63,447 research outputs found
A wall-crossing formula for Gromov-Witten invariants under variation of git quotient
We prove a quantum version of Kalkman's wall-crossing formula comparing
Gromov-Witten invariants on geometric invariant theory (git) quotients related
by a change in polarization. The wall-crossing terms are gauged Gromov-Witten
invariants with smaller structure group. As an application, we show that the
graph Gromov-Witten potentials of quotients related by wall-crossings of
crepant type are equivalent up to a distribution in the quantum parameter that
is almost everywhere zero. This is a version of the crepant transformation
conjecture of Li-Ruan, Bryan-Graber, Coates-Ruan etc. in cases where the
crepant transformation is obtained by variation of git.Comment: 64 pages, 1 figure. Expanded and clarified exposition in a number of
places in response to referee comment
Drugs and violence in Colombia: a VECM analysis
It has been widely argued that the production of illegal drugs, particularly cocaine, has financed guerrilla activity in Colombia. This paper uses quarterly time-series data for Colombia 1994-2005 to examine the interaction between legal agricultural production, illegal agricultural production of drugs and the number of guerrilla attacks. The time series analysis suggests that drug production acts as a weakly exogenous stochastic trend which has a negative effect on legal agricultural production and a positive effect on guerrilla attacks; with a long-run elasticity of attacks to drug production very close to unity
Voronoi Cell Patterns: theoretical model and applications
We use a simple fragmentation model to describe the statistical behavior of
the Voronoi cell patterns generated by a set of points in 1D and in 2D. In
particular, we are interested in the distribution of sizes of these Voronoi
cells. Our model is completely defined by two probability distributions in 1D
and again in 2D, the probability to add a new point inside an existing cell and
the probability that this new point is at a particular position relative to the
preexisting point inside this cell. In 1D the first distribution depends on a
single parameter while the second distribution is defined through a
fragmentation kernel; in 2D both distributions depend on a single parameter.
The fragmentation kernel and the control parameters are closely related to the
physical properties of the specific system under study. We use our model to
describe the Voronoi cell patterns of several systems. Specifically, we study
the island nucleation with irreversible attachment, the 1D car parking problem,
the formation of second-level administrative divisions, and the pattern formed
by the Paris M\'etro stations.Comment: 12 pages, 9 figure
Growing dust grains in protoplanetary discs - I. Radial drift with toy growth models
In a series of papers, we present a comprehensive analytic study of the
global motion of growing dust grains in protoplanetary discs, addressing both
the radial drift and the vertical settling of the particles. Here we study how
the radial drift of dust particles is affected by grain growth. In a first
step, toy models in which grain growth can either be constant, accelerate or
decelerate are introduced. The equations of motion are analytically integrable
and therefore the grains dynamics is easy to understand.
The radial motion of growing grains is governed by the relative efficiency of
the growth and migration processes which is expressed by the dimensionless
parameter Lambda, as well as the exponents for the gas surface density and
temperature profiles, denoted p and q respectively. When Lambda is of order
unity, growth and migration are strongly coupled, providing the most efficient
radial drift. For the toy models considered, grains pile up when -p+q+1/2<0.
Importantly, we show the existence of a second process which can help discs to
retain their solid materials. For accelerating growth, grains end up their
migration at a finite radius, thus avoiding being accreted onto the central
star.Comment: 12 pages, 9 figures. Accepted for publication in MNRAS. v2: typos
correcte
A Tale of Two Distributions: From Few To Many Vortices In Quasi-Two-Dimensional Bose-Einstein Condensates
Motivated by the recent successes of particle models in capturing the
precession and interactions of vortex structures in quasi-two-dimensional
Bose-Einstein condensates, we revisit the relevant systems of ordinary
differential equations. We consider the number of vortices as a parameter
and explore the prototypical configurations ("ground states") that arise in the
case of few or many vortices. In the case of few vortices, we modify the
classical result of Havelock [Phil. Mag. , 617 (1931)] illustrating
that vortex polygons in the form of a ring are unstable for .
Additionally, we reconcile this modification with the recent identification of
symmetry breaking bifurcations for the cases of . We also briefly
discuss the case of a ring of vortices surrounding a central vortex (so-called
configuration). We finally examine the opposite limit of large and
illustrate how a coarse-graining, continuum approach enables the accurate
identification of the radial distribution of vortices in that limit.Comment: 15 pages, 2 figure
Governance in Southeast Asia: Issues and Options
This paper attempts to analyze governance systems in Southeast Asia and proposes some policy suggestions that can improve governance practices in the region. It also discusses the links between governance and official development assistance (ODA) and the role of the Japan Bank for International Cooperation. To put the discussion on governance systems in a proper context, the paper discusses the governance and growth nexus in Southeast Asia; describes the operating governance systems in Southeast Asia; analyzes economic governance, more specifically in the areas of economic management and growth, revenue generation, social spending, access to services, cost of doing business, and corporate governance; and examines political governance, focusing on the rule of law and judicial independence, conflict management, and voice participation.governance, development program, corruption
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