2,807 research outputs found
Smoothness for Simultaneous Composition of Mechanisms with Admission
We study social welfare of learning outcomes in mechanisms with admission. In
our repeated game there are bidders and mechanisms, and in each round
each mechanism is available for each bidder only with a certain probability.
Our scenario is an elementary case of simple mechanism design with incomplete
information, where availabilities are bidder types. It captures natural
applications in online markets with limited supply and can be used to model
access of unreliable channels in wireless networks.
If mechanisms satisfy a smoothness guarantee, existing results show that
learning outcomes recover a significant fraction of the optimal social welfare.
These approaches, however, have serious drawbacks in terms of plausibility and
computational complexity. Also, the guarantees apply only when availabilities
are stochastically independent among bidders.
In contrast, we propose an alternative approach where each bidder uses a
single no-regret learning algorithm and applies it in all rounds. This results
in what we call availability-oblivious coarse correlated equilibria. It
exponentially decreases the learning burden, simplifies implementation (e.g.,
as a method for channel access in wireless devices), and thereby addresses some
of the concerns about Bayes-Nash equilibria and learning outcomes in Bayesian
settings. Our main results are general composition theorems for smooth
mechanisms when valuation functions of bidders are lattice-submodular. They
rely on an interesting connection to the notion of correlation gap of
submodular functions over product lattices.Comment: Full version of WINE 2016 pape
A Striking Confluence Between Theory and Observations of High-Mass X-ray Binary Pulsars
We analyse the most powerful X-ray outbursts from neutron stars in ten
Magellanic high-mass X-ray binaries and three pulsating ultraluminous X-ray
sources. Most of the outbursts rise to which is about the level of
the Eddington luminosity, while the rest and more powerful outbursts also
appear to recognize that limit when their emissions are assumed to be
anisotropic and beamed toward our direction. We use the measurements of pulsar
spin periods and their derivatives to calculate the X-ray
luminosities in their faintest accreting ("propeller") states. In four
cases with unknown , we use the lowest observed X-ray luminosities,
which only adds to the heterogeneity of the sample. Then we calculate the
ratios and we obtain an outstanding confluence of theory and
observations from which we conclude that work done on both fronts is accurate
and the results are trustworthy: sources known to reside on the lowest
Magellanic propeller line are all located on/near that line, whereas other
sources jump higher and reach higher-lying propeller lines. These jumps can be
interpreted in only one way, higher-lying pulsars have stronger surface
magnetic fields in agreement with empirical results in which and
values were not used.Comment: Added LMC X-4 and commented on the cyclotron absorption line of SMC
X-2. 4 pages, 1 figure, 2 tables, submitted to MNRAS
Spherically symmetric perfect fluid in area-radial coordinates
We study the spherically symmetric collapse of a perfect fluid using
area-radial coordinates. We show that analytic mass functions describe a static
regular centre in these coordinates. In this case, a central singularity can
not be realized without an infinite discontinuity in the central density. We
construct mass functions involving fluid dynamics at the centre and investigate
the relationship between those and the nature of the singularities.Comment: Accepted by CQG. LaTex file, 14 pages, no figure
On static shells and the Buchdahl inequality for the spherically symmetric Einstein-Vlasov system
In a previous work \cite{An1} matter models such that the energy density
and the radial- and tangential pressures and
satisfy were considered in the context of
Buchdahl's inequality. It was proved that static shell solutions of the
spherically symmetric Einstein equations obey a Buchdahl type inequality
whenever the support of the shell, satisfies
Moreover, given a sequence of solutions such that then the
limit supremum of was shown to be bounded by
In this paper we show that the hypothesis
that can be realized for Vlasov matter, by constructing a
sequence of static shells of the spherically symmetric Einstein-Vlasov system
with this property. We also prove that for this sequence not only the limit
supremum of is bounded, but that the limit is
since for Vlasov matter.
