1,332 research outputs found
Absorbing random-walk centrality: Theory and algorithms
We study a new notion of graph centrality based on absorbing random walks.
Given a graph and a set of query nodes , we aim to
identify the most central nodes in with respect to . Specifically,
we consider central nodes to be absorbing for random walks that start at the
query nodes . The goal is to find the set of central nodes that
minimizes the expected length of a random walk until absorption. The proposed
measure, which we call absorbing random-walk centrality, favors diverse
sets, as it is beneficial to place the absorbing nodes in different parts
of the graph so as to "intercept" random walks that start from different query
nodes.
Although similar problem definitions have been considered in the literature,
e.g., in information-retrieval settings where the goal is to diversify
web-search results, in this paper we study the problem formally and prove some
of its properties. We show that the problem is NP-hard, while the objective
function is monotone and supermodular, implying that a greedy algorithm
provides solutions with an approximation guarantee. On the other hand, the
greedy algorithm involves expensive matrix operations that make it prohibitive
to employ on large datasets. To confront this challenge, we develop more
efficient algorithms based on spectral clustering and on personalized PageRank.Comment: 11 pages, 11 figures, short paper to appear at ICDM 201
Markov Chain Monitoring
In networking applications, one often wishes to obtain estimates about the
number of objects at different parts of the network (e.g., the number of cars
at an intersection of a road network or the number of packets expected to reach
a node in a computer network) by monitoring the traffic in a small number of
network nodes or edges. We formalize this task by defining the 'Markov Chain
Monitoring' problem.
Given an initial distribution of items over the nodes of a Markov chain, we
wish to estimate the distribution of items at subsequent times. We do this by
asking a limited number of queries that retrieve, for example, how many items
transitioned to a specific node or over a specific edge at a particular time.
We consider different types of queries, each defining a different variant of
the Markov chain monitoring. For each variant, we design efficient algorithms
for choosing the queries that make our estimates as accurate as possible. In
our experiments with synthetic and real datasets we demonstrate the efficiency
and the efficacy of our algorithms in a variety of settings.Comment: 13 pages, 10 figures, 1 tabl
Nanoflare Activity in the Solar Chromosphere
We use ground-based images of high spatial and temporal resolution to search
for evidence of nanoflare activity in the solar chromosphere. Through close
examination of more than 10^9 pixels in the immediate vicinity of an active
region, we show that the distributions of observed intensity fluctuations have
subtle asymmetries. A negative excess in the intensity fluctuations indicates
that more pixels have fainter-than-average intensities compared with those that
appear brighter than average. By employing Monte Carlo simulations, we reveal
how the negative excess can be explained by a series of impulsive events,
coupled with exponential decays, that are fractionally below the current
resolving limits of low-noise equipment on high-resolution ground-based
observatories. Importantly, our Monte Carlo simulations provide clear evidence
that the intensity asymmetries cannot be explained by photon-counting
statistics alone. A comparison to the coronal work of Terzo et al. (2011)
suggests that nanoflare activity in the chromosphere is more readily occurring,
with an impulsive event occurring every ~360s in a 10,000 km^2 area of the
chromosphere, some 50 times more events than a comparably sized region of the
corona. As a result, nanoflare activity in the chromosphere is likely to play
an important role in providing heat energy to this layer of the solar
atmosphere.Comment: 7 pages, 3 figures, accepted into Ap
European Union in need of a “Relaunch”
In its simplest sense, the winner is the one to come first. In the same sense, for the front-runner of last European elections, Jean Claude Juncker, the votes of the EPP signalled a win. However, a win in politics – and especially European politics – is much more than this. It depends on the context and the perspective
Alfv\'en waves in simulations of solar photospheric vortices
Using advanced numerical magneto-hydrodynamic simulations of the magnetised
solar photosphere, including non-grey radiative transport and a non-ideal
equation of state, we analyse plasma motions in photospheric magnetic vortices.
We demonstrate that apparent vortex-like motions in photospheric magnetic field
concentrations do not exhibit "tornado"-like behaviour or a "bath-tub" effect.
While at each time instance the velocity field lines in the upper layers of the
solar photosphere show swirls, the test particles moving with the
time-dependent velocity field do not demonstrate such structures. Instead, they
move in a wave-like fashion with rapidly changing and oscillating velocity
field, determined mainly by magnetic tension in the magnetised intergranular
downflows. Using time-distance diagrams, we identify horizontal motions in the
magnetic flux tubes as torsional Alfv\'en perturbations propagating along the
nearly vertical magnetic field lines with local Alfv\'en speed.Comment: 5 pages, 4 figures, accepted to ApJ
- …