Showing papers by "Australian National University published in 1996"

Harmonic Vibrational Frequencies: An Evaluation of Hartree−Fock, Møller−Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors

Abstract: Scaling factors for obtaining fundamental vibrational frequencies, low-frequency vibrations, zero-point vibrational energies (ZPVE), and thermal contributions to enthalpy and entropy from harmonic frequencies determined at 19 levels of theory have been derived through a least-squares approach. Semiempirical methods (AM1 and PM3), conventional uncorrelated and correlated ab initio molecular orbital procedures [Hartree−Fock (HF), Moller−Plesset (MP2), and quadratic configuration interaction including single and double substitutions (QCISD)], and several variants of density functional theory (DFT: B-LYP, B-P86, B3-LYP, B3-P86, and B3-PW91) have been examined in conjunction with the 3-21G, 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-311G(d,p), and 6-311G(df,p) basis sets. The scaling factors for the theoretical harmonic vibrational frequencies were determined by a comparison with the corresponding experimental fundamentals utilizing a total of 1066 individual vibrations. Scaling factors suitable for low-frequency vib...

Interleukin 5 deficiency abolishes eosinophilia, airways hyperreactivity, and lung damage in a mouse asthma model.

Abstract: Airways inflammation is thought to play a central role in the pathogenesis of asthma. However, the precise role that individual inflammatory cells and mediators play in the development of airways hyperreactivity and the morphological changes of the lung during allergic pulmonary inflammation is unknown. In this investigation we have used a mouse model of allergic pulmonary inflammation and interleukin (IL) 5-deficient mice to establish the essential role of this cytokine and eosinophils in the initiation of aeroallergen-induced lung damage and the development of airways hyperreactivity. Sensitization and aerosol challenge of mice with ovalbumin results in airways eosinophilia and extensive lung damage analogous to that seen in asthma. Aeroallergen-challenged mice also display airways hyperreactivity to beta-methacholine. In IL-5-deficient mice, the eosinophilia, lung damage, and airways hyperreactivity normally resulting from aeroallergen challenge were abolished. Reconstitution of IL-5 production with recombinant vaccinia viruses engineered to express this factor completely restored aeroallergen-induced eosinophilia and airways dysfunction. These results indicate that IL-5 and eosinophils are central mediators in the pathogenesis of allergic lung disease.

Evidence for life on Earth before 3,800 million years ago.

Abstract: It is unknown when life first appeared on Earth. The earliest known microfossils (approx. 3,500 Myr before present) are structurally complex, and if it is assumed that the associated organisms required a long time to develop this degree of complexity, then the existence of life much earlier than this can be argued. But the known examples of crustal rocks older than approx. 3,500 Myr have experienced intense metamorphism, which would have obliterated any fragile microfossils contained therein. It is therefore necessary to search for geochemical evidence of past biotic activity that has been preserved within minerals that are resistant to metamorphism. Here we report ion-microprobe measurements of the carbon-isotope composition of carbonaceous inclusions within grains of apatite (basic calcium phosphate) from the oldest known sediment sequences a approx. 3,800 Myr-old banded iron formation from the Isua supracrustal belt, West Greenland, and a similar formation from the nearby Akilia island that is possibly older than 3,850 Myr. The carbon in the carbonaceous inclusions is isotopically light, indicative of biological activity; no known abiotic process can explain the data. Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present.

Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study

Abstract: Zircons from a metasedimentary and a meta-igneous quartz-feldspar granulite from the Val Sesia and Val Mastallone area of the Ivrea Zone (Southern Alps) differ in their response to granulite facies metamorphism with respect to crystal morphology and U/Pb ages. Detrital zircons in the metasediment developed an isometric overgrowth by the addition of Zr derived mainly from co-existing minerals, most probably biotite, decomposing during anatectic melting. The overgrowth started in the pelitic layer of the metasediment in the Late Carboniferous at approximately 296 Ma, significantly earlier than in the adjacent psammitic layer where it started only at 261 ± 4 Ma (95% confidence level). These ages are ascribed to the differential initiation of anatexis in the two layers. The delay of melting in the psammitic layer was probably due to the different position and less steep slope of its solidus in P-T-space, as compared to the solidus in the pelitic layer. Accordingly, the melting in the psammitic layer at 261 Ma was initiated by a thermal pulse and/or by a decompression event. Decompression melting is supported by a characteristic shell morphology of the zircon overgrowth in the psammitic layer, which might have grown under shear movements during high-temperature extensional faulting. The typically magmatic zircon population of the meta-igneous granulite crystallized at 355 ± 6 Ma (95% confidence level). The morphology of the zircons and the chemistry of the rock suggest that the magma was calcalkaline. A minor subpopulation of crystals is morphologically similar to the zircons in the pelitic layer of the metasediment. This points to the admixture of a minor sediment component and, thus, to a largely volcaniclastic origin of the protolith. In contrast to the detrital zircons in the metasediment, the magmatic zircons show rare and little overgrowth and, instead, have been strongly resorbed by anatectic melt. In addition, they became partially recrystallized and the rejuvenated ages from the most thoroughly recrystallized domains indicate that the rock was subject to prograde metamorphism after 279 Ma. This may correspond to the regional temperature increase prior to the climax of metamorphism or to a local thermal pulse due to nearby mafic intrusions. An Upper Triassic event at 226 ± 5 Ma is reflected by distinct peripheral zones in the overgrowths of some zircons in the metapelite. These are interpreted as a second metamorphic pulse, possibly induced by the infiltration of fluids.

The theory of externalities, public goods, and club goods: Homogeneous clubs and local public goods

Abstract: A club is a voluntary group of individuals who derive mutual benefit from sharing one or more of the following: production costs, the members' characteristics, or a good characterized by excludable benefits. When production costs are shared and the good is purely private, a private good club is being analyzed (McGuire 1972; Wiseman 1957). If membership characteristics differ and motivate sharing, then membership fees will differ among members (DeSerpa 1977; Scotchmer 1994b; Scotchmer and Wooders 1987). Such fees are nonanonymous , inasmuch as a fee structure is related to the identity and attributes of a member. The focus of our analysis is the sharing of an excludable (rivalrous) public good, which we term a club good . Unless otherwise specified, crowding is assumed to be independent of the individual and hence anonymous. A number of aspects of the club definition deserve highlighting. Privately owned and operated clubs must be voluntary; members choose to belong because they anticipate a net benefit from membership. Thus, the utility jointly derived from membership and from the consumption of other goods must exceed the utility associated with nonmembership status. Furthermore, the net gain in utility from membership must exceed or equal membership fees or toll payments. This voluntarism serves as the first characteristic by which to distinguish between pure public goods and club goods. In the case of a pure public good, voluntarism may be absent, since the good might harm some recipients (e.g., defense to a pacifist, fluoridation to someone who opposes its use).

IFN-gamma plays a critical down-regulatory role in the induction and effector phase of myelin oligodendrocyte glycoprotein-induced autoimmune encephalomyelitis.

Abstract: 129/Sv mice are resistant to induction of experimental autoimmune encephalomyelitis (EAE) induced with myelin oligodendrocyte glycoprotein peptide (MOG35-55). Mice of this strain lacking the gene coding for the ligand-binding chain of the IFN-gamma receptor develop EAE with high morbidity and mortality. Spleen cells from sensitized IFN-gammaR-/- mice proliferated extensively when stimulated with MOG peptide in culture and produced high levels of IFN-gamma and TNF but no detectable IL-4. Transfer of spleen cells from sensitized IFN-gammaR-/- mice produced EAE in both IFN-gammaR+/+ and IFN-gammaR-/- recipients. Disease was severe in IFN-gammaR-/- recipients and mortality high (77%). Surviving mice remained moribund until termination of the experiments. IFN-gammaR+/+ recipients developed disease of equal severity, but with no mortality, and recovered significantly. These results indicate that IFN-gamma is not essential for the generation or function of anti-MOG35-55 effector cells but does play an important role in down-regulating EAE at both the effector and induction phase of disease.