Thus, static shells of Vlasov matter can have arbitrary close to
which is interesting in view of \cite{AR2}, where numerical evidence is
presented that 8/9 is an upper bound of of any static solution of the
spherically symmetric Einstein-Vlasov system.Comment: 20 pages, Late
Exact solution for scalar field collapse
We give an exact spherically symmetric solution for the Einstein-scalar field
system. The solution may be interpreted as an inhomogeneous dynamical scalar
field cosmology. The spacetime has a timelike conformal Killing vector field
and is asymptotically conformally flat. It also has black or white hole-like
regions containing trapped surfaces. We describe the properties of the apparent
horizon and comment on the relevance of the solution to the recently discovered
critical behaviour in scalar field collapse.Comment: 10 pages(Latex) (2 figures available upon request), Alberta-Thy-4-9
The Great Pretenders Among the ULX Class
The recent discoveries of pulsed X-ray emission from three ultraluminous
X-ray (ULX) sources have finally enabled us to recognize a subclass within the
ULX class: the great pretenders, neutron stars (NSs) that appear to emit X-ray
radiation at isotropic luminosities ~erg~s~erg~s only because their emissions are strongly beamed toward
our direction and our sight lines are offset by only a few degrees from their
magnetic-dipole axes. The three known pretenders appear to be stronger emitters
than the presumed black holes of the ULX class, such as Holmberg II \& IX X-1,
IC10 X-1, and NGC300 X-1. For these three NSs, we have adopted a single
reasonable assumption, that their brightest observed outbursts unfold at the
Eddington rate, and we have calculated both their propeller states and their
surface magnetic-field magnitudes. We find that the results are not at all
different from those recently obtained for the Magellanic Be/X-ray pulsars: the
three NSs reveal modest magnetic fields of about 0.3-0.4~TG and beamed
propeller-line X-ray luminosities of ~erg~s,
substantially below the Eddington limit.Comment: To appear in Research in Astronomy and Astrophysic
How Formal and Informal Institutions of Middle Eastern Countries Influence Managerial Discretion: An Empirical Investigation
Managerial discretion is the focal theme bridging the clash between two schools of thoughts; whether executives have greater influence on their firms’ outcomes or other factors restrain their actions (Hambrick & Finkelstein, 1987). It is argued that constraints come from inertial, normative and environmental forces (e.g. DiMaggio & Powell, 1983). Of these restraints is the institutional environment in which a firm is headquartered. Our paper falls within this research stream and provides an extension for Crossland and Hambrick (2007, 2011) work. We investigate the national level of discretion in new cross-cultural contexts, provide deeper understanding of its concept, and shed the light on undiscovered discretion’s antecedents and consequences. We adopt a quantitative approach in which questionnaires represent our data collection instrument. We anticipate that in high discretion countries firms tend to follow what Miles & Snow (1978) labeled ‘Prospector’ strategy as opposed to low discretion countries in which firms incline to implement a ‘Defender’ strategy
Generic Cosmic Censorship Violation in anti de Sitter Space
We consider (four dimensional) gravity coupled to a scalar field with
potential V(\phi). The potential satisfies the positive energy theorem for
solutions that asymptotically tend to a negative local minimum. We show that
for a large class of such potentials, there is an open set of smooth initial
data that evolve to naked singularities. Hence cosmic censorship does not hold
for certain reasonable matter theories in asymptotically anti de Sitter
spacetimes. The asymptotically flat case is more subtle. We suspect that
potentials with a local Minkowski minimum may similarly lead to violations of
cosmic censorship in asymptotically flat spacetimes, but we do not have
definite results.Comment: 4 pages, v2: minor change
Improving the Price of Anarchy for Selfish Routing via Coordination Mechanisms
We reconsider the well-studied Selfish Routing game with affine latency
functions. The Price of Anarchy for this class of games takes maximum value
4/3; this maximum is attained already for a simple network of two parallel
links, known as Pigou's network. We improve upon the value 4/3 by means of
Coordination Mechanisms.
We increase the latency functions of the edges in the network, i.e., if
is the latency function of an edge , we replace it by
with for all . Then an
adversary fixes a demand rate as input. The engineered Price of Anarchy of the
mechanism is defined as the worst-case ratio of the Nash social cost in the
modified network over the optimal social cost in the original network.
Formally, if \CM(r) denotes the cost of the worst Nash flow in the modified
network for rate and \Copt(r) denotes the cost of the optimal flow in the
original network for the same rate then [\ePoA = \max_{r \ge 0}
\frac{\CM(r)}{\Copt(r)}.]
We first exhibit a simple coordination mechanism that achieves for any
network of parallel links an engineered Price of Anarchy strictly less than
4/3. For the case of two parallel links our basic mechanism gives 5/4 = 1.25.
Then, for the case of two parallel links, we describe an optimal mechanism; its
engineered Price of Anarchy lies between 1.191 and 1.192.Comment: 17 pages, 2 figures, preliminary version appeared at ESA 201
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