12,783 research outputs found
On convergence to a football
We show that spheres of positive constant curvature with () conic
points converge to a sphere of positive constant curvature with two conic
points (or called an (American) football) in Gromov-Hausdorff topology when the
corresponding singular divisors converge to a critical divisor in the sense of
Troyanov.
We prove this convergence in two different ways. Geometrically, the
convergence follows from Luo-Tian's explicit description of conic spheres as
boundaries of convex polytopes in . Analytically, regarding the
conformal factors as the singular solutions to the corresponding PDE, we derive
the required a priori estimates and convergence result after proper
reparametrization.Comment: 19 page
On the geometric flows solving K\"ahlerian inverse equations
In this note, we extend our previous work on the inverse problem.
Inverse problem is a fully nonlinear geometric PDE on compact
K\"ahler manifolds. Given a proper geometric condition, we prove that a large
family of nonlinear geometric flows converges to the desired solution of the
given PDE.Comment: to appear in Pacific Journal of Mathematic
Volume bounds of conic 2-spheres
We obtain sharp volume bound for a conic 2-sphere in terms of its Gaussian
curvature bound. We also give the geometric models realizing the extremal
volume. In particular, when the curvature is bounded in absolute value by ,
we compute the minimal volume of a conic sphere in the sense of Gromov. In
order to apply the level set analysis and iso-perimetric inequality as in our
previous works, we develop some new analytical tools to treat regions with
vanishing curvature.Comment: 19 pages, 1 figur
Dependence in Propositional Logic: Formula-Formula Dependence and Formula Forgetting -- Application to Belief Update and Conservative Extension
Dependence is an important concept for many tasks in artificial intelligence.
A task can be executed more efficiently by discarding something independent
from the task. In this paper, we propose two novel notions of dependence in
propositional logic: formula-formula dependence and formula forgetting. The
first is a relation between formulas capturing whether a formula depends on
another one, while the second is an operation that returns the strongest
consequence independent of a formula. We also apply these two notions in two
well-known issues: belief update and conservative extension. Firstly, we define
a new update operator based on formula-formula dependence. Furthermore, we
reduce conservative extension to formula forgetting.Comment: We find a mistake in this version and we need a period of time to fix
i
Escobar-Yamabe compactifications for Poincare-Einstein manifolds and rigidity theorems
Let be a Poincar\'{e}-Einstein manifold which is
conformally compact with conformal infinity . On the conformal compactification via some boundary defining function , there are two types
of Yamabe constants: and
. (See definitions (\ref{def.type1}) and
(\ref{def.type2})). In \cite{GH}, Gursky and Han gave an inequality between
and . In this
paper, we first show that the equality holds in Gursky-Han's theorem if and
only if is isometric to the standard hyperbolic space
. Secondly, we derive an inequality between
and , and show
that the equality holds if and only if is isometric to
. Based on this, we give a simple proof of
the rigidity theorem for Poincar\'{e}-Einstein manifolds with conformal
infinity being conformally equivalent to the standard sphere.Comment: 1
The Obata equation with Robin boundary condition
We study the Obata equation with Robin boundary condition on manifolds with boundary, where . Dirichlet and Neumann boundary conditions were
previously studied by Reilly \cite{R}, Escobar \cite{Es} and Xia \cite{X}.