Integrable structure of conformal field theory, quantum KdV theory and thermodynamic Bethe ansatz

Abstract: We construct the quantum versions of the monodromy matrices of KdV theory. The traces of these quantum monodromy matrices, which will be called as “T-operators,” act in highest weight Virasoro modules. TheT-operators depend on the spectral parameter λ and their expansion around λ=∞ generates an infinite set of commuting Hamiltonians of the quantum KdV system. TheT-operators can be viewed as the continuous field theory versions of the commuting transfermatrices of integrable lattice theory. In particular, we show that for the values\(c = 1 - 3\frac{{3(2n + 1)^2 }}{{2n + 3}}\),n=1,2,3 .... of the Virasoro central charge the eigenvalues of theT-operators satisfy a closed system of functional equations sufficient for determining the spectrum. For the ground-state eigenvalue these functional equations are equivalent to those of the massless Thermodynamic Bethe Ansatz for the minimal conformal field theoryM2,2n+3; in general they provide a way to generalize the technique of the Thermodynamic Bethe Ansatz to the excited states. We discuss a generalization of our approach to the cases of massive field theories obtained by perturbing these Conformal Field Theories with the operator Φ1,3. The relation of theseT-operators to the boundary states is also briefly described.

Climate Change and Human Health

Abstract: The article “Apocalypse not” by Gary Taubes (News & Comment, 7 Nov., [p. 1004][1]) addresses the issue of fundamental differences of opinion among health scientists about the impact of climate on human health. While we acknowledge that there are strong differences in opinion about the potential consequences of future climate change on disease incidence and distribution, we share common concerns; we wish to emphasize that despite any differences, there are many areas where we agree. The key questions behind the climate/health research agendas are, How will climate change alter health risks, to what extent will risks be altered, and what can be done to mitigate any potential increase in health risks? At issue is not which is more important, climate factors or improved health measures; rather, it is important to assess how health risks might change in both industrialized and more vulnerable developing countries. The complexity of this public health issue entails far more uncertainty than many health hazards with which we are familiar. Impacts may occur indirectly through simultaneous disturbances of other sectors, including water supply, food production, or habitat. Thus far, scientists have found great difficult in communicating this extra level of uncertainty. We agree on the need to improve understanding of the complex relationships between climatic conditions and disease transmission dynamics. We also agree that disease incidence is influenced by multiple factors (none of us will argue that climate is the only or the most important factor). Well-designed research studies must be conducted to gain a better understanding of how these multiple factors relate to each other and how all might be influenced by climate. Identifying risk factors that influence disease transmission is a key to public health planning, and as more data from climate/health research studies become available, the influence of weather will be better understood. We recognize that extreme weather events such as those that may accompany this year's El Nino place an extra burden on sanitation and general public health systems. The early regional forecasts obtained from El Nino exemplify important new predictive capabilities that public health officials can use in their public health planning. Interdisciplinary research and interagency cooperation can go far toward improving the health risk assessment associated with climate change. Ecology-based research and monitoring combined with advances in climate forecasting will enhance our understanding of complex environmental health hazards and may provide the public with early warning systems that allow timely public health interventions. The signatories of this letter agree that public health is of great importance and that public health infrastructure and services must be improved worldwide. We recognize that environmental and socioeconomic conditions underpin health status; effective and sustainable public health prevention will ultimately require improvement in these underlying conditions. It is important to realize, however, that the projected climate change may have a profound influence on an aspects of human ecology, and we strongly recommend that research be supported to allow development of effective prevention strategies that will help mitigate its effect on public health. [1]: /lookup/doi/10.1126/science.278.5340.1004

Extremely Metal-Poor Stars. II. Elemental Abundances and the Early Chemical Enrichment of The Galaxy*

Abstract: We have obtained high-resolution spectra of 23 very metal-poor stars and present an abundance analysis for 19 of these for elements between Mg and Eu. The sample comprises roughly equal numbers of dwarfs and giants. All stars have [Fe/H] 0 deserve further study. CS 22897–008 has high Sr, Y, and C abundances for its [Fe/H] but normal Ba. This signature may have arisen from the weak s-process in M > 15 M stars or by r-processing. By combining an analytic description of gaseous supernova remnants with supernova yields, we show that enrichment of the interstellar medium is influenced more by supernova physics (explosive energy) than by environmental conditions (cloud density). If supernova iron-peak yields are correlated with explosion energy, we can accommodate the well-defined abundance trends with a chaotic picture for halo formation involving independently evolving clouds, as was envisaged by Searle & Zinn. We calculate that a typical enrichment in the protohalo will produce [Fe/H] = −2.7. This coincides with larger abundance variations in field stars of lower metallicity and the lower abundance limit for Galactic globular clusters.

