19,056 research outputs found
Modal Interface Automata
De Alfaro and Henzinger's Interface Automata (IA) and Nyman et al.'s recent
combination IOMTS of IA and Larsen's Modal Transition Systems (MTS) are
established frameworks for specifying interfaces of system components. However,
neither IA nor IOMTS consider conjunction that is needed in practice when a
component shall satisfy multiple interfaces, while Larsen's MTS-conjunction is
not closed and Bene\v{s} et al.'s conjunction on disjunctive MTS does not treat
internal transitions. In addition, IOMTS-parallel composition exhibits a
compositionality defect. This article defines conjunction (and also
disjunction) on IA and disjunctive MTS and proves the operators to be
'correct', i.e., the greatest lower bounds (least upper bounds) wrt. IA- and
resp. MTS-refinement. As its main contribution, a novel interface theory called
Modal Interface Automata (MIA) is introduced: MIA is a rich subset of IOMTS
featuring explicit output-must-transitions while input-transitions are always
allowed implicitly, is equipped with compositional parallel, conjunction and
disjunction operators, and allows a simpler embedding of IA than Nyman's. Thus,
it fixes the shortcomings of related work, without restricting designers to
deterministic interfaces as Raclet et al.'s modal interface theory does.Comment: 28 page
Online Bin Covering: Expectations vs. Guarantees
Bin covering is a dual version of classic bin packing. Thus, the goal is to
cover as many bins as possible, where covering a bin means packing items of
total size at least one in the bin.
For online bin covering, competitive analysis fails to distinguish between
most algorithms of interest; all "reasonable" algorithms have a competitive
ratio of 1/2. Thus, in order to get a better understanding of the combinatorial
difficulties in solving this problem, we turn to other performance measures,
namely relative worst order, random order, and max/max analysis, as well as
analyzing input with restricted or uniformly distributed item sizes. In this
way, our study also supplements the ongoing systematic studies of the relative
strengths of various performance measures.
Two classic algorithms for online bin packing that have natural dual versions
are Harmonic and Next-Fit. Even though the algorithms are quite different in
nature, the dual versions are not separated by competitive analysis. We make
the case that when guarantees are needed, even under restricted input
sequences, dual Harmonic is preferable. In addition, we establish quite robust
theoretical results showing that if items come from a uniform distribution or
even if just the ordering of items is uniformly random, then dual Next-Fit is
the right choice.Comment: IMADA-preprint-c
Young and intermediate-age massive star clusters
An overview of our current understanding of the formation and evolution of
star clusters is given, with main emphasis on high-mass clusters. Clusters form
deeply embedded within dense clouds of molecular gas. Left-over gas is cleared
within a few million years and, depending on the efficiency of star formation,
the clusters may disperse almost immediately or remain gravitationally bound.
Current evidence suggests that a few percent of star formation occurs in
clusters that remain bound, although it is not yet clear if this fraction is
truly universal. Internal two-body relaxation and external shocks will lead to
further, gradual dissolution on timescales of up to a few hundred million years
for low-mass open clusters in the Milky Way, while the most massive clusters (>
10^5 Msun) have lifetimes comparable to or exceeding the age of the Universe.
The low-mass end of the initial cluster mass function is well approximated by a
power-law distribution, dN/dM ~ M^{-2}, but there is mounting evidence that
quiescent spiral discs form relatively few clusters with masses M > 2 x 10^5
Msun. In starburst galaxies and old globular cluster systems, this limit
appears to be higher, at least several x 10^6 Msun. The difference is likely
related to the higher gas densities and pressures in starburst galaxies, which
allow denser, more massive giant molecular clouds to form. Low-mass clusters
may thus trace star formation quite universally, while the more long-lived,
massive clusters appear to form preferentially in the context of violent star
formation.Comment: 21 pages, 3 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 9 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
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Distributed tuplespace and location management - an integrated perspective using Bluetooth
Location based or "context aware" computing is becoming increasingly recognized as a vital part of a mobile computing environment. As a consequence, the need for location-management middleware is widely recognized and actively researched. Location management is frequently offered to the application through an API where the location is given in the form of coordinates. It is the opinion of the authors that a localization API should offer localized data (e.g. direction to the nearest pharmacy) directly through a transparent and integrated API. Our proposed middleware for location and context management is built on top of Mobispace. Mobispace is a distributed tuplespace made for J2me units where replication between local replicas takes place with a central server (over GPRS) or with other mobile units (using Bluetooth). Since a Bluetooth connection indicates physical proximity to another node, a set of stationary nodes may distribute locality information over Bluetooth connections, and this information may be retrieved through the ordinary tuplespace AP
Continuous star cluster formation in the spiral NGC 45
We determined ages for 52 star clusters with masses < 10^6 solar masses in
