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1999

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Articles 2221 - 2250 of 2555

Full-Text Articles in Physical Sciences and Mathematics

Giant Efficiency Of Fröhlich Interaction In Self-Assembled Quantum Dots, A. W. E. Minnaert, A. Yu. Silov, J. E. M. Haverkort, J. H. Wolter, A. Garcia, V. N. Gladilin, V. M. Fomn, J. T. Devreese Jan 1999

Giant Efficiency Of Fröhlich Interaction In Self-Assembled Quantum Dots, A. W. E. Minnaert, A. Yu. Silov, J. E. M. Haverkort, J. H. Wolter, A. Garcia, V. N. Gladilin, V. M. Fomn, J. T. Devreese

Turkish Journal of Physics

A transition is being observed from an inhomogeneously broadened photoluminescence band under non-resonant excitation of InAs/GaAs Self-Assembled quantum Dots (SAD's) into up to five phonon-assisted emission bands under selective excitation. A similar effect is obtained from photoluminescence excitation experiments (PLE). We interpret the phonon-assisted PL as being due to a giant efficiency of the Fröhlich interaction between an exciton polarized by strain in the SAD and LO-phonons. The model is consistent with the pronounced p-type polarization of the emission observed in our cleaved-side PL-measurements. Further support is obtained from our calculations in which a different localization of the electrons and …

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Electron Liquid Beyond The Random Phase Approximation, Mehmet Tomak, Ceyhun Bulutay Jan 1999

Electron Liquid Beyond The Random Phase Approximation, Mehmet Tomak, Ceyhun Bulutay

Turkish Journal of Physics

A deeper understanding of low-dimensional and small-scale electronic systems requires an accurate characterization of the many-electron effects. Based on the electron liquid (EL) model, these many-body effects in the three-dimensional (3D) and two-dimensional (2D) electronic systems are dealt going beyond the random phase approximation within the zero-temperature framework. The dielectric formulation of the many-body problem is recapitulated and the longitudinal dielectric function of the EL is obtained using the self-consistent local-field correction scheme, known as STLS, under a general spin-valley degeneracy. Extensive explicit expressions are provided for the use of other researchers. The performance of this formalism is compared in …

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Energy Relaxation Rate Of Hot Electrons In N-Type Gan Epilayers Using Heat Pulse Techniques, P. Hawker, A. J. Kent, T. S. Cheng, C. T. Foxon Jan 1999

Energy Relaxation Rate Of Hot Electrons In N-Type Gan Epilayers Using Heat Pulse Techniques, P. Hawker, A. J. Kent, T. S. Cheng, C. T. Foxon

Turkish Journal of Physics

We have measured the energy relaxation rate of hot electrons in MBE grown bulk GaN epilayers on GaAs and sapphire substrates over the electron temperature range 1 - 130 K. The measurements were made using heat pulse techniques. For layers grown on GaAs substrates the results show that the carriers reside in the substrate, probably as in a GaAs/AlGaAs heterojunction. For layers grown on sapphire substrates we obtain a P \propto T^4_e dependence for the relaxation rate in the low temperature limit, consistent with piezoelectric coupling in the so-called `dirty' regime, changing to a linear dependence in the high temperature, …

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Feasibility Of The Gainp/Ingaas/Gaas System For Modulation Doped Field-Effect Transistors, C. Besi̇kci̇, Y. Ci̇van Jan 1999

Feasibility Of The Gainp/Ingaas/Gaas System For Modulation Doped Field-Effect Transistors, C. Besi̇kci̇, Y. Ci̇van

Turkish Journal of Physics

Si-doped single Ga_{0.51}In_{0.49}P layers and GaInP/InGaAs/GaAs modulation doped field-effect transistor structures grown by gas source molecular beam epitaxy were characterized in detail through Hall-Effect and Deep Level Transient Spectroscopy techniques. Electrical characterization of the undoped and moderately Si-doped (N_D = 3 x 10^{17} cm^{-3}) GaInP layers yielded an electron trap with an activation energy of 0.75 eV and a temperature dependent capture cross section with a capture barrier of 0.593 eV showing DX center properties. The trap was not detected in highly Si doped (N_D = 4 x 10^{18} cm^{-3}) as grown layers suggesting that it is a defect complex …

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The Concept Of Free Electromagnetic Field In Quantum Domain, Alexander Shumovsky, Özgür Müstecaplioğlu Jan 1999

The Concept Of Free Electromagnetic Field In Quantum Domain, Alexander Shumovsky, Özgür Müstecaplioğlu

Turkish Journal of Physics

By virtue of the consideration of polarization and phase properties of dipole radiation in the quantum domain, it is shown that the concept of free electromagnetic field should be considered as a quite risky approximation in the description of quantum fluctuations of some physical observables.

