9,597 research outputs found
Superdiffusion in a class of networks with marginal long-range connections
A class of cubic networks composed of a regular one-dimensional lattice and a
set of long-range links is introduced. Networks parametrized by a positive
integer k are constructed by starting from a one-dimensional lattice and
iteratively connecting each site of degree 2 with a th neighboring site of
degree 2. Specifying the way pairs of sites to be connected are selected,
various random and regular networks are defined, all of which have a power-law
edge-length distribution of the form with the marginal
exponent s=1. In all these networks, lengths of shortest paths grow as a power
of the distance and random walk is super-diffusive. Applying a renormalization
group method, the corresponding shortest-path dimensions and random-walk
dimensions are calculated exactly for k=1 networks and for k=2 regular
networks; in other cases, they are estimated by numerical methods. Although,
s=1 holds for all representatives of this class, the above quantities are found
to depend on the details of the structure of networks controlled by k and other
parameters.Comment: 10 pages, 9 figure
Detecting the Baryons in Matter Power Spectra
We examine power spectra from the Abell/ACO rich cluster survey and the 2dF
Galaxy Redshift Survey (2dfGRS) for observational evidence of features produced
by the baryons. A non-negligible baryon fraction produces relatively sharp
oscillatory features at specific wavenumbers in the matter power spectrum.
However, the mere existence of baryons will also produce a global suppression
of the power spectrum. We look for both of these features using the false
discovery rate (FDR) statistic. We show that the window effects on the
Abell/ACO power spectrum are minimal, which has allowed for the discovery of
discrete oscillatory features in the power spectrum. On the other hand, there
are no statistically significant oscillatory features in the 2dFGRS power
spectrum, which is expected from the survey's broad window function. After
accounting for window effects, we apply a scale-independent bias to the 2dFGRS
power spectrum, P_{Abell}(k) = b^2P_{2dF}(k) and b = 3.2. We find that the
overall shapes of the Abell/ACO and the biased 2dFGRS power spectra are
entirely consistent over the range 0.02 <= k <= 0.15hMpc^-1. We examine the
range of Omega_{matter} and baryon fraction for which these surveys could
detect significant suppression in power. The reported baryon fractions for both
the Abell/ACO and 2dFGRS surveys are high enough to cause a detectable
suppression in power (after accounting for errors, windows and k-space
sampling). Using the same technique, we also examine, given the best fit baryon
density obtained from BBN, whether it is possible to detect additional
suppression due to dark matter-baryon interaction. We find that the limit on
dark matter cross section/mass derived from these surveys are the same as those
ruled out in a recent study by Chen, Hannestad and Scherrer.Comment: 11 pages of text, 6 figures. Submitted to Ap
Harmonic Labeling of Graphs
Which graphs admit an integer value harmonic function which is injective and
surjective onto ? Such a function, which we call harmonic labeling, is
constructed when the graph is the square grid. It is shown that for any
finite graph containing at least one edge, there is no harmonic labeling of
The Inverse Shapley Value Problem
For a weighted voting scheme used by voters to choose between two
candidates, the \emph{Shapley-Shubik Indices} (or {\em Shapley values}) of
provide a measure of how much control each voter can exert over the overall
outcome of the vote. Shapley-Shubik indices were introduced by Lloyd Shapley
and Martin Shubik in 1954 \cite{SS54} and are widely studied in social choice
theory as a measure of the "influence" of voters. The \emph{Inverse Shapley
Value Problem} is the problem of designing a weighted voting scheme which
(approximately) achieves a desired input vector of values for the
Shapley-Shubik indices. Despite much interest in this problem no provably
correct and efficient algorithm was known prior to our work.
We give the first efficient algorithm with provable performance guarantees
for the Inverse Shapley Value Problem. For any constant \eps > 0 our
algorithm runs in fixed poly time (the degree of the polynomial is
independent of \eps) and has the following performance guarantee: given as
input a vector of desired Shapley values, if any "reasonable" weighted voting
scheme (roughly, one in which the threshold is not too skewed) approximately
matches the desired vector of values to within some small error, then our
algorithm explicitly outputs a weighted voting scheme that achieves this vector
of Shapley values to within error \eps. If there is a "reasonable" voting
scheme in which all voting weights are integers at most \poly(n) that
approximately achieves the desired Shapley values, then our algorithm runs in
time \poly(n) and outputs a weighted voting scheme that achieves the target
vector of Shapley values to within error $\eps=n^{-1/8}.
Scaling behavior of the contact process in networks with long-range connections
We present simulation results for the contact process on regular, cubic
networks that are composed of a one-dimensional lattice and a set of long edges
with unbounded length. Networks with different sets of long edges are
considered, that are characterized by different shortest-path dimensions and
random-walk dimensions. We provide numerical evidence that an absorbing phase
transition occurs at some finite value of the infection rate and the
corresponding dynamical critical exponents depend on the underlying network.
