43,312 research outputs found
Criteria for strong and weak random attractors
The theory of random attractors has different notions of attraction, amongst
them pullback attraction and weak attraction. We investigate necessary and
sufficient conditions for the existence of pullback attractors as well as of
weak attractors
Climate dynamics and fluid mechanics: Natural variability and related uncertainties
The purpose of this review-and-research paper is twofold: (i) to review the
role played in climate dynamics by fluid-dynamical models; and (ii) to
contribute to the understanding and reduction of the uncertainties in future
climate-change projections. To illustrate the first point, we focus on the
large-scale, wind-driven flow of the mid-latitude oceans which contribute in a
crucial way to Earth's climate, and to changes therein. We study the
low-frequency variability (LFV) of the wind-driven, double-gyre circulation in
mid-latitude ocean basins, via the bifurcation sequence that leads from steady
states through periodic solutions and on to the chaotic, irregular flows
documented in the observations. This sequence involves local, pitchfork and
Hopf bifurcations, as well as global, homoclinic ones. The natural climate
variability induced by the LFV of the ocean circulation is but one of the
causes of uncertainties in climate projections. Another major cause of such
uncertainties could reside in the structural instability in the topological
sense, of the equations governing climate dynamics, including but not
restricted to those of atmospheric and ocean dynamics. We propose a novel
approach to understand, and possibly reduce, these uncertainties, based on the
concepts and methods of random dynamical systems theory. As a very first step,
we study the effect of noise on the topological classes of the Arnol'd family
of circle maps, a paradigmatic model of frequency locking as occurring in the
nonlinear interactions between the El Nino-Southern Oscillations (ENSO) and the
seasonal cycle. It is shown that the maps' fine-grained resonant landscape is
smoothed by the noise, thus permitting their coarse-grained classification.
This result is consistent with stabilizing effects of stochastic
parametrization obtained in modeling of ENSO phenomenon via some general
circulation models.Comment: Invited survey paper for Special Issue on The Euler Equations: 250
Years On, in Physica D: Nonlinear phenomen
A class of spatial econometric methods in the empirical analysis of clusters of firms in the space
In this paper we aim at identifying stylized facts in order to suggest adequate models of spatial co–agglomeration of industries. We describe a class of spatial statistical methods to be used in the empirical analysis of spatial clusters. Compared to previous contributions using point pattern methods, the main innovation of the present paper is to consider clustering for bivariate (rather than univariate) distributions, which allows uncovering co–agglomeration and repulsion phenomena between the different industrial sectors. Furthermore we present the results of an empirical application of such methods to a set of European Patent Office (EPO) data and we produce a series of empirical evidences referred to the the pair–wise intra–sectoral spatial distribution of patents in Italy in the nineties. In this analysis we are able to identify some distinctive joint patterns of location between patents of different sectors and to propose some possible economic interpretations
Accidents in (0,2) Landau-Ginzburg theories
We study the role of accidental symmetries in two-dimensional (0,2)
superconformal field theories obtained by RG flow from (0,2) Landau-Ginzburg
theories. These accidental symmetries are ubiquitous, and, unlike in the case
of (2,2) theories, their identification is key to correctly identifying the IR
fixed point and its properties. We develop a number of tools that help to
identify such accidental symmetries in the context of (0,2) Landau-Ginzburg
models and provide a conjecture for a toric structure of the SCFT moduli space
in a large class of models. We also give a self-contained discussion of aspects
of (0,2) conformal perturbation theory.Comment: 37 pages; expanded conformal perturbation theory discussion in v2;
fixed an accident in section 3.5 in v
Spreading of an infectious disease between different locations
The endogenous adaptation of agents, that may adjust their local contact
network in response to the risk of being infected, can have the perverse effect
of increasing the overall systemic infectiveness of a disease. We study a
dynamical model over two geographically distinct but interacting locations, to
better understand theoretically the mechanism at play. Moreover, we provide
empirical motivation from the Italian National Bovine Database, for the period
2006-2013.Comment: 22 pages (and 10 pages for appendix); 11 figures (2 in appendix
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