2,280 research outputs found
Extreme Walrasian Dynamics: The Gale Example in the Lab
We study the classic Gale (1963) economy using laboratory markets. Tatonnement theory
predicts prices will diverge from an equitable interior equilibrium towards infinity or zero
depending only on initial prices. The inequitable equilibria determined by these dynamics
give all gains from exchange to one side of the market. Our results show surprisingly strong
support for these predictions. In most sessions one side of the market eventually outgains the
other by more than twenty times, leaving the disadvantaged side to trade for mere pennies.
We also find preliminary evidence that these dynamics are sticky, resisting exogenous
interventions designed to reverse their trajectories
An Unsplit, Cell-Centered Godunov Method for Ideal MHD
We present a second-order Godunov algorithm for multidimensional, ideal MHD.
Our algorithm is based on the unsplit formulation of Colella (J. Comput. Phys.
vol. 87, 1990), with all of the primary dependent variables centered at the
same location. To properly represent the divergence-free condition of the
magnetic fields, we apply a discrete projection to the intermediate values of
the field at cell faces, and apply a filter to the primary dependent variables
at the end of each time step. We test the method against a suite of linear and
nonlinear tests to ascertain accuracy and stability of the scheme under a
variety of conditions. The test suite includes rotated planar linear waves, MHD
shock tube problems, low-beta flux tubes, and a magnetized rotor problem. For
all of these cases, we observe that the algorithm is second-order accurate for
smooth solutions, converges to the correct weak solution for problems involving
shocks, and exhibits no evidence of instability or loss of accuracy due to the
possible presence of non-solenoidal fields.Comment: 37 Pages, 9 Figures, submitted to Journal of Computational Physic
The death of massive stars - II. Observational constraints on the progenitors of type Ibc supernovae
The progenitors of many type II core-collapse supernovae have now been
identified directly on pre-discovery imaging. Here we present an extensive
search for the progenitors of type Ibc supernovae in all available
pre-discovery imaging since 1998. There are 12 type Ibc supernovae with no
detections of progenitors in either deep ground-based or Hubble Space Telescope
archival imaging. The deepest absolute BVR magnitude limits are between -4 and
-5. We compare these limits with the observed Wolf-Rayet population in the
Large Magellanic Cloud and estimate a 16 per cent probability we have failed to
detect such a progenitor by chance. Alternatively the progenitors evolve
significantly before core-collapse or we have underestimated the extinction
towards the progenitors. Reviewing the relative rates and ejecta mass estimates
from lightcurve modelling of Ibc SNe, we find both incompatible with Wolf-Rayet
stars with initial masses >25Msun being the only progenitors. We present binary
evolution models that fit these observational constraints. Stars in binaries
with initial masses <20Msun lose their hydrogen envelopes in binary
interactions to become low mass helium stars. They retain a low mass hydrogen
envelope until approximately 10,000 years before core-collapse; hence it is not
surprising that galactic analogues have been difficult to identify.Comment: Accepted by MNRAS. 31 pages, 12 figures, 8 table
A dc-to-pulse-width converter
Operation, design, and linearity of dc-to-pulse width converter for analog-to-digital converter application
Extinctions of aculeate pollinators in Britain and the role of large-scale agricultural changes
Pollinators are fundamental to maintaining both biodiversity and agricultural productivity, but habitat destruction, loss of flower resources, and increased use of pesticides are causing declines in their abundance and diversity. Using historical records we assessed the rate of extinction of bee and flower-visiting wasp species in Britain, from the mid 19th century to the present. The most rapid phase of extinction appears to be related to changes in agricultural policy and practice beginning in the 1920s, before the agricultural intensification prompted by the Second World War, often cited as the most important driver of biodiversity loss in Britain. Slowing of the extinction rate from the 1960s onwards may be due to prior loss of the most sensitive species and/or effective conservation programs
Excellence: capturing Aristotelian notions of meaning and purpose.
In this paper, we argue that conventional models for excellence are too narrow, too internally focused, too instrumental and too functionalist to achieve excellence in dealing with emerging broad socioeconomic concerns. We try to show that Aristotle’s theory of virtue has useful contemporary relevance and can be developed and used to understand a fuller notion of business excellence. We develop a qualitative research instrument to discover and illustrate excellence and to establish its modern nature. Our application of virtue demonstrated that purpose was the key to achieving excellence. We find that the virtue theory can be a useful and practical guide to producing excellence
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