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Bands Of Localized Electromagnetic Waves In Random Collections Of Dielectric Particles, Marian Rusek, Arkadiusz Orlowski Jan 1999

Bands Of Localized Electromagnetic Waves In Random Collections Of Dielectric Particles, Marian Rusek, Arkadiusz Orlowski

Turkish Journal of Physics

Anderson localization of electromagnetic waves in random arrays of dielectric cylinders is studied. An effective theoretical approach based on the finite size scaling analysis of transmission is developed. The disordered dielectric medium is modeled by a system of randomly distributed 2D electric dipoles. The appearance of the band of localized waves emerging in the limit of an infinite medium is discovered. It suggests deeper insight into existing experimental and theoretical results.

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Gauge Theory Of Phase And Scale, Marek Pawlowski Jan 1999

Gauge Theory Of Phase And Scale, Marek Pawlowski

Turkish Journal of Physics

Old Weyl's the idea of scale recalibration freedom and Infeld's and van der Waerden's (IW) ideas concerning geometrical interpretation of natural spinor phase gauge symmetry are discussed in the context of modern models of fundamental particle interactions. It is argued that (IW) gauge symmetry can be naturaly identified with the U(1) symmetry of the Weinberg-Salam model. It is also argued that there are no serious reasons to reject Weyl's gauge theory from consideration. Its inclusion enriches the original Weinberg-Salam theory and leads to prediction of new phenomena that do not contradict experiments.

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The Wigner Problem In Electrodynamics, Andrzej Horzela Jan 1999

The Wigner Problem In Electrodynamics, Andrzej Horzela

Turkish Journal of Physics

The relation between canonical commutation rules and a shape of acting force is analysed. It is shown that canonical commutation relations and Newtonian equations of motion for a single particle dynamics do imply the force to be of the Lorentz type but the inverse is not true. The example of a single particle motion in a constant magnetic field shows that equations of motion allow an alternative to canonical commutation relations to exist and it is a Lie algebra. The algebra found is of the type of algebras found in studies leading towards noncommutative geometry approaches to physical problems.

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Effect Of Ionizing Radiation On The Silicon Impatt Diode Characteristics, M. B. Tagaev Jan 1999

Effect Of Ionizing Radiation On The Silicon Impatt Diode Characteristics, M. B. Tagaev

Turkish Journal of Physics

We investigated the effect of ^{60}Co \gamma-irradiation (doses from 10^2 to 2 \times 10^6 Gy), both without and with heat annealing, on silicon IMPATT diode parameters. It is shown that such treatments improve the diode characteristics (particularly decrease the reverse current and increase both the output and the diffusion length of the minority charge carriers) due to radiation-enhanced processes.

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Cyclic Nature Of The Orbital Period Variations Of Some Algol-Type Binaries, Seli̇m O. Selam, Osman Demi̇rcan Jan 1999

Cyclic Nature Of The Orbital Period Variations Of Some Algol-Type Binaries, Seli̇m O. Selam, Osman Demi̇rcan

Turkish Journal of Physics

The long-term period variations of the classical algol-type binaries XZ And, U Cep, \beta Per, RT Per, ST Per and TX UMa were studied by using all available observed times of eclipse minima. The (O-C) diagrams formed by the times of eclipse minima were found to be representable by one or more cyclic variations superimposed on a secular parabolic variation. The secular variations in the orbital periods should be caused by the mass transfer between the component stars and the mass loss from the systems. For our sample of Algol systems, none of the cyclic variations were found caused by …

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Band-Gap Renormalization In Quantum Wires, K. Güven, C. R. Bennett, B. Tanatar Jan 1999

Band-Gap Renormalization In Quantum Wires, K. Güven, C. R. Bennett, B. Tanatar

Turkish Journal of Physics

Improved techniques in semiconductor fabrication increased the interest in quantum wire structures, because of their opto-electronic device application possibilities. Many-body interactions among the electrons and holes in the wire lead to the band-gap renormalization (BGR), which in turn affect the optical properties of the system. We study the BGR within the random-phase approximation incorporating the dynamical effects, and investigate the density dependence.