Quasi-steady-state photoconductance, a new method for solar cell material and device characterization

Abstract: This paper describes a new method for minority-carrier lifetime determination using a contactless photoconductance instrument in a quasi-steady-state mode. Compared to the more common transient photoconductance decay approach, the new technique permits the use of simpler electronics and light sources, yet has the capability to measure lifetimes in the nanosecond to millisecond range. In addition, by analyzing the quasi-steady-state photoconductance as a function of incident light intensity, an implicit I/sub SC/-V/sub OV/ curve can be obtained for noncontacted silicon wafers and solar cell precursors.

Regulation of protein transport to the nucleus: Central role of phosphorylation

Abstract: Nuclear protein transport is integral to eukaryotic cell processes such as differentiation, transformation, and the control of gene expression. Although the targeting role of nuclear localization signals (NLSs) has been known for some time, more recent results indicate that NLS-dependent nuclear protein import is precisely regulated. Phosphorylation appears to be the main mechanism controlling the nuclear transport of a number of proteins, including transcription factors such as NFkappaB, c-rel, dorsal, and SWI5 from yeast. Cytoplasmic retention factors, intra- and intermolecular NLS masking, and NLS masking by phosphorylation are some of the mechanisms by which phosphorylation specifically regulates nuclear transport. Even nuclear localization of the archetypal NLS-containing simian virus 40 large tumor antigen (T-ag) is regulated, namely by the "CcN motif," which comprises the T-ag NLS ("N") determining ultimate subcellular destination, a casein kinase II site ("C") 13 amino acids NH2-terminal to the NLS modulating the rate of nuclear import, and a cyclin-dependent kinase site ("c") adjacent to the NLS regulating the maximal level of nuclear accumulation. The CcN motif appears to be a special form of phosphorylation-regulated NLS (prNLS), where phosphorylation at site(s) close to the NLS specifically regulates NLS function. The regulation of nuclear transport through phosphorylation and prNLSs appears to be common in eukaryotic cells from yeast and plants to higher mammals.

Hubble Space Telescope Observations of the Disk and Jet of HH 30

Abstract: HH 30 in Taurus has been imaged with the Hubble Space Telescope WFPC2. The images show in reflected light a flared disk with a radius of about 250 AU that obscures the protostar. The disk resembles detailed accretion disk models that constrain its density distribution and show that its inclination is less than 10 degrees. There are bipolar emission-line jets perpendicular to the disk, a very clear demonstration of the standard paradigm for accretion disk and jet systems. However, asymmetries in the light distribution show that the disk has not completely settled into a quasi-equilibrium accretion state, or that some of the observed scattering is from an asymmetric envelope. The emission-line jet itself is resolved into a number of knots with typical lengths and separations of 0".4, much smaller and more numerous than indicated by lower resolution ground-based studies. There are indications of still finer structures in the jet all the way to the resolution limit of 0".1. The knots have proper motions ranging from 100 to 300 km s^(-1) and are therefore generated at the surprisingly high rate of about 0.4 knots per jet per year. The jet appears to be collimated within a cone of opening angle 3o and can be seen to within 30 AU of the star. Both single- and multiple-scattering disk models have a range of possible solutions, but by requiring pressure support and temperature equilibrium, a self-consistent model emerges. There is evidence for pressure support because the disk appears to have a Gaussian height profile. The temperature at each point in the disk is determined by the disk geometry, which in turn fixes the temperature in a self-consistent manner. The extinction to the protostar is unknown but constrained to be greater than 24 mag. The optical properties of the scattering grains in the disk are determined and found to imply a large scattering asymmetry, but they seem to follow the interstellar reddening law. The absolute magnitude and colors of the unseen protostar, which has a brightness in the I bandpass of about 0.16 times solar and is very red, are obtained. The disk mass is about 0.006 times solar and has an expected lifetime of about 10^5 yr.

