51,388 research outputs found
Relaxing the Irrevocability Requirement for Online Graph Algorithms
Online graph problems are considered in models where the irrevocability
requirement is relaxed. Motivated by practical examples where, for example,
there is a cost associated with building a facility and no extra cost
associated with doing it later, we consider the Late Accept model, where a
request can be accepted at a later point, but any acceptance is irrevocable.
Similarly, we also consider a Late Reject model, where an accepted request can
later be rejected, but any rejection is irrevocable (this is sometimes called
preemption). Finally, we consider the Late Accept/Reject model, where late
accepts and rejects are both allowed, but any late reject is irrevocable. For
Independent Set, the Late Accept/Reject model is necessary to obtain a constant
competitive ratio, but for Vertex Cover the Late Accept model is sufficient and
for Minimum Spanning Forest the Late Reject model is sufficient. The Matching
problem has a competitive ratio of 2, but in the Late Accept/Reject model, its
competitive ratio is 3/2
Dynamical Mean Field Theory of Nickelate Superlattices
Dynamical mean field methods are used to calculate the phase diagram,
many-body density of states, relative orbital occupancy and Fermi surface shape
for a realistic model of -based superlattices. The model is derived
from density functional band calculations and includes oxygen orbitals. The
combination of the on-site Hunds interaction and charge-transfer between the
transition metal and the oxygen orbitals is found to reduce the orbital
polarization far below the levels predicted either by band structure
calculations or by many-body analyses of Hubbard-type models which do not
explicitly include the oxygen orbitals. The findings indicate that
heterostructuring is unlikely to produce one band model physics and demonstrate
the fundamental inadequacy of modeling the physics of late transition metal
oxides with Hubbard-like models.Comment: Values of orbitals polarizations reported in Fig. 2 corrected. We
thank E. Benckiser and M. Wu for pointing out the error
A scheme for demonstration of fractional statistics of anyons in an exactly solvable model
We propose a scheme to demonstrate fractional statistics of anyons in an
exactly solvable lattice model proposed by Kitaev that involves four-body
interactions. The required many-body ground state, as well as the anyon
excitations and their braiding operations, can be conveniently realized through
\textit{dynamic}laser manipulation of cold atoms in an optical lattice. Due to
the perfect localization of anyons in this model, we show that a quantum
circuit with only six qubits is enough for demonstration of the basic braiding
statistics of anyons. This opens up the immediate possibility of
proof-of-principle experiments with trapped ions, photons, or nuclear magnetic
resonance systems.Comment: 4 pages, 3 figure
Sharp norm estimates of layer potentials and operators at high frequency
In this paper, we investigate single and double layer potentials mapping boundary data to interior functions of a domain at high frequency λ2ââ. For single layer potentials, we find that the L2(âΩ)âL2(Ω) norms decay in λ. The rate of decay depends on the curvature of âΩ: The norm is λâ3/4 in general domains and λâ5/6 if the boundary âΩ is curved. The double layer potential, however, displays uniform L2(âΩ)âL2(Ω) bounds independent of curvature. By various examples, we show that all our estimates on layer potentials are sharp.
The appendix by Galkowski gives bounds L2(âΩ)âL2(âΩ) for the single and double layer operators at high frequency that are sharp modulo logλ. In this case, both the single and double layer operator bounds depend upon the curvature of the boundary
Shortcuts through Colocation Facilities
Network overlays, running on top of the existing Internet substrate, are of
perennial value to Internet end-users in the context of, e.g., real-time
applications. Such overlays can employ traffic relays to yield path latencies
lower than the direct paths, a phenomenon known as Triangle Inequality
Violation (TIV). Past studies identify the opportunities of reducing latency
using TIVs. However, they do not investigate the gains of strategically
selecting relays in Colocation Facilities (Colos). In this work, we answer the
following questions: (i) how Colo-hosted relays compare with other relays as
well as with the direct Internet, in terms of latency (RTT) reductions; (ii)
what are the best locations for placing the relays to yield these reductions.
To this end, we conduct a large-scale one-month measurement of inter-domain
paths between RIPE Atlas (RA) nodes as endpoints, located at eyeball networks.
We employ as relays Planetlab nodes, other RA nodes, and machines in Colos. We
examine the RTTs of the overlay paths obtained via the selected relays, as well
as the direct paths. We find that Colo-based relays perform the best and can
achieve latency reductions against direct paths, ranging from a few to 100s of
milliseconds, in 76% of the total cases; 75% (58% of total cases) of these
reductions require only 10 relays in 6 large Colos.Comment: In Proceedings of the ACM Internet Measurement Conference (IMC '17),
London, GB, 201
Evaluation of management strategies for the operation of high-speed railways in China
High-Speed Train (HST) operations have recently been introduced in rail passenger transportation markets worldwide. Although the technologies for such operations have levelled at speeds of around 300 km/h, the operating parameters to be adopted in each application will differ from country to country. The operating environment will be one of the crucial success factors for the implementation of HST operations in China. This paper compares three different management/ownership models which might be used in China. The paper analyzes the characteristics of each model and proposes an optimal plan of an operational system to develop HST operations in China by using a hierarchy goals achievement matrix approach
The mass function of hydrogen-rich white dwarfs: robust observational evidence for a distinctive high-mass excess near 1Msun
The mass function of hydrogen-rich atmosphere white dwarfs has been
frequently found to reveal a distinctive high-mass excess near 1Msun. However,
a significant excess of massive white dwarfs has not been detected in the mass
function of the largest white dwarf catalogue to date from the Sloan Digital
Sky Survey. Hence, whether a high-mass excess exists or not has remained an
open question. In this work we build the mass function of the latest catalogue
of data release 10 SDSS hydrogen-rich white dwarfs, including the cool and
faint population (i.e. effective temperatures 6,000 <~ Teff <~ 12,000 K,
equivalent to 12 mag <~ Mbol <~ 13 mag). We show that the high-mass excess is
clearly present in our mass function, and that it disappears only if the
hottest (brightest) white dwarfs (those with Teff >~ 12,000 K, Mbol <~ 12 mag)
are considered. This naturally explains why previous SDSS mass functions failed
at detecting a significant excess of high-mass white dwarfs. Thus, our results
provide additional and robust observational evidence for the existence of a
distinctive high-mass excess near 1Msun. We investigate possible origins of
this feature and argue that the most plausible scenario that may lead to an
observed excess of massive white dwarfs is the merger of the degenerate core of
a giant star with a main sequence or a white dwarf companion during or shortly
after a common envelope event.Comment: Accepted for publication by MNRA
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