1,212 research outputs found
Stable Lengths on the pants graph are rational
For the pants graph, there is little known about the behaviour of geodesics,
as opposed to quasigeodesics. Brock-Masur-Minsky showed that geodesics or
geodesic segments connecting endpoints satisfying a bounded combinatorics
condition, such as the stable/unstable laminations of a pseudo-Anosov, all have
bounded combinatorics, \textit{outside of annuli}. In this paper it is shown
that there exist geodesics that also have bounded combinatorics within annuli.
These geodesics are shown to have finiteness properties analogous to those of
tight geodesics in the complex of curves, from which rationality of stable
lengths of pseudo-Anosovs acting on the pants graph then follows from the
arguments of Bowditch for the curve complex.Comment: No mathematical changes; had to add the number of the grant that
funded this wor
Examples of covering properties of boundary points of space-times
The problem of classifying boundary points of space-time, for example
singularities, regular points and points at infinity, is an unexpectedly subtle
one. Due to the fact that whether or not two boundary points are identified or
even "nearby" is dependant on the way the space-time is embedded, difficulties
occur when singularities are thought of as an inherently local aspect of a
space-time, as an analogy with electromagnetism would imply. The completion of
a manifold with respect to a pseudo-Riemannian metric can be defined
intrinsically, [SS94]. This is done via an equivalence relation, formalising
which boundary sets cover other sets. This paper works through the
possibilities, providing examples to show that all covering relations not
immediately ruled out by the definitions are possible
Downlink and Uplink Decoupling: a Disruptive Architectural Design for 5G Networks
Cell association in cellular networks has traditionally been based on the
downlink received signal power only, despite the fact that up and downlink
transmission powers and interference levels differed significantly. This
approach was adequate in homogeneous networks with macro base stations all
having similar transmission power levels. However, with the growth of
heterogeneous networks where there is a big disparity in the transmit power of
the different base station types, this approach is highly inefficient. In this
paper, we study the notion of Downlink and Uplink Decoupling (DUDe) where the
downlink cell association is based on the downlink received power while the
uplink is based on the pathloss. We present the motivation and assess the gains
of this 5G design approach with simulations that are based on Vodafone's LTE
field trial network in a dense urban area, employing a high resolution
ray-tracing pathloss prediction and realistic traffic maps based on live
network measurements.Comment: 6 pages, 7 figures, conference paper, submitted to IEEE GLOBECOM 201
Resonant scattering by magnetic impurities as a model for spin relaxation in bilayer graphene
We propose that the observed spin-relaxation in bilayer graphene is due to
resonant scattering by magnetic impurities. We analyze a resonant scattering
model due to adatoms on both dimer and non-dimer sites, finding that only the
former give narrow resonances at the charge neutrality point. Opposite to
single-layer graphene, the measured spin-relaxation rate in graphene bilayer
increases with carrier density. Although it has been commonly argued that a
different mechanism must be at play for the two structures, our model explains
this behavior rather naturally in terms of different broadening scales for the
same underlying resonant processes. Not only our results---using robust and
first-principles inspired parameters---agree with experiment, they also predict
an experimentally testable sharp decrease of the spin-relaxation rate at high
carrier densities.Comment: 6 pages, 3 figures + 2 pages Suppl. Materia
The Differential Topology of the Thurston Spine of Teichm\"uller Space
In \cite{Thurston}, a short, simple and elegant construction of a mapping
class group-equivariant deformation retraction of Teichm\"uller space of a
closed compact surface was given. The preprint \cite{Thurston}, which
unfortunately is not online, has not been broadly accepted. The purpose of this
paper is to go through the construction in detail, resolve any questions that
have arisen in the literature and in personal communications. An explicit
example is given to show that one of the claims needs to be modified, and
details of how to do this are given. A mapping class group-equivariant
deformation retraction of the Thurston spine onto a complex of dimension equal
to the virtual cohomological dimension of the mapping class group is then
constructed.Comment: A complete reformulation. A shorter proof of the vcd result was
given, and a claim in the Thurston construction was fixed. Some of the
material was moved to the paper submit/534164
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