559 research outputs found
Curvature estimates for stable marginally trapped surfaces
We derive integral and sup-estimates for the curvature of stably marginally
outer trapped surfaces in a sliced space-time. The estimates bound the shear of
a marginally outer trapped surface in terms of the intrinsic and extrinsic
curvature of a slice containing the surface. These estimates are well adapted
to situations of physical insterest, such as dynamical horizons.Comment: 28 pages. This is a major rework of the previous version. It extends
the curvature estimates to no longer require global area bounds. In addition
some mistakes were correcte
Large isoperimetric surfaces in initial data sets
We study the isoperimetric structure of asymptotically flat Riemannian
3-manifolds (M,g) that are C^0-asymptotic to Schwarzschild of mass m>0.
Refining an argument due to H. Bray we obtain an effective volume comparison
theorem in Schwarzschild. We use it to show that isoperimetric regions exist in
(M, g) for all sufficiently large volumes, and that they are close to centered
coordinate spheres. This implies that the volume-preserving stable constant
mean curvature spheres constructed by G. Huisken and S.-T. Yau as well as R. Ye
as perturbations of large centered coordinate spheres minimize area among all
competing surfaces that enclose the same volume. This confirms a conjecture of
H. Bray. Our results are consistent with the uniqueness results for
volume-preserving stable constant mean curvature surfaces in initial data sets
obtained by G. Huisken and S.-T. Yau and strengthened by J. Qing and G. Tian.
The additional hypotheses that the surfaces be spherical and far out in the
asymptotic region in their results are not necessary in our work.Comment: 29 pages. All comments welcome! This is the final version to appear
in J. Differential Geo
Jenkins-Serrin type results for the Jang equation
Let (M, g, k) be an initial data set for the Einstein equations of general
relativity. We prove that there exist solutions of the Plateau problem for
marginally outer trapped surfaces (MOTSs) that are stable in the sense of
MOTSs. This answers a question of G. Galloway and N. O'Murchadha and is an
ingredient in the proof of the spacetime positive mass theorem given by L.-H.
Huang, D. Lee, R. Schoen and the first author. We show that a canonical
solution of the Jang equation exists in the complement of the union of all
weakly future outer trapped regions in the initial data set with respect to a
given end, provided that this complement contains no weakly past outer trapped
regions. The graph of this solution relates the area of the horizon to the
global geometry of the initial data set in a non-trivial way. We prove the
existence of a Scherk-type solution of the Jang equation outside the union of
all weakly future or past outer trapped regions in the initial data set. This
result is a natural exterior analogue for the Jang equation of the classical
Jenkins--Serrin theory. We extend and complement existence theorems by
Jenkins-Serrin, Spruck, Nelli-Rosenberg, Hauswirth-Rosenberg-Spruck, and
Pinheiro for Scherk-type constant mean curvature graphs over polygonal domains
in (M, g), where (M, g) is a complete Riemannian surface. We can dispense with
the a priori assumptions that a sub solution exists and that (M, g) has
particular symmetries. Also, our method generalizes to higher dimensions.Comment: All comments welcome. This is the final version to appear in J.
Differential Geo
The time evolution of marginally trapped surfaces
In previous work we have shown the existence of a dynamical horizon or
marginally trapped tube (MOTT) containing a given strictly stable marginally
outer trapped surface (MOTS). In this paper we show some results on the global
behavior of MOTTs assuming the null energy condition. In particular we show
that MOTSs persist in the sense that every Cauchy surface in the future of a
given Cauchy surface containing a MOTS also must contain a MOTS. We describe a
situation where the evolving outermost MOTS must jump during the coalescence of
two seperate MOTSs. We furthermore characterize the behavior of MOTSs in the
case that the principal eigenvalue vanishes under a genericity assumption. This
leads to a regularity result for the tube of outermost MOTSs under the
genericity assumption. This tube is then smooth up to finitely many jump times.
Finally we discuss the relation of MOTSs to singularities of a space-time.Comment: 21 pages. This revision corrects some typos and contains more
detailed proofs than the original versio
An AI Chatbot for Explaining Deep Reinforcement Learning Decisions of Service-oriented Systems
Deep Reinforcement Learning (Deep RL) is increasingly used to cope with the
open-world assumption in service-oriented systems. Deep RL was successfully
applied to problems such as dynamic service composition, job scheduling, and
offloading, as well as service adaptation. While Deep RL offers many benefits,
understanding the decision-making of Deep RL is challenging because its learned
decision-making policy essentially appears as a black box. Yet, understanding
the decision-making of Deep RL is key to help service developers perform
debugging, support service providers to comply with relevant legal frameworks,
and facilitate service users to build trust. We introduce Chat4XAI to
facilitate the understanding of the decision-making of Deep RL by providing
natural-language explanations. Compared with visual explanations, the reported
benefits of natural-language explanations include better understandability for
non-technical users, increased user acceptance and trust, as well as more
efficient explanations. Chat4XAI leverages modern AI chatbot technology and
dedicated prompt engineering. Compared to earlier work on natural-language
explanations using classical software-based dialogue systems, using an AI
chatbot eliminates the need for eliciting and defining potential questions and
answers up-front. We prototypically realize Chat4XAI using OpenAI's ChatGPT API
and evaluate the fidelity and stability of its explanations using an adaptive
service exemplar.Comment: To be published at 21st Int'l Conference on Service-Oriented
Computing (ICSOC 2023), Rome, Italy, November 28-December 1, 2023, ser. LNCS,
F. Monti, S. Rinderle-Ma, A. Ruiz Cortes, Z. Zheng, M. Mecella, Eds.,
Springer, 202
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