12,531 research outputs found
Cumulant expansion of the periodic Anderson model in infinite dimension
The diagrammatic cumulant expansion for the periodic Anderson model with
infinite Coulomb repulsion () is considered here for an hypercubic
lattice of infinite dimension (). The same type of simplifications
obtained by Metzner for the cumulant expansion of the Hubbard model in the
limit of , are shown to be also valid for the periodic Anderson
model.Comment: 13 pages, 7 figures.ps. To be published in J. Phys. A: Mathematical
and General (1997
Financing Constraints and Corporate Investment
Most empirical models of investment rely on the assumption that firms are able to respond to prices set in centralized securities markets (through the "cost of capital" or "q"). An alternative approach emphasizes the importance of cash flow as a determinant of investment spending, because of a "financing hierarchy," in which internal finance has important cost advantages over external finance. We build on recent research concerning imperfections in markets for equity and debt. This work suggests that some firms do not have sufficient access to external capital markets to enable them to respond to changes in the cost of capital, asset prices, or tax-based investment incentives. To the extent that firms are constrained in their ability to raise funds externally, investment spending may be sensitive to the availability of internal finance. That is, investment may display "excess sensitivity" to movements in cash flow. In this paper, we work within the q theory of investment, and examine the importance of a financing hierarchy created by capital-market imperfections. Using panel data on individual manufacturing firms, we compare the investment behavior of rapidly growing firms that exhaust all of their internal finance with that of mature firms paying dividends. We find that q values remain very high for significant periods of time for firms paying no dividends, relative to those for mature firms. We also find that investment is more sensitive to cash flow for the group of firms that our model implies is most likely to face external finance constraints. These results are consistent with the augmented model we propose, which takes into account different financing regimes for different groups of firms. Some extensions and implications for public policy are discussed at the end.
Temperature Fluctuations driven by Magnetorotational Instability in Protoplanetary Disks
The magnetorotational instability (MRI) drives magnetized turbulence in
sufficiently ionized regions of protoplanetary disks, leading to mass
accretion. The dissipation of the potential energy associated with this
accretion determines the thermal structure of accreting regions. Until
recently, the heating from the turbulence has only been treated in an
azimuthally averaged sense, neglecting local fluctuations. However, magnetized
turbulence dissipates its energy intermittently in current sheet structures. We
study this intermittent energy dissipation using high resolution numerical
models including a treatment of radiative thermal diffusion in an optically
thick regime. Our models predict that these turbulent current sheets drive
order unity temperature variations even where the MRI is damped strongly by
Ohmic resistivity. This implies that the current sheet structures where energy
dissipation occurs must be well resolved to correctly capture the flow
structure in numerical models. Higher resolutions are required to resolve
energy dissipation than to resolve the magnetic field strength or accretion
stresses. The temperature variations are large enough to have major
consequences for mineral formation in disks, including melting chondrules,
remelting calcium-aluminum rich inclusions, and annealing silicates; and may
drive hysteresis: current sheets in MRI active regions could be significantly
more conductive than the remainder of the disk.Comment: 16 pages, 13 figures, ApJ In Press, updated to match proof
Market Structure and Cyclical Fluctuations in U.S. Manufacturing
The relevance of imperfect competition for models of aggregate economic fluctuations has received increased attention from researchers in both macroeconomics and industrial organization. Measuring properly the size of industry markups of price over marginal cost is important both for assessing the role of market structure and for determining the extent to which excess capacity is a significant feature accompanying imperfect competition in American industry. Using a panel data set on four-digit Census manufacturing industries, this paper expands recent work by Robert Hall on the importance of market structure for understanding cyclical fluctuations. We outline a methodology for estimating industry markups of price over cost and the influence of market structure on cyclical movements in total factor productivity. While we find evidence to support the proposition that price exceeds marginal cost in U.S. manufacturing, our results offer only limited support for the notion that markups are importantly related to differences in industry concentration, though the effect of unionization is important. Concentration effects are important only in industries producing durable goods or differentiated consumer goods. In addition, much of the estimated markup of price over marginal cost is accounted for by fixed costs related to overhead labor, advertising, and central office expenses; we do not find compelling evidence of substantial evidence of excess capacity in most industries.
Business Cycles and Oligopoly Supergames: Some Empirical Evidence on Prices and Margins
There has been a significant interest on a theoretical level in the application of supergames to oligopoly behavior. Implications for pricing behavior in trigger-strategy models in response to aggregate demand are of particular importance for public policy considerations. We contrast the predictions for the movements of industry prices over the business cycle of two such models -- put forth by Edward Green and Robert Porter and by Julio Rotemberg and Garth Saloner -- and test the predictions using a panel data set of U.S. manufacturing industries. Our principal findings are four. First, the levels of price-cost margins of concentrated, homogeneous-goods industries, while higher than those of unconcentrated counterparts, appear to be closer to those predicted by a single-period Cournot-Nash equilibrium than monopoly. Second, there is little evidence to support the idea that price-cost margins of these industries have different cyclical patterns from other industries apart from effects by level of industry concentration. Maximum price declines for concentrated industries give little support for the occurrence of price wars during either recessions or booms. Finally, consistent with the predictions of the Rotemberg-Saloner model, the industries with high price-cost margins have more countercyclical price movements than those exhibited by other industries. That gradual price adjustment is quantitatively important for those industries, suggests, however, that other factors may lie behind the apparent rigidity of prices.
Compressibility of the Two-Dimensional infinite-U Hubbard Model
We study the interactions between the coherent quasiparticles and the
incoherent Mott-Hubbard excitations and their effects on the low energy
properties in the Hubbard model. Within the framework of a
systematic large-N expansion, these effects first occur in the next to leading
order in 1/N. We calculate the scattering phase shift and the free energy, and
determine the quasiparticle weight Z, mass renormalization, and the
compressibility. It is found that the compressibility is strongly renormalized
and diverges at a critical doping . We discuss the nature
of this zero-temperature phase transition and its connection to phase
separation and superconductivity.Comment: 4 pages, 3 eps figures, final version to appear in Phys. Rev. Let
Breadboard stellar tracker system test report, volume 1
The performance of a star tracker equipped with a focal plane detector was evaluated. The CID board is an array of 256 x 256 pixels which are 20 x 20 micrometers in dimension. The tracker used for test was a breadboard tracker system developed by BASD. Unique acquisition and tracking algorithms are employed to enhance performance. A pattern recognition process is used to test for proper image spread function and to avoid false acquisition on noise. A very linear, high gain, interpixel transfer function is derived for interpolating star position. The lens used in the tracker has an EFL of 100 mm. The tracker has an FOV of 2.93 degrees resulting in a pixel angular subtense of 41.253 arc sec in each axis. The test procedure used for the program presented a star to the tracker in a circular pattern of positions; the pattern was formed by projecting a simulated star through a rotatable deviation wedge. Further tests determined readout noise, Noise Equivalent Displacement during track, and spatial noise during acquisition by taking related data and reducing it
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