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Carrier Transport And Band Offsets In Two Dimensional Heterostructures, Hi̇lmi̇ Ünlü Jan 1999

Carrier Transport And Band Offsets In Two Dimensional Heterostructures, Hi̇lmi̇ Ünlü

Turkish Journal of Physics

Advances in the low dimensional heterojunction device technology cannot be complete without adequate and reliable analytic model for determining interface properties such as band offsets needed to study the carrier transport and to evaluate the device performance at high temperatures and pressures. In this article, using the extended universal tight binding model of semiconductors, a new way of determining the band offsets in heterostructures is presented. In this model the band offsets are first determined by aligning the vacuum level, defined relative to valence band maximum which is screened by optical dielectric constant of semiconductors, at the interface at absolute …

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Multitime Generalization Of Maxwell Electrodynamics And Gravity, Vladilen Barashenkov Jan 1999

Multitime Generalization Of Maxwell Electrodynamics And Gravity, Vladilen Barashenkov

Turkish Journal of Physics

System of equations of motion for interacting electromagnetic field and charged bodies is derived by means of an action principle in six-dimensional space with a three-dimensional time vector. The influence of the additional (hidden) components of time on the body motion is considered. Macroscopic experiments and astrophysical observations which can reveal the time multi-dimensionality are discussed. At present there are no facts which would be in contradiction with the hypothesis of the multi-dimensionality of time.

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New Aspects Of Vavilov-Cherenkov Radiation, Georgy Afanasiev, Vladimir Kartavenko Jan 1999

New Aspects Of Vavilov-Cherenkov Radiation, Georgy Afanasiev, Vladimir Kartavenko

Turkish Journal of Physics

The effects arising from accelerated and decelerated motion of charged point particles inside a medium are studied. It is shown explicitly that, in addition to the bremsstrahlung and Cherenkov shock wave, the electromagnetic shock wave arising from the charge velocity exceeding the medium light velocity should be observed. This shock wave has the same singularity as the Cherenkov one and therefore, it is more singular than the bremsstrahlung shock wave. The space-time regions where these shock waves exist and conditionsunder which they appear are determined.

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Projectile Fragmentation In Emulsion At (4.1-4.5) A Gev/C, A. El-Naghy, S. A. H. Abou-Steit, M. Mohery Jan 1999

Projectile Fragmentation In Emulsion At (4.1-4.5) A Gev/C, A. El-Naghy, S. A. H. Abou-Steit, M. Mohery

Turkish Journal of Physics

The fragmentation process has been investigated for 4.5A GeV/c^{28}Si nuclei in emulsion and compared with the results of 4.5A GeV/c^{24}Mg and 4.1A GeV/c^{22}Ne in order to test the validity of the different theoretical models. It has been found that a single parameter distribution is insufficient to explain exactly the fragmentation process. Correlation studies have shown to be necessary for distinguishing between the different theoretical models for the fragmentation. The impact parameter, which defines the nature of the collision, has been found to influence considerably the shape of the charge yield distribution. The angular distributions of the projectile fragments can be …

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Assessment Of The Degree Of Mix Of Powder Mixtures, Orhan Vai̇zoğlu Jan 1999

Assessment Of The Degree Of Mix Of Powder Mixtures, Orhan Vai̇zoğlu

Turkish Journal of Physics

The older methods for the determination of the degree of a random mix was solely based on the calculation of a simple standard deviation. Physical characteristics like, density, average particle weight and size of each component was not considered. Therefore, the evaluation, when the state of a random mix is reached, was somewhat superficial. Poole, Taylor, and Wall [1] have derived equations, where the physical characteristics of the components were taken into consideration. By this means, it is possible to calculate, in advance, the theoretical coefficient of variation and the corresponding confidence limits. One can then experimentally follow under which …

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Spectral Broadening Functions, Slavek Rucinski Jan 1999

Spectral Broadening Functions, Slavek Rucinski

Turkish Journal of Physics

Many astrophysical phenomena involving velocity fields produce broadening of spectral lines. Frequently, the cross-correlation function (CCF) is used to extract information about this broadening from segments of high resolution spectra. CCF has become the standard tool for handing extraction of radial-velocity and broadening information from high resolution spectra as it permits integration of information which is common to many spectral lines into one function which is easy to calculate, visualize and interpret. However, it can be argued that this is not the best tool for applications such as determinations of metallicities ([Fe/H]), for finding locations of star spots on active …

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Virginia Game Fish Tagging Program Annual Report 1998, John A. Lucy, C.M. Bain, M. D. Arendt Jan 1999

Virginia Game Fish Tagging Program Annual Report 1998, John A. Lucy, C.M. Bain, M. D. Arendt

Reports

The Virginia Game Fish Tagging Program (VGFTP), a cooperative project of the Virginia Marine Resources Commission and the Virginia Institute of Marine Science (VIMS), set records for fish tagged and fish recaptured during 1998, its fourth year of operation.

