferentially inhibit the survival of cancer tells under low nutrient conditions represents a novel antiausterity strategy in anticancer drug discovery. In this investigation, a methanol extract of the rhizomes of Boesenbergia pandurata showed potent preferential cytotoxicity against PANC-1 human pancreatic cancer cells under nutrient-deprived conditions, with a PC50 value of 6.6 mu g/mL. Phytochemical investigation of this, extract led to the isolation of 15 compounds, including eight new cyclohexene chalcones (1-8). The structures of the new compounds were elucidated by NMR spectroscopic data analysis. Among the isolated compounds obtained, isopanduratin A1 (14) and nicolaioidesin C (15) exhibited potent preferential cytotoxicity against PANC-1 human pancreatic canter cells under nutrition deprived conditions, with PC50 values of 1.0 and 0.84 mu M, respectively.

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DNA barcoding of East Asian Amentotaxus (Taxaceae): Potential new species and implications for conservation

DNAbarcoding is a useful tool for species identification using standardized genomic DNA fragments. The genusAmentotaxus, consisting offive or six species, is confined to South China, Northeast India, Laos, and Vietnam. All species have been assessed as globally or nationally threatened. However, there is uncertainty about the number of species involved, especially in the border areas of southern China, northern Vietnam, and Laos. We selectedfive DNA barcodes (rbcL,matK,trnH-psbA,trnL-F, and internal transcribed spacer (ITS)) to evaluate their discrimination ability in this genus, and to investigate the current taxonomy ofAmentotaxus. Our results indicate that all the selected barcoding regions showed a high level of universality for PCR and sequencing. When six species are recognized, thenuclear ribosomal DNA region ITS and the chloroplast DNA regiontrnL-Fusedon their ownprovided the highest identification success (60%). Two barcode combinations that included either ITS ortrnL-Fhad the same species discrimination ability. Combinations using additional barcodes did not improve the species identification success. When only five species are recognized, withA. hatuyenensisT. H. Nguyen treated as a synonym of A. yunnanensisH. L. Li, the discrimination rate rises to 100%. Our results also indicate that recent collections from Yunnan province, China, Lao Cai province, Vietnam, and Laos may represent a potential new species. Thefindings from this study will be very useful for the formulation of appropriate conservation strategies for threatened Amentotaxusspecies in national and trans-boundary regions.
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Polygonum multiflorum root extract as a potential candidate for treatment of early graying hair

 Despite Polygonum multiflorum (PM) has been experiencely used as a drug to treat early graying hair phenomenon in Asian countries for a long time, there is limited study examined the real biological effects of PM on hair graying in vitro and in vivo. In this study, we investigated the effects of PM root extract (PM-RE) on melanin synthesis in human melanoma SKMEL-28 cells and embryos/larvae of wild-type strain AB zebrafish. We also preliminary revealed the molecular mechanism of early hair graying phenomenon in both in vitro and in vivo models. Our results showed that PM-RE significantly induced melanin synthesis in melanin-producing SKMEL-28 melanoma cells and also in zebrafish embryos/larvae at 4-day postfertilization through activation of MC1R/MITF/tyrosinase-signaling pathway. We also investigated the differences in genotype between graying hair follicle and black hair follicle of young peoples and found that early hair graying phenomenon may be related to downregulation of MC1R/MITF/tyrosinase pathway. Taken together, we suggested that PM-RE at safe doses could be used as a potential agent for the treatment of early hair graying and other loss pigmentation-related diseases.
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Contamination of drinking water resources in the Mekong delta floodplains: Arsenic and other trace metals pose serious health risks to population