Carbon Dioxide Diffusion inside Leaves

Abstract: CO2 reaches Rubisco by diffusion down a concentration gradient The gradient forms during photosynthesis as a result of restrictions to free movement imposed by stomata and leaf anatomy Recent techniques have enabled the internal conductance, gi, of leaves, that is, from sub-stomatal cavities to the sites of carboxylation, to be measured (1) A strong correlation has been found between gi and the rate of CO2 assimilation under high irradiance and ambient CO2 partial pressure, Ca (2,3,4) Unfortunately, the title of the paper by Lloyd et al (3), low conductances for CO, diffusion from stomata to the sites of carboxylation in leaves of woody species’ has been taken to mean that there is a correspondingly low partial pressure of CO2 in the chloroplast, which contributes to the low CO2 assimilation rate of sclerophyllous leaves The claim was repeated by Epron et al (5) ‘high internal resistances to CO2 transfer may account for the low net CO2 assimilation rates that often characterise tree leaves

How to reconcile body-wave and normal-mode reference earth models

Abstract: SUMMARY Reference earth models can be retrieved from either body waves or normal-mode eigenperiods. However, there is a large discrepancy between different reference earth models, which arises partly from the type of data set used in their construction and partly from differences in parametrization. Reference models derived from body-wave observations do not give access to density, attenuation factor and radial anisotropy. Conversely, reference models derived from normal modes cannot provide the correct locations for the depth of seismic discontinuities, nor the associated velocity jump. Eigenperiods derived from reference models constructed using body-wave data together with classifical attenuation models differ significantly from the observed eigenperiods. The body-wave and normal-mode approaches can be reconciled. The V’ and V, velocities given by body-wave models are considered as constraints, and an inversion is performed for parameters that cannot be extracted from body waves in the context of a radially anisotropic model, i.e. the density p, the quality factor Q, and the anisotropy parameters 5, (b and q. The influence of anelasticity is very large, although insufficient by itself to reconcile the two types of model. However, by including in the inversion procedure the density and the three anisotropic parameters, body-wave models can be brought into complete agreement with eigenperiod data. A number of reference models derived from body waves were tested and used as starting models: iasp91, sp6, and two new models ak303 and ak135. A number of robust features can be extracted from the inversions based on these different models. The quality factor Q,, is found to be much larger in the lower mantle than in previous models (e.g. prern). Anisotropy, in the form of transverse isotropy with a vertical symmetry axis, is significant in the whole upper mantle, but very small in the lower mantle except in the lower transition zone (between the 660 km discontinuity and 1000 km depth) and in the D”-layer. Compared with prem there is an increase of density in the D”-layer and a decrease in the lower transition zone. The attenuation estimates have been derived using velocity dispersion information, but are in agreement with available direct measurements of normal-mode attenuation. Such attenuation data are still of limited quality, and the present results emphasize the need for improved attenuation measurements.

B cell differentiation and isotype switching is related to division cycle number.

Abstract: The mature, resting immunoglobulin (Ig) M, IgD+ B lymphocyte can be induced by T cells to proliferate, switch isotype, and differentiate into Ig-secreting or memory cells. Furthermore, B cell activation results in the de novo expression or loss of a number of cell surface molecules that function in cell recirculation or further interaction with T cells. Here, a novel fluorescent technique reveals that T-dependent B cell activation induces cell surface changes that correlate with division cycle number. Furthermore, striking stepwise changes are often centered on a single round of cell division. Particularly marked was the consistent increase in IgG1+ B cells after the second division cycle, from an initial level of < 3% IgG1+ to a plateau of approximately 40% after six cell divisions. The relationship between the percentage of IgG1+ B cells and division number was independent of time after stimulation, indicating a requirement for cell division in isotype switching. IgD expression became negative after four divisions, and a number of changes centered on the sixth division, including the loss of IgM, CD23, and B220. The techniques used here should prove useful for tracking other differentiation pathways and for future analysis of the molecular events associated with stepwise differentiation at the single cell level.

Autofib Redshift Survey — I. Evolution of the galaxy luminosity function

Abstract: We present a detailed determination of the restframe B-band galaxy luminosity function (LF) as a function of redshift and star formation activity from z=0 to z=0.75. The dataset used for this purpose is a combined sample of over 1700 redshifts spanning a wide range in apparent magnitude, 11.5

Evidence of oxidant-induced injury to epithelial cells during inflammatory bowel disease.

