10,167 research outputs found
Abnormal oscillation modes in a waning light bridge
A sunspot acts as a waveguide in response to the dynamics of the solar
interior; the trapped waves and oscillations could reveal its thermal and
magnetic structures.
We study the oscillations in a sunspot intruded by a light bridge, the
details of the oscillations could reveal the fine structure of the magnetic
topology.
We use the Solar Dynamics Observatory/Atmospheric Imaging Assembly data to
analyse the oscillations in the emission intensity of light bridge plasma at
different temperatures and investigate their spatial distributions.
The extreme ultraviolet emission intensity exhibits two persistent
oscillations at five-minute and sub-minute ranges. The spatial distribution of
the five-minute oscillation follows the spine of the bridge; whereas the
sub-minute oscillations overlap with two flanks of the bridge. Moreover, the
sub-minute oscillations are highly correlated in spatial domain, however, the
oscillations at the eastern and western flanks are asymmetric with regard to
the lag time. In the meanwhile, jet-like activities are only found at the
eastern flank.
Asymmetries in forms of oscillatory pattern and jet-like activities
\textbf{are} found between two flanks of a granular light bridge. Based on our
study and recent findings, we propose a new model of twisted magnetic field for
a light bridge and its dynamic interactions with the magnetic field of a
sunspot.Comment: 5 figures, Accepted version in A&
Equivariant Perturbation in Gomory and Johnson's Infinite Group Problem. VII. Inverse semigroup theory, closures, decomposition of perturbations
In this self-contained paper, we present a theory of the piecewise linear
minimal valid functions for the 1-row Gomory-Johnson infinite group problem.
The non-extreme minimal valid functions are those that admit effective
perturbations. We give a precise description of the space of these
perturbations as a direct sum of certain finite- and infinite-dimensional
subspaces. The infinite-dimensional subspaces have partial symmetries; to
describe them, we develop a theory of inverse semigroups of partial bijections,
interacting with the functional equations satisfied by the perturbations. Our
paper provides the foundation for grid-free algorithms for the Gomory-Johnson
model, in particular for testing extremality of piecewise linear functions
whose breakpoints are rational numbers with huge denominators.Comment: 67 pages, 21 figures; v2: changes to sections 10.2-10.3, improved
figures; v3: additional figures and minor updates, add reference to IPCO
abstract. CC-BY-S
Simple and Near-Optimal Mechanisms For Market Intermediation
A prevalent market structure in the Internet economy consists of buyers and
sellers connected by a platform (such as Amazon or eBay) that acts as an
intermediary and keeps a share of the revenue of each transaction. While the
optimal mechanism that maximizes the intermediary's profit in such a setting
may be quite complicated, the mechanisms observed in reality are generally much
simpler, e.g., applying an affine function to the price of the transaction as
the intermediary's fee. Loertscher and Niedermayer [2007] initiated the study
of such fee-setting mechanisms in two-sided markets, and we continue this
investigation by addressing the question of when an affine fee schedule is
approximately optimal for worst-case seller distribution. On one hand our work
supplies non-trivial sufficient conditions on the buyer side (i.e. linearity of
marginal revenue function, or MHR property of value and value minus cost
distributions) under which an affine fee schedule can obtain a constant
fraction of the intermediary's optimal profit for all seller distributions. On
the other hand we complement our result by showing that proper affine
fee-setting mechanisms (e.g. those used in eBay and Amazon selling plans) are
unable to extract a constant fraction of optimal profit in the worst-case
seller distribution. As subsidiary results we also show there exists a constant
gap between maximum surplus and maximum revenue under the aforementioned
conditions. Most of the mechanisms that we propose are also prior-independent
with respect to the seller, which signifies the practical implications of our
result.Comment: To appear in WINE'14, the 10th conference on Web and Internet
Economic
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