24,788 research outputs found
The Planning Spectrum - One, Two, Three, Infinity
Linear Temporal Logic (LTL) is widely used for defining conditions on the
execution paths of dynamic systems. In the case of dynamic systems that allow
for nondeterministic evolutions, one has to specify, along with an LTL formula
f, which are the paths that are required to satisfy the formula. Two extreme
cases are the universal interpretation A.f, which requires that the formula be
satisfied for all execution paths, and the existential interpretation E.f,
which requires that the formula be satisfied for some execution path.
When LTL is applied to the definition of goals in planning problems on
nondeterministic domains, these two extreme cases are too restrictive. It is
often impossible to develop plans that achieve the goal in all the
nondeterministic evolutions of a system, and it is too weak to require that the
goal is satisfied by some execution.
In this paper we explore alternative interpretations of an LTL formula that
are between these extreme cases. We define a new language that permits an
arbitrary combination of the A and E quantifiers, thus allowing, for instance,
to require that each finite execution can be extended to an execution
satisfying an LTL formula (AE.f), or that there is some finite execution whose
extensions all satisfy an LTL formula (EA.f). We show that only eight of these
combinations of path quantifiers are relevant, corresponding to an alternation
of the quantifiers of length one (A and E), two (AE and EA), three (AEA and
EAE), and infinity ((AE)* and (EA)*). We also present a planning algorithm for
the new language that is based on an automata-theoretic approach, and study its
complexity
Simulation of Acoustic Black Hole in a Laval Nozzle
A numerical simulation of fluid flows in a Laval nozzle is performed to
observe formations of acoustic black holes and the classical counterpart to
Hawking radiation under a realistic setting of the laboratory experiment. We
determined the Hawking temperature of the acoustic black hole from obtained
numerical data. Some noteworthy points in analyzing the experimental data are
clarified through our numerical simulation.Comment: 26 pages, published versio
Quality Measures of Parameter Tuning for Aggregated Multi-Objective Temporal Planning
Parameter tuning is recognized today as a crucial ingredient when tackling an
optimization problem. Several meta-optimization methods have been proposed to
find the best parameter set for a given optimization algorithm and (set of)
problem instances. When the objective of the optimization is some scalar
quality of the solution given by the target algorithm, this quality is also
used as the basis for the quality of parameter sets. But in the case of
multi-objective optimization by aggregation, the set of solutions is given by
several single-objective runs with different weights on the objectives, and it
turns out that the hypervolume of the final population of each single-objective
run might be a better indicator of the global performance of the aggregation
method than the best fitness in its population. This paper discusses this issue
on a case study in multi-objective temporal planning using the evolutionary
planner DaE-YAHSP and the meta-optimizer ParamILS. The results clearly show how
ParamILS makes a difference between both approaches, and demonstrate that
indeed, in this context, using the hypervolume indicator as ParamILS target is
the best choice. Other issues pertaining to parameter tuning in the proposed
context are also discussed.Comment: arXiv admin note: substantial text overlap with arXiv:1305.116
Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440
After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright
transient X-ray source turning on in August 2001, as noted with the RXTE
All-Sky Monitor. We carried out a short target of opportunity observation with
the Chandra X-ray Observatory and are able to associate the transient with the
brightest of 24 X-ray sources detected during quiescence in July 2000 with
Chandra. Furthermore, we securely identify the optical counterpart and
determine that the 1998 X-ray outburst in NGC 6440 was from the same object.
This is the first time that an optical counterpart to a transient in a globular
cluster is securely identified. Since the transient is a type I X-ray burster,
it is established that the compact accretor is a neutron star. Thus, this
transient provides an ideal case to study the quiescent emission in the optical
and X-ray of a transiently accreting neutron star while knowing the distance
and reddening accurately. One model that fits the quiescent spectrum is an
absorbed power law plus neutron star hydrogen atmosphere model. We find an
intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric
luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is
consistent with predictions for a cooling neutron star.Comment: Accepted for publication in ApJ Letter
Tadpole versus anomaly cancellation in D=4,6 compact IIB orientifolds
It is often stated in the literature concerning D=4,6 compact Type IIB
orientifolds that tadpole cancellation conditions i) uniquely fix the gauge
group (up to Wilson lines and/or moving of branes) and ii) are equivalent to
gauge anomaly cancellation. We study the relationship between tadpole and
anomaly cancellation conditions and qualify both statements. In general the
tadpole cancellation conditions imply gauge anomaly cancellation but are
stronger than the latter conditions in D=4, N=1 orientifolds. We also find that
tadpole cancellation conditions in Z_N D=4,6 compact orientifolds do not
completely fix the gauge group and we provide new solutions different from
those previously reported in the literature.Comment: 28 pages, Latex. Minor corrections, updated reference
The social welfare function and individual responsibility: Some theoretical issues and empirical evidence from health
The literature on income distribution has attempted to quantitatively analyse different degrees of inequality using a social welfare function (SWF) approach. However, it has largely ignored the source of such inequalities, and has thus failed to consider different degrees of inequity. The literature on egalitarianism has addressed issues of equity, largely in relation to individual responsibility. This paper brings these two literatures together by introducing the concept of individual responsibility into the SWF approach. The results from an empirical study of people’s preferences in relation to the distribution of health benefits are presented to illustrate how the parameter values in such a SWF might be determined
Numerical Implementation of the QuEST Function
This paper deals with certain estimation problems involving the covariance
matrix in large dimensions. Due to the breakdown of finite-dimensional
asymptotic theory when the dimension is not negligible with respect to the
sample size, it is necessary to resort to an alternative framework known as
large-dimensional asymptotics. Recently, Ledoit and Wolf (2015) have proposed
an estimator of the eigenvalues of the population covariance matrix that is
consistent according to a mean-square criterion under large-dimensional
asymptotics. It requires numerical inversion of a multivariate nonrandom
function which they call the QuEST function. The present paper explains how to
numerically implement the QuEST function in practice through a series of six
successive steps. It also provides an algorithm to compute the Jacobian
analytically, which is necessary for numerical inversion by a nonlinear
optimizer. Monte Carlo simulations document the effectiveness of the code.Comment: 35 pages, 8 figure
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