Abstract: Evidence of in vivo oxidant-induced injury in inflammatory bowel disease (IBD) is largely indirect. Colon epithelial crypt cells (CEC) from paired specimens of histologically normal and inflamed bowel from IBD patients with active disease were examined for altered protein thiol redox status as an indicator of oxidative damage. When CEC preparations from 22 IBD patients were labeled with the reduced-thiol-specific probe [14C]-iodoacetamide (IAM), there was decreased labeling of a number of proteins indicating oxidation of thiol groups in CEC from inflamed mucosa compared to paired normal mucosa, especially the loss of thiol labeling of a 37-kD protein which was almost completely lost. The loss of reduced protein thiol status for the 37-kD band was paralleled by loss of epithelial cell glyceraldehyde-3-phosphate dehydrogenase (GAPDH, EC 1.2.1.12) enzyme activity, an enzyme known to contain an essential reduced cysteine (Cys149) at the active site. The identity of the 37-kD protein as GADPH monomer was confirmed by NH2-terminal amino acid sequence analysis. To examine whether this type of in vivo injury could be attributed to biologically relevant oxidants produced by inflammatory cells, CEC prepared from normal mucosa were exposed to H2O2, OCl-, nitric oxide (NO), and a model chloramine molecule chloramine T (ChT) in vitro. Dose-dependent loss of IAM labeling and GAPDH enzyme activity was observed. The efficacy (IC50) against IAM labeling was OCl- >> ChT > H2O2 > NO (52 +/- 3, 250 +/- 17, 420 +/- 12, 779 +/- 120 microM oxidant) and OCl- >> ChT > NO > H2O2 (89 +/- 17, 256 +/- 11, 407 +/- 105, 457 +/- 75 microM oxidant), respectively, for GAPDH enzyme activity. This study provides direct evidence of in vivo oxidant injury in CEC from inflamed mucosa of IBD patients. Oxidation and inhibition of essential protein function by inflammatory cells is a potential mechanism of tissue injury that may contribute to the pathogenesis of the disease and supports the exploration of compounds with antioxidant activity as new therapies for IBD.

The theory of externalities, public goods, and club goods: Externalities and private information

Abstract: The analysis in Chapter 4 suggests that in a world of zero transaction costs, where participants in an exchange know the relevant characteristics of each other's preferences, technology, and endowments, mutually advantageous bargaining opportunities will not be ignored. The individuals may be expected to agree on a Pareto-efficient allocation. The traditional Pigouvian tax/subsidy literature, by contrast, assumes at the outset that equilibrium outcomes may indeed be inefficient. In particular, it presupposes that existing markets will not function in such a way as to eliminate all potential sources of inefficiency and that the introduction of taxes and subsidies into a market system can help by confronting individuals with those costs and benefits that their consumption and production activities generate for other parties and that are not internalized by the market mechanism. In order to justify the possibility of inefficiency in equilibrium, we need to examine more closely the idea of positive transaction costs. We hinted earlier that one way of thinking about such costs is to see them as arising from the presence of private information. Participants in an exchange may know their own preferences, endowments, and technology but may not be so well informed about the relevant characteristics of other potential participants. In analyzing the bargaining process that leads to an equilibrium, it is important to consider carefully the precise nature of the process, the information available to each party, and the implications of such matters for the properties of the equilibrium.

The Theory of Externalities, Public Goods, and Club Goods

Abstract: This book presents a theoretical treatment of externalities (i.e. uncompensated interdependencies), public goods, and club goods. The new edition updates and expands the discussion of externalities and their implications, coverage of asymmetric information, underlying game-theoretic formulations, and intuitive and graphical presentations. Aimed at well-prepared undergraduates and graduate students making a serious foray into this branch of economics, the analysis should also interest professional economists wishing to survey recent advances in the field. No other single source for the range of materials explored is currently available. Topics investigated include Nash equilibrium, Lindahl equilibria, club theory, preference-revelation mechanism, Pigouvian taxes, the commons, Coase Theorem, and static and repeated games. The authors use mathematical techniques only as much as necessary to pursue the economic argument. They develop key principles of public economics that are useful for subfields such as public choice, labor economics, economic growth, international economics, environmental and natural resource economics, and industrial organization.