140 research outputs found
Approximate well-supported Nash equilibria in symmetric bimatrix games
The -well-supported Nash equilibrium is a strong notion of
approximation of a Nash equilibrium, where no player has an incentive greater
than to deviate from any of the pure strategies that she uses in
her mixed strategy. The smallest constant currently known for
which there is a polynomial-time algorithm that computes an
-well-supported Nash equilibrium in bimatrix games is slightly
below . In this paper we study this problem for symmetric bimatrix games
and we provide a polynomial-time algorithm that gives a
-well-supported Nash equilibrium, for an arbitrarily small
positive constant
Energy and helicity budgets of solar quiet regions
We investigate the free magnetic energy and relative magnetic helicity
budgets of solar quiet regions. Using a novel non-linear force-free method
requiring single solar vector magnetograms we calculate the instantaneous free
magnetic energy and relative magnetic helicity budgets in 55 quiet-Sun vector
magnetograms. As in a previous work on active regions, we construct here for
the first time the (free) energy-(relative) helicity diagram of quiet-Sun
regions. We find that quiet-Sun regions have no dominant sense of helicity and
show monotonic correlations a) between free magnetic energy/relative helicity
and magnetic network area and, consequently, b) between free magnetic energy
and helicity. Free magnetic energy budgets of quiet-Sun regions represent a
rather continuous extension of respective active-region budgets towards lower
values, but the corresponding helicity transition is discontinuous due to the
incoherence of the helicity sense contrary to active regions. We further
estimate the instantaneous free magnetic-energy and relative magnetic-helicity
budgets of the entire quiet Sun, as well as the respective budgets over an
entire solar cycle. Derived instantaneous free magnetic energy budgets and, to
a lesser extent, relative magnetic helicity budgets over the entire quiet Sun
are comparable to the respective budgets of a sizeable active region, while
total budgets within a solar cycle are found higher than previously reported.
Free-energy budgets are comparable to the energy needed to power fine-scale
structures residing at the network, such as mottles and spicules
Polylogarithmic Supports are required for Approximate Well-Supported Nash Equilibria below 2/3
In an epsilon-approximate Nash equilibrium, a player can gain at most epsilon
in expectation by unilateral deviation. An epsilon well-supported approximate
Nash equilibrium has the stronger requirement that every pure strategy used
with positive probability must have payoff within epsilon of the best response
payoff. Daskalakis, Mehta and Papadimitriou conjectured that every win-lose
bimatrix game has a 2/3-well-supported Nash equilibrium that uses supports of
cardinality at most three. Indeed, they showed that such an equilibrium will
exist subject to the correctness of a graph-theoretic conjecture. Regardless of
the correctness of this conjecture, we show that the barrier of a 2/3 payoff
guarantee cannot be broken with constant size supports; we construct win-lose
games that require supports of cardinality at least Omega((log n)^(1/3)) in any
epsilon-well supported equilibrium with epsilon < 2/3. The key tool in showing
the validity of the construction is a proof of a bipartite digraph variant of
the well-known Caccetta-Haggkvist conjecture. A probabilistic argument shows
that there exist epsilon-well-supported equilibria with supports of cardinality
O(log n/(epsilon^2)), for any epsilon> 0; thus, the polylogarithmic cardinality
bound presented cannot be greatly improved. We also show that for any delta >
0, there exist win-lose games for which no pair of strategies with support
sizes at most two is a (1-delta)-well-supported Nash equilibrium. In contrast,
every bimatrix game with payoffs in [0,1] has a 1/2-approximate Nash
equilibrium where the supports of the players have cardinality at most two.Comment: Added details on related work (footnote 7 expanded
A persistent quiet-Sun small-scale tornado. II. Oscillations
Recently, the characteristics, and dynamics of a persistent 1.7 h vortex
flow, resembling a small-scale tornado, have been investigated with ground-base
and space-based observations and for the first time in the Ha line centre. The
vortex flow showed significant substructure in the form of several intermittent
chromospheric swirls.
We investigate the oscillatory behaviour of various physical parameters in
the vortex area, with a 2D wavelet analysis performed within the vortex flow
area and in a quiet-Sun region (for comparison), using the same high spatial
and temporal resolution Ha and Ca II 8542 CRISP observations, as well as
Doppler velocities and FWHM derived from the Ha line profiles.
The vortex flow shows significant oscillatory power in the 3-5 min range that
peaks around 4 min and behaves differently than the reference quiet-Sun region.
Oscillations reflect the cumulative action of different components such as
swaying motions, rotation, and waves. The derived swaying motion periods are in
the range of 200-220 s, and the rotation periods are ~270 s for Ha and ~215 s
for Ca II. Periods increase with atmospheric height and seem to decrease with
radial distance from the vortex centre, suggesting a deviation from a rigid
rotation. The behaviour of power within the vortex flow as a function of period
and height implies the existence of evanescent waves and the excitation of
different types of waves, such as magnetoacoustic (e.g. kink) or Alfven waves.