the low surface brightness spiral galaxy NGC 45. Four of these candidates are
old globular clusters located in the bulge. The remaining ones span a large age
range. The cluster ages suggest a continuous star/cluster formation history
without evidence for bursts, consistent with the galaxy being located in a
relatively unperturbed environment in the outskirts of the Sculptor group.Comment: 4 pages, 3 figures. To appear in "Island Universes - Structure and
Evolution of Disk Galaxies", Terschelling (Netherlands), July 200
A Young Globular Cluster in the Galaxy NGC 6946
A globular cluster ~15 My old that contains 5x10^5 Msun of stars inside an 11
pc radius has been found in the nearby spiral galaxy NGC 6946, surrounded by
clouds of dust and smaller young clusters inside a giant circular bubble 300 pc
in radius. At the edge of the bubble is an arc of regularly-spaced clusters
that could have been triggered during the bubble's formation. The region is at
the end of a spiral arm, suggesting an origin by the asymmetric collapse of
spiral arm gas. The globular is one of the nearest examples of a cluster that
is similar to the massive old globulars in the Milky Way. We consider the
energetics of the bubble and possible formation mechanisms for the globular
cluster, including the coalescence of smaller clusters.Comment: 20 pages, 7 figures, accepted for Astrophysical Journal Vol 535, June
1 200
Ca_3AlSb_3: an inexpensive, non-toxic thermoelectric material for waste heat recovery
Thermoelectric materials directly convert thermal energy into electrical energy, offering a promising solid-state solution for waste
heat recovery. For thermoelectric devices to make a significant impact
on energy and the environment the major impediments are the efficiency,
availability and toxicity of current thermoelectric materials.
Typically, efficient thermoelectric materials contain heavy elements
such as lead and tellurium that are toxic and not earth abundant. Many
materials with unusual structures containing abundant and benign
elements are known, but remain unexplored for thermoelectric
applications. In this paper we demonstrate, with the discovery of high
thermoelectric efficiency in Ca_3AlSb_3, the use of elementary
solid-state chemistry and physics to guide the search and optimization
of such materials
Area products for stationary black hole horizons
Area products for multi-horizon stationary black holes often have intriguing
properties, and are often (though not always) independent of the mass of the
black hole itself (depending only on various charges, angular momenta, and
moduli). Such products are often formulated in terms of the areas of inner
(Cauchy) horizons and outer (event) horizons, and sometimes include the effects
of unphysical "virtual" horizons. But the conjectured mass-independence
sometimes fails. Specifically, for the Schwarzschild-de Sitter [Kottler] black
hole in (3+1) dimensions it is shown by explicit exact calculation that the
product of event horizon area and cosmological horizon area is not mass
independent. (Including the effect of the third "virtual" horizon does not
improve the situation.) Similarly, in the Reissner-Nordstrom-anti-de Sitter
black hole in (3+1) dimensions the product of inner (Cauchy) horizon area and
event horizon area is calculated (perturbatively), and is shown to be not mass
independent. That is, the mass-independence of the product of physical horizon
areas is not generic. In spherical symmetry, whenever the quasi-local mass m(r)
is a Laurent polynomial in aerial radius, r=sqrt{A/4\pi}, there are
significantly more complicated mass-independent quantities, the elementary
symmetric polynomials built up from the complete set of horizon radii (physical
and virtual). Sometimes it is possible to eliminate the unphysical virtual
horizons, constructing combinations of physical horizon areas that are mass
independent, but they tend to be considerably more complicated than the simple
products and related constructions currently being mooted in the literature.Comment: V1: 16 pages; V2: 9 pages (now formatted in PRD style). Minor change
in title. Extra introduction, background, discussion. Several additional
references; other references updated. Minor typos fixed. This version
accepted for publication in PRD; V3: Minor typos fixed. Published versio
Reduction of lattice thermal conductivity from planar faults in the layered Zintl compound SrZnSb_2
The layered Zintl compound SrZnSb_2 is investigated using transmission electron microscopy (TEM) to understand the low lattice thermal conductivity. The material displays out-of-phase boundaries with a spacing from 100 down to 2 nm. Density functional theory calculations confirm that the TEM-derived defect structure is energetically reasonable. The impact of these defects on phonon scattering is analyzed within the DebyeâCallaway model, which reveals a significant reduction in the acoustic phonon mean free path. This enhancement in phonon scattering leads to an ~30% reduction in lattice thermal conductivity at 300 K
von Neuman algebras of strongly connected higher-rank graphs
We investigate the factor types of the extremal KMS states for the preferred
dynamics on the Toeplitz algebra and the Cuntz--Krieger algebra of a strongly
connected finite -graph. For inverse temperatures above 1, all of the
extremal KMS states are of type I. At inverse temperature 1, there is
a dichotomy: if the -graph is a simple -dimensional cycle, we obtain a
finite type I factor; otherwise we obtain a type III factor, whose Connes
invariant we compute in terms of the spectral radii of the coordinate matrices
and the degrees of cycles in the graph.Comment: 16 pages; 1 picture prepared using TikZ. Version 2: this version to
appear in Math. An
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