7,099 research outputs found
Stratifications of Newton polygon strata and Traverso's conjectures for p-divisible groups
The isomorphism number (resp. isogeny cutoff) of a p-divisible group D over
an algebraically closed field is the least positive integer m such that D[p^m]
determines D up to isomorphism (resp. up to isogeny). We show that these
invariants are lower semicontinuous in families of p-divisible groups of
constant Newton polygon. Thus they allow refinements of Newton polygon strata.
In each isogeny class of p-divisible groups, we determine the maximal value of
isogeny cutoffs and give an upper bound for isomorphism numbers, which is shown
to be optimal in the isoclinic case. In particular, the latter disproves a
conjecture of Traverso. As an application, we answer a question of Zink on the
liftability of an endomorphism of D[p^m] to D.Comment: 50 pages, to appear in Annals of Mathematic
Level m stratifications of versal deformations of p-divisible groups
Let be an algebraically closed field of characteristic . Let
be positive integers. Let be a -divisible group of codimension and
dimension over . Let \scrD be a versal deformation of over a
smooth -scheme \scrA which is equidimensional of dimension . We show
that there exists a reduced, locally closed subscheme \grs_D(m) of \scrA
that has the following property: a point y\in\scrA(k) belongs to
\grs_D(m)(k) if and only if y^*(\scrD)[p^m] is isomorphic to . We
prove that \grs_D(m) is {\it regular and equidimensional} of {\it dimension}
. We give a proof of {\it Traverso's
formula} which for computes the codimension of \grs_D(m) in \scrA
(i.e., ) in terms of the Newton polygon of .
We also provide a criterion of when \grs_D(m) satisfies the {\it purity
property} (i.e., it is an affine \scrA-scheme). Similar results are proved
for {\it quasi Shimura -varieties of Hodge type} that generalize the special
fibres of good integral models of Shimura varieties of Hodge type in unramified
mixed characteristic .Comment: 35 pages. Accepted (in final form) for publication in J. Alg. Geo
Enabling Distributed Knowledge Management: Managerial and Technological Implications
In this paper we show that the typical architecture of current KM systems re.ects an objectivistic epistemology and a traditional managerial control paradigm. We argue that such an objectivistic epistemology is inconsistent with many theories on the nature of knowledge, in which subjectivity and sociality are taken as essential features of knowledge creation and sharing. We show that adopting such a new epistemological view has dramatic consequences at an architectural, managerial and technological level
Graph- versus Vector-Based Analysis of a Consensus Protocol
The Paxos distributed consensus algorithm is a challenging case-study for
standard, vector-based model checking techniques. Due to asynchronous
communication, exhaustive analysis may generate very large state spaces already
for small model instances. In this paper, we show the advantages of graph
transformation as an alternative modelling technique. We model Paxos in a rich
declarative transformation language, featuring (among other things) nested
quantifiers, and we validate our model using the GROOVE model checker, a
graph-based tool that exploits isomorphism as a natural way to prune the state
space via symmetry reductions. We compare the results with those obtained by
the standard model checker Spin on the basis of a vector-based encoding of the
algorithm.Comment: In Proceedings GRAPHITE 2014, arXiv:1407.767
Modeling and Evaluation of Multisource Streaming Strategies in P2P VoD Systems
In recent years, multimedia content distribution has largely been moved to the Internet, inducing broadcasters, operators and service providers to upgrade with large expenses their infrastructures. In this context, streaming solutions that rely on user devices such as set-top boxes (STBs) to offload dedicated streaming servers are particularly appropriate. In these systems, contents are usually replicated and scattered over the network established by STBs placed at users' home, and the video-on-demand (VoD) service is provisioned through streaming sessions established among neighboring STBs following a Peer-to-Peer fashion. Up to now the majority of research works have focused on the design and optimization of content replicas mechanisms to minimize server costs. The optimization of replicas mechanisms has been typically performed either considering very crude system performance indicators or analyzing asymptotic behavior. In this work, instead, we propose an analytical model that complements previous works providing fairly accurate predictions of system performance (i.e., blocking probability). Our model turns out to be a highly scalable, flexible, and extensible tool that may be helpful both for designers and developers to efficiently predict the effect of system design choices in large scale STB-VoD system
The chiral anomaly in real space
The chiral anomaly is based on a non-conserved chiral charge and can happen
in Dirac fermion systems under the influence of external electromagnetic
fields. In this case, the spectral flow leads to a transfer of right- to
left-moving excitations or vice versa. The corresponding transfer of chiral
particles happens in momentum space. We here describe an intriguing way to
introduce the chiral anomaly into real space. Our system consists of two
quantum dots that are formed at the helical edge of a quantum spin Hall
insulator on the basis of three magnetic impurities. Such a setup gives rise to
fractional charges which we show to be sharp quantum numbers for large barrier
strength. Interestingly, it is possible to map the system onto a quantum spin
Hall ring in the presence of a flux pierced through the ring where the relative
angle between the magnetization directions of the impurities takes the role of
the flux. The chiral anomaly in this system is then directly related to the
excess occupation of particles in the two quantum dots. This analogy allows us
to predict an observable consequence of the chiral anomaly in real space.Comment: 7 pages, 5 figure
Model Checking Paxos in Spin
We present a formal model of a distributed consensus algorithm in the
executable specification language Promela extended with a new type of guards,
called counting guards, needed to implement transitions that depend on majority
voting. Our formalization exploits abstractions that follow from reduction
theorems applied to the specific case-study. We apply the model checker Spin to
automatically validate finite instances of the model and to extract
preconditions on the size of quorums used in the election phases of the
protocol.Comment: In Proceedings GandALF 2014, arXiv:1408.556
Probing Wigner correlations in a suspended carbon nanotube
The influence of the electron-vibron coupling on the transport properties of
a strongly interacting quantum dot built in a suspended carbon nanotube is
analyzed. The latter is probed by a charged AFM tip scanned along the axis of
the CNT which induces oscillations of the chemical potential and of the linear
conductance. These oscillations are due to the competition between finite-size
effects and the formation of a Wigner molecule for strong interactions. Such
oscillations are shown to be suppressed by the electron-vibron coupling. The
suppression is more pronounced in the regime of weak Coulomb interactions,
which ensures that probing Wigner correlations in such a system is in principle
possible
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