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Land Use In The Karstic Lands In The Mediterranean Region, Ibrahim Atalay Jan 1999

Land Use In The Karstic Lands In The Mediterranean Region, Ibrahim Atalay

International Journal of Speleology

Karstic lands have special importance in terms of soil formation and land-use. Soil appears only on the flat and slightly undulating karstic lands, while soils are found along the cracks and bedding surfaces between the layers on the hilly karst areas although these lands are rocky in appearance. Karstic lands in the hilly area are not conducive to cultivation. But rocky areas create a favourable habitat for the growth of forests except in an arid climate. Because the tree roots easily follow and develop along the cracks in the limestone. As a general rule soil erosion does not occur on …

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Synthesis Of Novel Sesquiterpene Lactones: Heritianin, Vallapianin And Vallapin, Kostas Gavardinas '99 Jan 1999

Synthesis Of Novel Sesquiterpene Lactones: Heritianin, Vallapianin And Vallapin, Kostas Gavardinas '99

Honors Projects

For decades considerable research has focused on improving the efficiency of crop protection. Many naturally occurring biodegradable compounds have often been used as pesticides and with increased environmental concerns, the identification of biologically active natural products has attracted significant attention. Fish toxicity has often been used as a measure of pesticidal activity. Several Southeast Asian mangrove species have been shown to possess ichthyotoxic properties. A bioassay of the extracts obtained from Heritiera littoralis, a Philippine mangrove plant, indicated toxicity toward fish thus suggesting the possibility of their use as pesticides. Three sesquiterpene lactones, Heritianin, Vallapin, and Vallapianin have been isolated …

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Oxidation Of Benzoins To Benzils Using Bismuth(Iii) Nitrate-Copper(Ii) Acetate, Ram Mohan, Steven Tymonko, Bryce Nattier Jan 1999

Oxidation Of Benzoins To Benzils Using Bismuth(Iii) Nitrate-Copper(Ii) Acetate, Ram Mohan, Steven Tymonko, Bryce Nattier

Scholarship

Benzoins are oxidized to benzils in excellent yields by 0.4 equivalents of Bi(NO3)3.5H20 and 4 mol% of Cu(OAc)23)3.5H20 is a stable, inexpensive, commercially available solid. The relatively low cost and low toxicity of bismuth(Ill) nitrate makes this procedure a particularly attractive method for oxidation of benzoins.

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Classification Of Torbanite And Cannel Coal. I. Insights From Petrographic Analysis Of Density Fractions., Zhiwen Han, Michael A. Kruge, John C. Crelling, David F. Bensley Jan 1999

Classification Of Torbanite And Cannel Coal. I. Insights From Petrographic Analysis Of Density Fractions., Zhiwen Han, Michael A. Kruge, John C. Crelling, David F. Bensley

Department of Earth and Environmental Studies Faculty Scholarship and Creative Works

Torbanite and cannel coal are considered to be coals because of their low mineral content and overall physical morphology. However, the texture and composition of the organic matter in torbanite and cannel coal are similar to the kerogen occurring in oil shales and lacustrine source rocks. Therefore, understanding the nature and origin of organic components in torbanite and cannel coal is of significance in the study of kerogen and petroleum formation. In this research, a set of torbanites and cannel coals from different locations throughout the world were petrographically characterized and processed using a density gradient centrifugation (DGC) technique. Microscopically, …

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Performance Surface Prediction For Wan-Based Clusters, Mark J. Clement, J. Kelly Flanagan, Glenn M. Judd, Bryan S. Morse, Joy L. Peterson Jan 1999

Performance Surface Prediction For Wan-Based Clusters, Mark J. Clement, J. Kelly Flanagan, Glenn M. Judd, Bryan S. Morse, Joy L. Peterson

Faculty Publications

The last five years have been a period of exponential growth in the number of machines connected to the Internet and the speed at which these machines communicate. The infrastructure is now in place to consider a nationwide cluster of workstations as a viable parallel processing platform. In order to achieve acceptable performance on this kind of a machine, performance prediction tools must provide information on where to place computational objects. Incorrect object placement can result in poor performance and congestion in the network. This research develops a new paradigm for predicting performance in the Wide Area Network (WAN) based …

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Molecular Organic Geochemistry Of New York Bight Sediments. Sources Of Biogenic Organic Matter And Polycyclic Aromatic Hydrocarbons, Michael A. Kruge Jan 1999

Molecular Organic Geochemistry Of New York Bight Sediments. Sources Of Biogenic Organic Matter And Polycyclic Aromatic Hydrocarbons, Michael A. Kruge

