33,923 research outputs found
Coarsening of vortex ripples in sand
The coarsening of an array of vortex ripples prepared in an unstable state is
discussed within the framework of a simple mass transfer model first introduced
by K.H. Andersen et al. [Phys. Rev. E 63, 066308 (2001)]. Two scenarios for the
selection of the final pattern are identified. When the initial state is
homogeneous with uniform random perturbations, a unique final state is reached
which depends only on the shape of the interaction function . A
potential formulation of the dynamics suggests that the final wavelength is
determined by a Maxwell construction applied to , but comparison
with numerical simulations shows that this yields only an upper bound. In
contrast, the evolution from a perfectly homogeneous state with a localized
perturbation proceeds through the propagation of wavelength doubling fronts.
The front speed can be predicted by standard marginal stability theory. In this
case the final wavelength depends on the initial wavelength in a complicated
manner which involves multiplication by factors of 2 and rational ratios such
as 4/3.Comment: 7 pages, 4 figure
Universal Constants, Standard Models and Fundamental Metrology
Taking into account four universal constants, namely the Planck's constant
, the velocity of light , the constant of gravitation and the
Boltzmann's constant leads to structuring theoretical physics in terms of
three theories each taking into account a pair of constants: the quantum theory
of fields ( and ), the general theory of relativity ( and ) and
quantum statistics ( and ). These three theories are not yet unified but,
together, they underlie the standard models that allow a satisfactory
phenomenological description of all experimental or observational data, in
particle physics and in cosmology and they provide, through the modern
interpretation of quantum physics, fundamental metrology with a reliable
theoretical basis
A New Rational Algorithm for View Updating in Relational Databases
The dynamics of belief and knowledge is one of the major components of any
autonomous system that should be able to incorporate new pieces of information.
In order to apply the rationality result of belief dynamics theory to various
practical problems, it should be generalized in two respects: first it should
allow a certain part of belief to be declared as immutable; and second, the
belief state need not be deductively closed. Such a generalization of belief
dynamics, referred to as base dynamics, is presented in this paper, along with
the concept of a generalized revision algorithm for knowledge bases (Horn or
Horn logic with stratified negation). We show that knowledge base dynamics has
an interesting connection with kernel change via hitting set and abduction. In
this paper, we show how techniques from disjunctive logic programming can be
used for efficient (deductive) database updates. The key idea is to transform
the given database together with the update request into a disjunctive
(datalog) logic program and apply disjunctive techniques (such as minimal model
reasoning) to solve the original update problem. The approach extends and
integrates standard techniques for efficient query answering and integrity
checking. The generation of a hitting set is carried out through a hyper
tableaux calculus and magic set that is focused on the goal of minimality.Comment: arXiv admin note: substantial text overlap with arXiv:1301.515
- …