Furthermore, the time-dependent quantities exhibit log-periodic oscillations in
agreement with the discrete scale invariance of the networks. In case of
spreading from an initial active seed, the critical exponents are found to
depend on the location of the initial seed and break the hyper-scaling law of
the directed percolation universality class due to the inhomogeneity of the
networks. However, if the cluster spreading quantities are averaged over
initial sites the hyper-scaling law is restored.Comment: 9 pages, 10 figure
A new source detection algorithm using FDR
The False Discovery Rate (FDR) method has recently been described by Miller
et al (2001), along with several examples of astrophysical applications. FDR is
a new statistical procedure due to Benjamini and Hochberg (1995) for
controlling the fraction of false positives when performing multiple hypothesis
testing. The importance of this method to source detection algorithms is
immediately clear. To explore the possibilities offered we have developed a new
task for performing source detection in radio-telescope images, Sfind 2.0,
which implements FDR. We compare Sfind 2.0 with two other source detection and
measurement tasks, Imsad and SExtractor, and comment on several issues arising
from the nature of the correlation between nearby pixels and the necessary
assumption of the null hypothesis. The strong suggestion is made that
implementing FDR as a threshold defining method in other existing
source-detection tasks is easy and worthwhile. We show that the constraint on
the fraction of false detections as specified by FDR holds true even for highly
correlated and realistic images. For the detection of true sources, which are
complex combinations of source-pixels, this constraint appears to be somewhat
less strict. It is still reliable enough, however, for a priori estimates of
the fraction of false source detections to be robust and realistic.Comment: 17 pages, 7 figures, accepted for publication by A
On a random walk with memory and its relation to Markovian processes
We study a one-dimensional random walk with memory in which the step lengths
to the left and to the right evolve at each step in order to reduce the
wandering of the walker. The feedback is quite efficient and lead to a
non-diffusive walk. The time evolution of the displacement is given by an
equivalent Markovian dynamical process. The probability density for the
position of the walker is the same at any time as for a random walk with
shrinking steps, although the two-time correlation functions are quite
different.Comment: 10 pages, 4 figure
P-values for high-dimensional regression
Assigning significance in high-dimensional regression is challenging. Most
computationally efficient selection algorithms cannot guard against inclusion
of noise variables. Asymptotically valid p-values are not available. An
exception is a recent proposal by Wasserman and Roeder (2008) which splits the
data into two parts. The number of variables is then reduced to a manageable
size using the first split, while classical variable selection techniques can
be applied to the remaining variables, using the data from the second split.
This yields asymptotic error control under minimal conditions. It involves,
however, a one-time random split of the data. Results are sensitive to this
arbitrary choice: it amounts to a `p-value lottery' and makes it difficult to
reproduce results. Here, we show that inference across multiple random splits
can be aggregated, while keeping asymptotic control over the inclusion of noise
variables. We show that the resulting p-values can be used for control of both
family-wise error (FWER) and false discovery rate (FDR). In addition, the
proposed aggregation is shown to improve power while reducing the number of
falsely selected variables substantially.Comment: 25 pages, 4 figure
From brain to earth and climate systems: Small-world interaction networks or not?
We consider recent reports on small-world topologies of interaction networks
derived from the dynamics of spatially extended systems that are investigated
in diverse scientific fields such as neurosciences, geophysics, or meteorology.
With numerical simulations that mimic typical experimental situations we have
identified an important constraint when characterizing such networks:
indications of a small-world topology can be expected solely due to the spatial
sampling of the system along with commonly used time series analysis based
approaches to network characterization
ExplainIt! -- A declarative root-cause analysis engine for time series data (extended version)
We present ExplainIt!, a declarative, unsupervised root-cause analysis engine
that uses time series monitoring data from large complex systems such as data
centres. ExplainIt! empowers operators to succinctly specify a large number of
causal hypotheses to search for causes of interesting events. ExplainIt! then
ranks these hypotheses, reducing the number of causal dependencies from
hundreds of thousands to a handful for human understanding. We show how a
declarative language, such as SQL, can be effective in declaratively
enumerating hypotheses that probe the structure of an unknown probabilistic
graphical causal model of the underlying system. Our thesis is that databases
are in a unique position to enable users to rapidly explore the possible causal
mechanisms in data collected from diverse sources. We empirically demonstrate
how ExplainIt! had helped us resolve over 30 performance issues in a commercial
product since late 2014, of which we discuss a few cases in detail.Comment: SIGMOD Industry Track 201
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