The vortex flow seems to be dominated by two motions: a transverse (swaying)
motion, and a rotational motion while oscillations point to the propagation of
waves within it. Nearby fibril-like flows could play an important role in the
rotational modulation of the vortex flow. Indirect evidence exists that the
structure is magnetically supported while the central swirl seems to be acting
as a "central engine" to the vortex flow
Chromospheric swirls I. Automated detection in H observations and their statistical properties
Chromospheric swirls are considered to play a significant role in the
dynamics and heating of the upper solar atmosphere. It is important to
automatically detect and track them in chromospheric observations and determine
their properties. We applied a recently developed automated chromospheric swirl
detection method to time-series observations of a quiet region of the solar
chromosphere obtained in the H-0.2 \r{A} wavelength of the H
spectral line by the CRISP instrument at the Swedish 1-m Solar Telescope. The
algorithm exploits the morphological characteristics of swirling events in high
contrast chromospheric observations and results in the detection of these
structures in each frame of the time series and their tracking over time. We
conducted a statistical analysis to determine their various properties,
including a survival analysis for deriving the mean lifetime. A mean number of
146 9 swirls was detected within the FOV at any given time. The mean
surface density is found equal to 0.08 swirlsMm and the
occurrence rate is 10 swirlsMm min. These values
are much higher than those previously reported from chromospheric observations.
The radii of the detected swirls range between 0.5 and 2.5 Mm, with a mean
value equal to 1.3 0.3 Mm, which is slightly higher than previous
reports. The lifetimes range between 1.5 min and 33.7 min with an arithmetic
mean value of 8.5 min. A survival analysis of the lifetimes, however,
using the Kaplan-Meier estimator in combination with a parametric model results
in a mean lifetime of 10.3 0.6 min. An automated method sheds more light
on their abundance than visual inspection, while higher cadence, higher
resolution observations will most probably result in the detection of a higher
number of such features on smaller scales and with shorter lifetimes
State–of–the–art report on nonlinear representation of sources and channels
This report consists of two complementary parts, related to the modeling of two important sources of nonlinearities in a communications system. In the first part, an overview of important past work related to the estimation, compression and processing of sparse data through the use of nonlinear models is provided. In the second part, the current state of the art on the representation of wireless channels in the presence of nonlinearities is summarized. In addition to the characteristics of the nonlinear wireless fading channel, some information is also provided on recent approaches to the sparse representation of such channels
Emergence of small-scale magnetic flux in the quiet Sun
We study the evolution of a small-scale emerging flux region (EFR) in the
quiet Sun, from its emergence to its decay. We track processes and phenomena
across all atmospheric layers, explore their interrelations and compare our
findings with recent numerical modelling studies. We used imaging, spectral and
spectropolarimetric observations from space-borne and ground-based instruments.
The EFR appears next to the chromospheric network and shows all characteristics
predicted by numerical simulations. The total magnetic flux of the EFR exhibits
distinct evolutionary phases, namely an initial subtle increase, a fast
increase and expansion of the region area, a more gradual increase, and a slow
decay. During the initial stages, bright points coalesce, forming clusters of
positive- and negative-polarity in a largely bipolar configuration. During the
fast expansion, flux tubes make their way to the chromosphere, producing
pressure-driven absorption fronts, visible as blueshifted chromospheric
features. The connectivity of the quiet-Sun network gradually changes and part
of the existing network forms new connections with the EFR. A few minutes after
the bipole has reached its maximum magnetic flux, it brightens in soft X-rays
forming a coronal bright point, exhibiting episodic brightenings on top of a
long smooth increase. These coronal brightenings are also associated with
surge-like chromospheric features, which can be attributed to reconnection with
adjacent small-scale magnetic fields and the ambient magnetic field. The
emergence of magnetic flux even at the smallest scales can be the driver of a
series of energetic phenomena visible at various atmospheric heights and
temperature regimes. Multi-wavelength observations reveal a wealth of
mechanisms which produce diverse observable effects during the different
evolutionary stages of these small-scale structures.Comment: Accepted for publication in Astronomy & Astrophysics 14 pages, 14
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A Direct Reduction from k-Player to 2-Player Approximate Nash Equilibrium
We present a direct reduction from k-player games to 2-player games that
preserves approximate Nash equilibrium. Previously, the computational
equivalence of computing approximate Nash equilibrium in k-player and 2-player
games was established via an indirect reduction. This included a sequence of
works defining the complexity class PPAD, identifying complete problems for
this class, showing that computing approximate Nash equilibrium for k-player
games is in PPAD, and reducing a PPAD-complete problem to computing approximate
Nash equilibrium for 2-player games. Our direct reduction makes no use of the
concept of PPAD, thus eliminating some of the difficulties involved in
following the known indirect reduction.Comment: 21 page
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