8,090 research outputs found
Parallel algorithms and concentration bounds for the Lovasz Local Lemma via witness DAGs
The Lov\'{a}sz Local Lemma (LLL) is a cornerstone principle in the
probabilistic method of combinatorics, and a seminal algorithm of Moser &
Tardos (2010) provides an efficient randomized algorithm to implement it. This
can be parallelized to give an algorithm that uses polynomially many processors
and runs in time on an EREW PRAM, stemming from
adaptive computations of a maximal independent set (MIS). Chung et al. (2014)
developed faster local and parallel algorithms, potentially running in time
, but these algorithms require more stringent conditions than the
LLL.
We give a new parallel algorithm that works under essentially the same
conditions as the original algorithm of Moser & Tardos but uses only a single
MIS computation, thus running in time on an EREW PRAM. This can
be derandomized to give an NC algorithm running in time as well,
speeding up a previous NC LLL algorithm of Chandrasekaran et al. (2013).
We also provide improved and tighter bounds on the run-times of the
sequential and parallel resampling-based algorithms originally developed by
Moser & Tardos. These apply to any problem instance in which the tighter
Shearer LLL criterion is satisfied
Charge-localization and isospin-blockade in vertical double quantum dots
Charge localization seems unlikely to occur in two vertically coupled
symmetric quantum dots even if a small bias voltage breaks the exact
isospin-symmetry of the system. However for a three-electron double quantum dot
we find a strong localization of charges at certain vertically applied magnetic
fields. The charge localization is directly connected to new ground state
transitions between eigenstates differing only in parity. The transitions are
driven by magnetic field dependent Coulomb correlations between the electrons
and give rise to strong isospin blockade signatures in transport through the
double dot system.Comment: 10 pages, 4 figure
A unified quark-nuclear matter equation of state from the cluster virial expansion within the generalized Beth-Uhlenbeck approach
We consider a cluster expansion for strongly correlated quark matter where
the clusters are baryons with spectral properties that are described within the
generalized Beth-Uhlenbeck approach by a medium dependent phase shift. We
employ a simple ansatz for the phase shift which fulfils the Levinson theorem
by describing an on-shell bound state with an effective mass and models the
continuum by an anti-bound state located at the mass of the three-quark
threshold. The quark and baryon interactions are accounted for by the coupling
to scalar and vector meson mean fields modelled by density functionals. At
increasing density and temperature, due to the different medium-dependence of
quark and baryon masses, the Mott dissociation of baryons occurs and the
nuclear cluster contributions to the thermodynamics vanish. It is demonstrated
on this simple example that this unified approach to quark-nuclear matter is
capable of describing crossover as well as first order phase transition
behaviour in the phase diagram with a critical endpoint. Changing the meson
mean field, the case of a "crossover all over" in the phase diagram is also
obtained.Comment: 7 pages, 5 figure
Uncovering the Temporal Dynamics of Diffusion Networks
Time plays an essential role in the diffusion of information, influence and
disease over networks. In many cases we only observe when a node copies
information, makes a decision or becomes infected -- but the connectivity,
transmission rates between nodes and transmission sources are unknown.
Inferring the underlying dynamics is of outstanding interest since it enables
forecasting, influencing and retarding infections, broadly construed. To this
end, we model diffusion processes as discrete networks of continuous temporal
processes occurring at different rates. Given cascade data -- observed
infection times of nodes -- we infer the edges of the global diffusion network
and estimate the transmission rates of each edge that best explain the observed
data. The optimization problem is convex. The model naturally (without
heuristics) imposes sparse solutions and requires no parameter tuning. The
problem decouples into a collection of independent smaller problems, thus
scaling easily to networks on the order of hundreds of thousands of nodes.
Experiments on real and synthetic data show that our algorithm both recovers
the edges of diffusion networks and accurately estimates their transmission
rates from cascade data.Comment: To appear in the 28th International Conference on Machine Learning
(ICML), 2011. Website: http://www.stanford.edu/~manuelgr/netrate
Increased variability of microbial communities in restored salt marshes nearly 30 years after tidal flow restoration
We analyzed microbial diversity and community composition from four salt marsh sites that were impounded for 40–50 years and subsequently restored and four unimpounded sites in southeastern Connecticut over one growing season. Community composition and diversity were assessed by terminal restriction fragment length polymorphism (TRFLP) and sequence analysis of 16S ribosomal RNA (rRNA) genes. Our results indicated diverse communities, with sequences representing 14 different bacterial divisions. Proteobacteria, Bacteroidetes, and Planctomycetes dominated clone libraries from both restored and unimpounded sites. Multivariate analysis of the TRFLP data suggest significant site, sample date, and restoration status effects, but the exact causes of these effects are not clear. Composition of clone libraries and abundance of bacterial 16S rRNA genes were not significantly different between restored sites and unimpounded sites, but restored sites showed greater temporal and spatial variability of bacterial communities based on TRFLP profiles compared with unimpounded sites, and variability was greatest at sites more recently restored. In summary, our study suggests there may be long-lasting effects on stability of bacterial communities in restored salt marshes and raises questions about the resilience and ultimate recovery of the communities after chronic disturbance
Quantifying causal influences
Many methods for causal inference generate directed acyclic graphs (DAGs)
that formalize causal relations between variables. Given the joint
distribution on all these variables, the DAG contains all information about how
intervening on one variable changes the distribution of the other
variables. However, quantifying the causal influence of one variable on another
one remains a nontrivial question. Here we propose a set of natural, intuitive
postulates that a measure of causal strength should satisfy. We then introduce
a communication scenario, where edges in a DAG play the role of channels that
can be locally corrupted by interventions. Causal strength is then the relative
entropy distance between the old and the new distribution. Many other measures
of causal strength have been proposed, including average causal effect,
transfer entropy, directed information, and information flow. We explain how
they fail to satisfy the postulates on simple DAGs of nodes. Finally,
we investigate the behavior of our measure on time-series, supporting our
claims with experiments on simulated data.Comment: Published in at http://dx.doi.org/10.1214/13-AOS1145 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
- …