 This study presents a transnational groundwater survey of the 62,000 km 2 Mekong delta floodplain (Southern Vietnam and bordering Cambodia) and assesses human health risks associated with elevated concentrations of dissolved toxic elements. The lower Mekong delta generally features saline groundwater. However, where groundwater salinity isb1gL −1 Total Dissolved Solids (TDS), the rural population started exploiting shallow groundwater as drinking water in replacement of microbially contaminated surface water. In groundwater used as drinking water, arsenic concentrations ranged from 0.1–1340 µg L −1 , with 37% of the studied wells exceeding the WHO guidelines of 10 µg L −1 arsenic. In addition, 50% exceeded the manganese WHO guideline of 0.4 mg L −1 , with concentrations being particularly high in Vietnam (range 1.0–34 mg L −1 ). Other elements of (minor) concern are Ba, Cd, Ni, Se, Pb and U. Our measurements imply that groundwater contamination is of geogenic origin and caused by natural anoxic conditions in the aquifers. Chronic arsenic poisoning is the most serious health risk for the ~2million people drinking this groundwater without treatment, followed by malfunction in children's development through excessive manganese uptake. Government agencies, water specialists and scientists must get aware of the serious situation. Mitigation measures are urgently needed toprotect the unaware people from such health problems. Published by Elsevier Ltd.

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Persistent organic pollutants in Vietnam: Environmental contamination and human exposure

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Extragradient algorithms extended to equilibrium problems

 We make use of the auxiliary problem principle to develop iterative algorithms for solving equilibrium problems. The first one is an extension of the extragradient algorithm to equilibrium problems. In this algorithm the equilibrium bifunction is not required to satisfy any monotonicity property, but it must satisfy a certain Lipschitz-type condition. To avoid this requirement we propose linesearch procedures commonly used in variational inequalities to obtain projection-type algorithms for solving equilibrium problems. Applications to mixed variational inequalities are discussed. A special class of equilibrium problems is investigated and some preliminary computational results are reported.

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NAD-Malic Enzyme Affects Nitrogen Fixing Activity of Bradyrhizobium japonicum USDA 110 Bacteroids in Soybean Nodules

 The NAD(+)-dependent malic enzyme (DME) has been reported to play a key role Supporting nitrogenase activity in bacteroids of Sinorhizobium meliloti. Genetic evidence for a similar role in Bradyrhizobium japonicum USDA110 was obtained by constructing a dine mutant. Soybean plants inoculated with a dme mutant did not show delayed nodulation, but formed small root nodules and exhibited significant nitrogen-deficiency symptoms. Nodule numbers and the acetylene reducting activity per nodule as a dry weight value 14 and 28 days after inoculation with the dme mutant were comparable to those of plants inoculated with wild-type B. japonicum. However, shoot dry weight and acetylene reducting activity per nodule decreased to ca. 30% of the values in plants with wild-type B. japonicum. The sucrose and organic acid (malate, succinate, acetate, alpha-ketoglutarate and lactate) contents of the nodules were investigated. Amounts of sucrose, malate and alpha-ketoglutarate increased on inoculation with the dine mutant, suggesting that the decreased DME and nitrogenase activities in the bacteroids resulted in a reduction in the consumption of these respiratory metabolites by the nodules. The data suggest that the DME activity of B. japonicum bacteroids plays a role in nodule metabolism and supports nitrogen fixation.

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New 3-level response surface designs constructed from incomplete block designs

Box and Behnken [1958. Some new three level second-order designs for surface fitting. Statistical Technical Research Group Technical Report No. 26. Princeton University, Princeton, NJ; 1960. Some new three level designs for the study of quantitative variables. Technometrics 2, 455-475.] introduced a class of 3-level second-order designs for fitting the second-order response surface model. These 17 Box-Behnken designs (BB designs) are available for 3-12 and 16 factors. Although BB designs were developed nearly 50 years ago, they and the central-composite designs of Box and Wilson [1951. On the experimental attainment of optimum conditions. J. Royal Statist. Soc., Ser. B 13, 1-45.] are still the most often recommended response surface designs. Of the 17 aforementioned BB designs, 10 were constructed from balanced incomplete block designs (BIBDs) and seven were constructed from partially BIBDs (PBIBDs). In this paper we show that these seven BB designs constructed from PBIBDs can be improved in terms of rotatability as well as average prediction variance, D- and G-efficiency. In addition, we also report new orthogonally blocked solutions for 5, 8, 9, 11 and 13 factors. Note that an 11-factor BB design is available but cannot be orthogonally blocked. All new designs can be found at http://www.math.montana.edu/-jobo/bbd/. (C) 2007 Elsevier B.V. All rights reserved.