Compared with their results, the sign of plays an important role here. The
new discovery shows besides spherical domains, there are other manifolds for
both and . We also consider the Obata equation with non-vanishing
Neumann condition .Comment: 36 pages, 5 figure
On a class of fully nonlinear flow in K\"ahler geometry
In this paper, we study a class of fully nonlinear metric flow on K\"ahler
manifolds, which includes the J-flow as a special case. We provide a sufficient
and necessary condition for the long time convergence of the flow, generalizing
the result of Song-Weinkove. As a consequence, under the given condition, we
solved the corresponding Euler equation, which is fully nonlinear of
Monge-Amp\`ere type. As an application, we also discuss a complex
Monge-Amp\`ere type equation including terms of mixed degrees, which was first
posed by Chen.Comment: Added the second Appendix, some minor mistakes corrected. To appear
in Crelle's Journa
Content-Preserving Image Stitching with Regular Boundary Constraints
This paper proposes an approach to content-preserving stitching of images
with regular boundary constraints, which aims to stitch multiple images to
generate a panoramic image with regular boundary. Existing methods treat image
stitching and rectangling as two separate steps, which may result in suboptimal
results as the stitching process is not aware of the further warping needs for
rectangling. We address these limitations by formulating image stitching with
regular boundaries in a unified optimization. Starting from the initial
stitching results produced by traditional warping-based optimization, we obtain
the irregular boundary from the warped meshes by polygon Boolean operations
which robustly handle arbitrary mesh compositions, and by analyzing the
irregular boundary construct a piecewise rectangular boundary. Based on this,
we further incorporate straight line preserving and regular boundary
constraints into the image stitching framework, and conduct iterative
optimization to obtain an optimal piecewise rectangular boundary, thus can make
the panoramic boundary as close as possible to a rectangle, while reducing
unwanted distortions. We further extend our method to panoramic videos and
selfie photography, by integrating the temporal coherence and portrait
preservation into the optimization. Experiments show that our method
efficiently produces visually pleasing panoramas with regular boundaries and
unnoticeable distortions.Comment: 12 figures, 13 page
An immersed boundary method for fluid--structure--acoustics interactions involving large deformations and complex geometries
This paper presents an immersed boundary (IB) method for
fluid--structure--acoustics interactions involving large deformations and
complex geometries. In this method, the fluid dynamics is solved by a finite
difference method where the temporal, viscous and convective terms are
respectively discretized by the third-order Runge-Kutta scheme, the
fourth-order central difference scheme and a fifth-order W/TENO
(Weighted/Targeted Essentially Non-oscillation) scheme. Without loss of
generality, a nonlinear flexible plate is considered here, and is solved by a
finite element method based on the absolute nodal coordinate formulation. The
no-slip boundary condition at the fluid--structure interface is achieved by
using a diffusion-interface penalty IB method. With the above proposed method,
the aeroacoustics field generated by the moving boundaries and the associated
flows are inherently solved. In order to validate and verify the current
method, several benchmark cases are conducted: acoustic waves scattered from a
stationary cylinder in a quiescent flow, sound generation by a stationary and a
rotating cylinder in a uniform flow, sound generation by an insect in hovering
flight, deformation of a red blood cell induced by acoustic waves and acoustic
waves scattered by a stationary sphere. The comparison of the sound scattered
by a cylinder shows that the present IB--WENO scheme, a simple approach, has an
excellent performance which is even better than the implicit IB--lattice
Boltzmann method. For the sound scattered by a sphere, the IB--TENO scheme has
a lower dissipation compared with the IB--WENO scheme. Applications of this
technique to model fluid-structure-acoustics interactions of flapping foils
mimicking an insect wing section during forward flight and flapping foil energy
harvester are also presented, considering the effects of foil shape and
flexibility
Utility Optimal Thread Assignment and Resource Allocation in Distributed Systems
Achieving high performance in many distributed systems, such as a web hosting
center or the cloud requires finding a good assignment of worker threads to
servers and also effectively allocating each server's resources to its assigned
threads. The assignment and allocation components of this problem have been
studied extensively but largely separately in the literature. In this paper, we
introduce the \emph{assign and allocate (AA)} problem, which seeks to
simultaneously find an assignment and allocation that maximizes the total
utility of the threads. Assigning and allocating the threads together can
result in substantially better overall utility than performing the steps
separately, as is traditionally done. We model each thread by a utility
function giving its utility as a function of its assigned resources. We first
prove that the AA problem is NP-hard. We then present a factor approximation algorithm for concave utility functions, which runs
in time for threads and servers with
amount of resource each. We further present a faster algorithm with the same
approximation ratio and lower time complexity of . We then
extend our algorithms to solve AA problem with nonconcave utility functions and
achieve an approximation ratio . We conduct extensive experiments
to test the performance of our algorithms on threads with both synthetic and
realistic utility functions, and find that it achieves over 92\% of the optimal
utility on average. We also compare our algorithm against several other
assignment and allocation algorithms, and find that it achieves up to 9 times
better total utility.Comment: 15 page
- β¦