2,316 research outputs found
Multiple relaxations in temporal planning
CRIKEY is a planner that separates out the scheduling from the classical parts of temporal planning. This can be seen as a relaxation of the temporal information during the classical planning phase. Relaxations in planning are used to guide the search. However, the quality of the relaxation greatly affects the performance of the planner, and in some cases can lead the search into a dead end. This can happen whilst separating out the planning and scheduling problems, leading to the production of an unschedulable plan. CRIKEY can detect these cases and change the relaxation accordingly
High-Dimensional Diffusive Growth
We consider a model of aggregation, both diffusion-limited and ballistic,
based on the Cayley tree. Growth is from the leaves of the tree towards the
root, leading to non-trivial screening and branch competition effects. The
model exhibits a phase transition between ballistic and diffusion-controlled
growth, with non-trivial corrections to cluster size at the critical point.
Even in the ballistic regime, cluster scaling is controlled by extremal
statistics due to the branching structure of the Cayley tree; it is the
extremal nature of the fluctuations that enables us to solve the model.Comment: 5 pages, 3 figures; reference adde
Multifractal Dimensions for Branched Growth
A recently proposed theory for diffusion-limited aggregation (DLA), which
models this system as a random branched growth process, is reviewed. Like DLA,
this process is stochastic, and ensemble averaging is needed in order to define
multifractal dimensions. In an earlier work [T. C. Halsey and M. Leibig, Phys.
Rev. A46, 7793 (1992)], annealed average dimensions were computed for this
model. In this paper, we compute the quenched average dimensions, which are
expected to apply to typical members of the ensemble. We develop a perturbative
expansion for the average of the logarithm of the multifractal partition
function; the leading and sub-leading divergent terms in this expansion are
then resummed to all orders. The result is that in the limit where the number
of particles n -> \infty, the quenched and annealed dimensions are {\it
identical}; however, the attainment of this limit requires enormous values of
n. At smaller, more realistic values of n, the apparent quenched dimensions
differ from the annealed dimensions. We interpret these results to mean that
while multifractality as an ensemble property of random branched growth (and
hence of DLA) is quite robust, it subtly fails for typical members of the
ensemble.Comment: 82 pages, 24 included figures in 16 files, 1 included tabl
Exact Multifractal Spectra for Arbitrary Laplacian Random Walks
Iterated conformal mappings are used to obtain exact multifractal spectra of
the harmonic measure for arbitrary Laplacian random walks in two dimensions.
Separate spectra are found to describe scaling of the growth measure in time,
of the measure near the growth tip, and of the measure away from the growth
tip. The spectra away from the tip coincide with those of conformally invariant
equilibrium systems with arbitrary central charge , with related
to the particular walk chosen, while the scaling in time and near the tip
cannot be obtained from the equilibrium properties.Comment: 4 pages, 3 figures; references added, minor correction
Branched Growth with Walkers
Diffusion-limited aggregation has a natural generalization to the
"-models", in which random walkers must arrive at a point on the
cluster surface in order for growth to occur. It has recently been proposed
that in spatial dimensionality , there is an upper critical
above which the fractal dimensionality of the clusters is D=1. I compute the
first order correction to for , obtaining . The
methods used can also determine multifractal dimensions to first order in
.Comment: 6 pages, 1 figur
Mechanics\u27 Liens
In 1977 and 1978 the Florida Legislature made extensive amendments to the Mechanics\u27 Lien Law. The statute, however, remains intricate and elusive for builder, lawyer and lienor alike. The authors review the amendments and case law of the past two years and offer suggestions on how to avoid the many pitfalls of the statute. In addition, the article contains a section devoted to the equitable lien
Transfer across Random versus Deterministic Fractal Interfaces
A numerical study of the transfer across random fractal surfaces shows that
their responses are very close to the response of deterministic model
geometries with the same fractal dimension. The simulations of several
interfaces with prefractal geometries show that, within very good
approximation, the flux depends only on a few characteristic features of the
interface geometry: the lower and higher cut-offs and the fractal dimension.
Although the active zones are different for different geometries, the electrode
reponses are very nearly the same. In that sense, the fractal dimension is the
essential "universal" exponent which determines the net transfer.Comment: 4 pages, 6 figure
Diffusion-Reorganized Aggregates: Attractors in Diffusion Processes?
A process based on particle evaporation, diffusion and redeposition is
applied iteratively to a two-dimensional object of arbitrary shape. The
evolution spontaneously transforms the object morphology, converging to
branched structures. Independently of initial geometry, the structures found
after long time present fractal geometry with a fractal dimension around 1.75.
The final morphology, which constantly evolves in time, can be considered as
the dynamic attractor of this evaporation-diffusion-redeposition operator. The
ensemble of these fractal shapes can be considered to be the {\em dynamical
equilibrium} geometry of a diffusion controlled self-transformation process.Comment: 4 pages, 5 figure
Multiphase PC/PL Relations: Comparison between Theory and observations
Cepheids are fundamental objects astrophysically in that they hold the key to
a CMB independent estimate of Hubble's constant. A number of researchers have
pointed out the possibilities of breaking degeneracies between Omega_Matter and
H0 if there is a CMB independent distance scale accurate to a few percent (Hu
2005). Current uncertainties in the distance scale are about 10% but future
observations, with, for example, the JWST, will be capable of estimating H0 to
within a few percent. A crucial step in this process is the Cepheid PL
relation. Recent evidence has emerged that the PL relation, at least in optical
bands, is nonlinear and that neglect of such a nonlinearity can lead to errors
in estimating H0 of up to 2 percent. Hence it is important to critically
examine this possible nonlinearity both observationally and theoretically.
Existing PC/PL relations rely exclusively on evaluating these relations at mean
light. However, since such relations are the average of relations at different
phases. Here we report on recent attempts to compare theory and observation in
the multiphase PC/PL planes. We construct state of the art Cepheid pulsations
models appropriate for the LMC/Galaxy and compare the resulting PC/PL relations
as a function of phase with observations. For the LMC, the (V-I) period-color
relation at minimum light can have quite a narrow dispersion (0.2-0.3 mags) and
thus could be useful in placing constraints on models. At longer periods, the
models predict significantly redder (by about 0.2-0.3 mags) V-I colors. We
discuss possible reasons for this and also compare PL relations at various
phases of pulsation and find clear evidence in both theory and observations for
a nonlinear PL relation.Comment: 5 pages, 8 figures, proceeding for "Stellar Pulsation: Challenges for
Theory and Observation", Santa Fe 200
Diffusion Limited Aggregation with Power-Law Pinning
Using stochastic conformal mapping techniques we study the patterns emerging
from Laplacian growth with a power-law decaying threshold for growth
(where is the radius of the particle cluster). For
the growth pattern is in the same universality class as diffusion
limited aggregation (DLA) growth, while for the resulting patterns
have a lower fractal dimension than a DLA cluster due to the
enhancement of growth at the hot tips of the developing pattern. Our results
indicate that a pinning transition occurs at , significantly
smaller than might be expected from the lower bound
of multifractal spectrum of DLA. This limiting case shows that the most
singular tips in the pruned cluster now correspond to those expected for a
purely one-dimensional line. Using multifractal analysis, analytic expressions
are established for both close to the breakdown of DLA universality
class, i.e., , and close to the pinning transition, i.e.,
.Comment: 5 pages, e figures, submitted to Phys. Rev.
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