68,799 research outputs found
Algorithms For Extracting Timeliness Graphs
We consider asynchronous message-passing systems in which some links are
timely and processes may crash. Each run defines a timeliness graph among
correct processes: (p; q) is an edge of the timeliness graph if the link from p
to q is timely (that is, there is bound on communication delays from p to q).
The main goal of this paper is to approximate this timeliness graph by graphs
having some properties (such as being trees, rings, ...). Given a family S of
graphs, for runs such that the timeliness graph contains at least one graph in
S then using an extraction algorithm, each correct process has to converge to
the same graph in S that is, in a precise sense, an approximation of the
timeliness graph of the run. For example, if the timeliness graph contains a
ring, then using an extraction algorithm, all correct processes eventually
converge to the same ring and in this ring all nodes will be correct processes
and all links will be timely. We first present a general extraction algorithm
and then a more specific extraction algorithm that is communication efficient
(i.e., eventually all the messages of the extraction algorithm use only links
of the extracted graph)
A low optical depth region in the inner disk of the HerbigAe star HR5999
Circumstellar disks surrounding young stars are known to be the birthplaces
of planets, and the innermost astronomical unit is of particular interest. We
present new long-baseline spectro-interferometric observations of the HerbigAe
star, HR5999, obtained in the H and K bands with the AMBER instrument at the
VLTI, and aim to produce near-infrared images at the sub-AU spatial scale. We
spatially resolve the circumstellar material and reconstruct images using the
MiRA algorithm. In addition, we interpret the interferometric observations
using models that assume that the near-infrared excess is dominated by the
emission of a circumstellar disk. We compare the images reconstructed from the
VLTI measurements to images obtained using simulated model data. The K-band
image reveals three main elements: a ring-like feature located at ~0.65 AU, a
low surface brightness region inside, and a central spot. At the maximum
angular resolution of our observations (1.3 mas), the ring is resolved while
the central spot is only marginally resolved, preventing us from revealing the
exact morphology of the circumstellar environment. We suggest that the ring
traces silicate condensation, i.e., an opacity change, in a circumstellar disk
around HR 5999. We build a model that includes a ring at the silicate
sublimation radius and an inner disk of low surface brightness responsible for
a large amount of the near-infrared continuum emission. The model successfully
fits the SED, visibilities, and closure phases, and provides evidence of a low
surface brightness region inside the silicate sublimation radius. This study
provides additional evidence that in HerbigAe stars, there is material in a low
surface brightness region, probably a low optical depth region, located inside
the silicate sublimation radius and of unknown nature.Comment: 11 pages, 10 figure
The radius of a subcategory of modules
We introduce a new invariant for subcategories X of finitely generated
modules over a local ring R which we call the radius of X. We show that if R is
a complete intersection and X is resolving, then finiteness of the radius
forces X to contain only maximal Cohen-Macaulay modules. We also show that the
category of maximal Cohen-Macaulay modules has finite radius when R is a
Cohen-Macaulay complete local ring with perfect coefficient field. We link the
radius to many well-studied notions such as the dimension of the stable
category of maximal Cohen-Macaulay modules, finite/countable Cohen-Macaulay
representation type and the uniform Auslander condition.Comment: Final version, to appear in Algebra and Number Theor
Homology of perfect complexes
It is proved that the sum of the Loewy lengths of the homology modules of a
finite free complex F over a local ring R is bounded below by a number
depending only on R. This result uncovers, in the structure of modules of
finite projective dimension, obstructions to realizing R as a closed fiber of
some flat local homomorphism. Other applications include, as special cases,
uniform proofs of known results on free actions of elementary abelian groups
and of tori on finite CW complexes. The arguments use numerical invariants of
objects in general triangulated categories, introduced here and called levels.
They allow one to track, through changes of triangulated categories,
homological invariants like projective dimension, as well as structural
invariants like Loewy length. An intermediate result sharpens, with a new
proof, the New Intersection Theorem for commutative algebras over fields. Under
additional hypotheses on the ring stronger estimates are proved for Loewy
lengths of modules of finite projective dimension.Comment: This version corrects an error in the statement (and proof) of
Theorem 7.4 in the published version of the paper [Adv. Math. 223 (2010)
1731--1781]. These changes do not affect any other results or proofs in the
paper. A corrigendum has been submitted
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