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Chemical constituents from Abutilon indicum

 The investigation on the chemical constituents of the whole plant of Abutilon indicum has resulted in the isolation of two new compounds, abutilin A (1) and (R)-N-(1 '-methoxycarbonyl- 2 '-phenylethyl)-4-hydroxybenzamide (2), as well as 28 known compounds. The structures of the two new compounds were established on the basis of the spectroscopic analysis, and the known compounds were identified by comparison of their spectroscopic and physical data with those reported in the literature.

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The higher Tamm-Dancoff approximation: Theoretical context and phenomenological aspects

 We present the key aspects of the theoretical foundations of the Higher Tamm-Dancoff Approximation which can be interpreted as a truncated shell-model approach based on a Hartree Fock solution, ensuring the conservation of the particle number. Then we discuss some phenomenological aspects of the residual interactions used, namely the delta interaction to describe the neutron neutron and proton proton pairing correlations and the quadrupole-quadrupole interaction to describe vibrational correlations.

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Criticizing in an L2: Pragmatic strategies used by Vietnamese EFL learners

 Criticizing has been a rather under-represented speech act in interlanguage pragmatics (ILP) literature. Native speakers (NSs) find this speech act challenging, often needing to pre-plan how to perform it (Murphy & Neu 1996). Thus, it can be expected that second-language (L2) learners will also experience considerable difficulty. This paper reports a study of the pragmatic strategies used by Vietnamese learners of English as a foreign language (EFL) when criticizing in English with a view to shedding light on the pragmatic properties of this under-researched act. Interlanguage data were collected from 36 adult learners via a peer-feedback task, a written questionnaire, and a retrospective interview. First and second language baseline data were collected from two respective groups of 12 Vietnamese NSs and 12 NSs of Australian English, via the same peer feedback task and questionnaire. Results showed that the English language learners criticized in significantly different ways from the Australian NSs in terms of their preference for realization strategies, their choice of semantic formulae, and their choice and frequency of use of mitigating devices. A number of interplaying factors might explain these differences: learners' limited L2 linguistic competence and lack of fluency, which seemed to load their processing capability under communicative pressure, their lack of L2 pragmatic knowledge, and the influence of L1 pragmatics.

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Streamlining the Design of MEMS Devices: An Acceleration Sensor
 A synthesis and optimization process is proposed and applied to the design of a specific MEMS device, namely an acceleration sensor. The design synthesis methodology exploits the fast and accurate simulation of the SUGAR tool (based on modified modal analysis) along with the full simulation capability of ANSYS (based on the finite element method). A three degrees-of-freedom piezoresistive acceleration sensor was designed to validate the proposed design flow. During the course of design, the modified nodal analysis and the finite element methods were combined in optimizing the sensor structure. In the latter, the piezoresistance effect was employed in sensing the acceleration in three dimensions.
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Holding the line: family responses to pregnancy and the desire for a child in the context of HIV in Vietnam

 Health services around the world offer many guidelines for HIV-positive women who are pregnant or who want to become pregnant, and for women with HIV infected partners. These guidelines are addressed to women and, increasingly, also to men, but pay little or no attention to the role of other members of the family in fertility decisions. This study looked at factors influencing decisions about fertility in families with an HIV-positive member. In Vietnam, the whole family takes a crucial role in deciding whether a woman should become pregnant and whether she will keep her child. This decision is taken in the context not only of the close family but also under the influence of ancestors and the weight given to them within the culture. Key in this regard is the need for parents and grandparents to have male offspring. Health workers share these ideas about preferred family composition and support men and women in the quest for male offspring. Policies and guidelines should take into account these additional family factors and goals as a basis for the design of appropriate programmes to reduce HIV transmission.

