28,506 research outputs found
Optimized Adaptive Streaming Representations based on System Dynamics
Adaptive streaming addresses the increasing and heterogenous demand of
multimedia content over the Internet by offering several encoded versions for
each video sequence. Each version (or representation) has a different
resolution and bit rate, aimed at a specific set of users, like TV or mobile
phone clients. While most existing works on adaptive streaming deal with
effective playout-control strategies at the client side, we take in this paper
a providers' perspective and propose solutions to improve user satisfaction by
optimizing the encoding rates of the video sequences. We formulate an integer
linear program that maximizes users' average satisfaction, taking into account
the network dynamics, the video content information, and the user population
characteristics. The solution of the optimization is a set of encoding
parameters that permit to create different streams to robustly satisfy users'
requests over time. We simulate multiple adaptive streaming sessions
characterized by realistic network connections models, where the proposed
solution outperforms commonly used vendor recommendations, in terms of user
satisfaction but also in terms of fairness and outage probability. The
simulation results further show that video content information as well as
network constraints and users' statistics play a crucial role in selecting
proper encoding parameters to provide fairness a mong users and to reduce
network resource usage. We finally propose a few practical guidelines that can
be used to choose the encoding parameters based on the user base
characteristics, the network capacity and the type of video content
Box Graphs and Singular Fibers
We determine the higher codimension fibers of elliptically fibered Calabi-Yau
fourfolds with section by studying the three-dimensional N=2 supersymmetric
gauge theory with matter which describes the low energy effective theory of
M-theory compactified on the associated Weierstrass model, a singular model of
the fourfold. Each phase of the Coulomb branch of this theory corresponds to a
particular resolution of the Weierstrass model, and we show that these have a
concise description in terms of decorated box graphs based on the
representation graph of the matter multiplets, or alternatively by a class of
convex paths on said graph. Transitions between phases have a simple
interpretation as `flopping' of the path, and in the geometry correspond to
actual flop transitions. This description of the phases enables us to enumerate
and determine the entire network between them, with various matter
representations for all reductive Lie groups. Furthermore, we observe that each
network of phases carries the structure of a (quasi-)minuscule representation
of a specific Lie algebra. Interpreted from a geometric point of view, this
analysis determines the generators of the cone of effective curves as well as
the network of flop transitions between crepant resolutions of singular
elliptic Calabi-Yau fourfolds. From the box graphs we determine all fiber types
in codimensions two and three, and we find new, non-Kodaira, fiber types for
E_6, E_7 and E_8.Comment: 107 pages, 44 figures, v2: added case of E7 monodromy-reduced fiber
Efficient data structures for masks on 2D grids
This article discusses various methods of representing and manipulating
arbitrary coverage information in two dimensions, with a focus on space- and
time-efficiency when processing such coverages, storing them on disk, and
transmitting them between computers. While these considerations were originally
motivated by the specific tasks of representing sky coverage and cross-matching
catalogues of astronomical surveys, they can be profitably applied in many
other situations as well.Comment: accepted by A&
Exponential Networks and Representations of Quivers
We study the geometric description of BPS states in supersymmetric theories
with eight supercharges in terms of geodesic networks on suitable spectral
curves. We lift and extend several constructions of Gaiotto-Moore-Neitzke from
gauge theory to local Calabi-Yau threefolds and related models. The
differential is multi-valued on the covering curve and features a new type of
logarithmic singularity in order to account for D0-branes and non-compact
D4-branes, respectively. We describe local rules for the three-way junctions of
BPS trajectories relative to a particular framing of the curve. We reproduce
BPS quivers of local geometries and illustrate the wall-crossing of finite-mass
bound states in several new examples. We describe first steps toward
understanding the spectrum of framed BPS states in terms of such "exponential
networks."Comment: 82 pages, 60 figures, typos fixe
When Rational Sections Become Cyclic: Gauge Enhancement in F-theory via Mordell--Weil Torsion
We explore novel gauge enhancements from abelian to non-simply-connected
gauge groups in F-theory. To this end we consider complex structure
deformations of elliptic fibrations with a Mordell--Weil group of rank one and
identify the conditions under which the generating section becomes torsional.
For the specific case of Z2 torsion we construct the generic solution to these
conditions and show that the associated F-theory compactification exhibits the
global gauge group [SU(2) x SU(4)]/Z2 x SU(2). The subsolution with gauge group
SU(2)/Z2 x SU(2), for which we provide a global resolution, is related by a
further complex structure deformation to a genus-one fibration with a bisection
whose Jacobian has a Z2 torsional section. While an analysis of the spectrum on
the Jacobian fibration reveals an SU(2)/Z2 x Z2 gauge theory, reproducing this
result from the bisection geometry raises some conceptual puzzles about
F-theory on genus-one fibrations.Comment: 51 page
- âŠ