Department of Earth and Environmental Studies Faculty Scholarship and Creative Works

As part of a multidisciplinary evaluation of the environmental impact of waste disposal in the New York Bight, a preliminary effort was undertaken to assess the molecular geochemistry of its sedimentary organic matter (OM). For this initial phase of the study, samples were taken from two New York Bight cores collected outside the area of known dumping [in Sandy Hook Bay, NJ], to establish the environmental context for future work on sediments from within dumpsite zones. The method employed was flash pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), which is an organic microanalysis technique suitable for solid samples, in this case dried, raw …

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Experimental Observation Of Individual Single Wall Nanotube Species By Raman Microscopy, G. S. Duesberg, W. Blau, Hugh Byrne, J. Muster, M. Burghard, S. Roth Jan 1999

Experimental Observation Of Individual Single Wall Nanotube Species By Raman Microscopy, G. S. Duesberg, W. Blau, Hugh Byrne, J. Muster, M. Burghard, S. Roth

Articles

Individual single-wall carbon nanotubes (SWNT) or small ropes of SWNTs with the same diameter have been characterised by Raman spectroscopy. Highly pure, length-selected SWNTs adsorbed onto substrates designed for surface-enhanced Raman spectroscopy were investigated with a Raman microscope. Various spectra, each with a distinct sharp peak in the radial breathing mode area and four C–C stretching modes could be observed. The modes also showed a strong variation in the relative intensities. In correlation with theoretical predictions this should establish Raman microscopy as a non-destructive nano-technological tool, capable of determining the diameter, symmetry and position of individual SWNTs.

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Direct Current Glow Discharges In Atmospheric Air, Robert H. Stark, Karl H. Schoenbach Jan 1999

Direct Current Glow Discharges In Atmospheric Air, Robert H. Stark, Karl H. Schoenbach

Bioelectrics Publications

Direct current glow discharges have been operated in atmospheric air by using 100 μm microhollow cathode discharges as plasma cathodes. The glow discharges were operated at currents of up to 22 mA, corresponding to current densities of 3.8 A/cm2 and at average electric fields of 1.2 kV/cm. Electron densities in the glow are in the range from 1012 to 1013  cm−3. Varying the current of the microhollow cathode discharge allows us to control the current in the atmospheric pressure glow discharge. Large volume atmospheric pressure air plasmas can be generated by operating microhollow cathode discharges …

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Analysis Of The Gal-Chen–Zhang Single-Doppler Velocity Retrieval, Steven Lazarus, Alan Shapiro, Kelvin Droegemeier Jan 1999

Analysis Of The Gal-Chen–Zhang Single-Doppler Velocity Retrieval, Steven Lazarus, Alan Shapiro, Kelvin Droegemeier

Ocean Engineering and Marine Sciences Faculty Publications

The authors present herein an analysis of a single-Doppler velocity retrieval (SDVR) technique whereby the unobserved wind components are determined from single-Doppler radar data. The analysis is designed to provide information about the behavior and/or sensitivity of the SDVR scheme as a function of various internal and external parameters as well as about observational errors and weights. Results presented for retrieval of both the mean and local flow indicate that the SDVR breaks down if the reflectivity gradient vanishes or if a reflectivity isoline is locally perpendicular to the radar beam. In the absence of reflectivity or radial velocity errors, …

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On-Road Remote Sensing Of Automobile Emissions In The Chicago Area: Year 2, Peter J. Popp, Gary A. Bishop, Donald H. Stedman Jan 1999

On-Road Remote Sensing Of Automobile Emissions In The Chicago Area: Year 2, Peter J. Popp, Gary A. Bishop, Donald H. Stedman

Fuel Efficiency Automobile Test Publications

No abstract provided.

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On-Road Remote Sensing Of Automobile Emissions In The Denver Area: Year 1, Peter J. Popp, Sajal S. Pokharel, Gary A. Bishop, Donald H. Stedman Jan 1999

On-Road Remote Sensing Of Automobile Emissions In The Denver Area: Year 1, Peter J. Popp, Sajal S. Pokharel, Gary A. Bishop, Donald H. Stedman

Fuel Efficiency Automobile Test Publications

No abstract provided.

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Rsd Studies In Austin, 1998, Sajal S. Pokharel, Donald H. Stedman, Gary A. Bishop Jan 1999

Rsd Studies In Austin, 1998, Sajal S. Pokharel, Donald H. Stedman, Gary A. Bishop

Fuel Efficiency Automobile Test Publications

No abstract provided.

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