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Dealing with a positive result: routine HIV testing of pregnant women in Vietnam

 HIV testing is an essential component of PMTCT. It can be offered to pregnant women through different testing models, ranging from voluntary counseling and testing (VCT) to routine and mandatory testing. This study was conducted in Hanoi, Vietnam, where HIV-prevalence is low among the general population, but high among young, urban, sexually active, male intravenous drug users. Women who want to deliver in a state hospital are routinely tested for HIV in the absence of well-defined opt-out procedures. In-depth interviews with a convenience sample of 38 seropositive pregnant women and mothers and 53 health workers explored the acceptability of routine testing. Patients and healthcare workers appeared to accept routine 'blood' tests (including HIV tests) because they feel uncomfortable discussing issues specific to HIV/AIDS. To avoid having to inform women directly about their HIV status, health workers at routine testing sites rely on the official notification system, shifting the responsibility from the hospitals to district and commune health staff. The notification system in Hanoi informs these local officials about the HIV status of people living in their catchment area without patients' consent. Our study shows that this non-confidential process can have serious social, economic and health consequences for the HIV-positive women and their children.

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Magnetic sensors based on piezoelectric-magnetostrictive composites

 Magnetoelectric (ME) composites have been fabricated by sandwiching a lead titanate (PZT) laminate between two magnetostrictive (Tb(Fe0.55Co0.45)1.5) (known as Terfecohan) films. Giant ME effect at low fields obtained is associated to large magnetostriction as well as high magnetostrictive susceptibility of the Terfecohan films. Magnetoelectric voltage coefficients,αE=(∂E/∂H), as large as 3350 and 9650 V m/kA m were achieved, respectively, for the as-deposited, and annealed films. The coefficientαEwas, however, highly dependent on the direction of the magnetic field with respect to the electrical polarization. On the basic of this magnetoelectric composite, a magnetic sensor operating in an ac magnetic field of 0.1 mT at a resonant frequency of 40 Hz has been prepared. The ME voltage response in applied magnetic fields (dVME/dH)as large as 130 mV/mT was obtained.

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Economic reforms and mangrove forests in central Vietnam

 Community-based natural resource management (CBNRM) is increasingly viewed as especially suitable for promoting the sustainable development of common-pool resources. It is considered an alternative to state control or privatization because it ostensibly brings about decentralization, meaningful participation, and conservation. Although attracting international attention, CBNRM has not been widely implemented in Vietnam, where the main strategies are centralized management by state agencies and assignment of management responsibility to individual households. Findings are presented of a study of mangrove forest use and management in a commune in central Vietnam since the introduction of economic reforms in 1986. This article argues that the promotion of nationalization or privatization, rather than solving problems of resource degradation and overexploitation, has deprived many rural households of their livelihoods. Further research is needed to define what mix of economic incentives and state and community regulation will best achieve and maintain sustainable and equitable management of local resources.
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Improved parallel-iterated pseudo two-step RK methods for nonstiff IVPs

 The aim of this paper is to consider a parallel predictor–corrector (PC) iteration scheme for a general class of pseudo two-step Runge–Kutta methods (PTRK methods) of arbitrary high-order for solving first-order nonstiff initial-value problems (IVPs) on parallel computers. Starting with ans-stage pseudo two-step RK method of orderp ∗ withwimplicit stages, we apply a highly parallel PC iteration process inPE(CE) m Emode. The resulting parallel PC method can be viewed as a parallel-iterated pseudo two-step Runge–Kutta method (PIPTRK method) with an improved (new) predictor formula and therefore will be called the improved PIPTRK method (IPIPTRK method). The IPIPTRK method uses an optimal number of processors equal tow p ∗ /2. Numerical experiments show that the IPIPTRK methods proposed in this paper are superior to the efficient sequentialDOPRI5andDOP853 codes and parallel PIRK methods available in the literature.

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