167,943 research outputs found
Dynamic and Multi-functional Labeling Schemes
We investigate labeling schemes supporting adjacency, ancestry, sibling, and
connectivity queries in forests. In the course of more than 20 years, the
existence of labeling schemes supporting each of these
functions was proven, with the most recent being ancestry [Fraigniaud and
Korman, STOC '10]. Several multi-functional labeling schemes also enjoy lower
or upper bounds of or
respectively. Notably an upper bound of for
adjacency+siblings and a lower bound of for each of the
functions siblings, ancestry, and connectivity [Alstrup et al., SODA '03]. We
improve the constants hidden in the -notation. In particular we show a lower bound for connectivity+ancestry and
connectivity+siblings, as well as an upper bound of for connectivity+adjacency+siblings by altering existing
methods.
In the context of dynamic labeling schemes it is known that ancestry requires
bits [Cohen, et al. PODS '02]. In contrast, we show upper and lower
bounds on the label size for adjacency, siblings, and connectivity of
bits, and to support all three functions. There exist efficient
adjacency labeling schemes for planar, bounded treewidth, bounded arboricity
and interval graphs. In a dynamic setting, we show a lower bound of
for each of those families.Comment: 17 pages, 5 figure
Percolation and Connectivity on the Signal to Interference Ratio Graph
A wireless communication network is considered where any two nodes are
connected if the signal-to-interference ratio (SIR) between them is greater
than a threshold. Assuming that the nodes of the wireless network are
distributed as a Poisson point process (PPP), percolation (unbounded connected
cluster) on the resulting SIR graph is studied as a function of the density of
the PPP. For both the path-loss as well as path-loss plus fading model of
signal propagation, it is shown that for a small enough threshold, there exists
a closed interval of densities for which percolation happens with non-zero
probability. Conversely, for the path-loss model of signal propagation, it is
shown that for a large enough threshold, there exists a closed interval of
densities for which the probability of percolation is zero. Restricting all
nodes to lie in an unit square, connectivity properties of the SIR graph are
also studied. Assigning separate frequency bands or time-slots proportional to
the logarithm of the number of nodes to different nodes for
transmission/reception is sufficient to guarantee connectivity in the SIR
graph.Comment: To appear in the Proceedings of the IEEE Conference on Computer
Communications (INFOCOM 2012), to be held in Orlando Florida Mar. 201
The Cost of Global Broadcast in Dynamic Radio Networks
We study the single-message broadcast problem in dynamic radio networks. We
show that the time complexity of the problem depends on the amount of stability
and connectivity of the dynamic network topology and on the adaptiveness of the
adversary providing the dynamic topology. More formally, we model communication
using the standard graph-based radio network model. To model the dynamic
network, we use a generalization of the synchronous dynamic graph model
introduced in [Kuhn et al., STOC 2010]. For integer parameters and
, we call a dynamic graph -interval -connected if for every
interval of consecutive rounds, there exists a -vertex-connected stable
subgraph. Further, for an integer parameter , we say that the
adversary providing the dynamic network is -oblivious if for constructing
the graph of some round , the adversary has access to all the randomness
(and states) of the algorithm up to round .
As our main result, we show that for any , any , and any
, for a -oblivious adversary, there is a distributed
algorithm to broadcast a single message in time
. We further show that even for large interval -connectivity,
efficient broadcast is not possible for the usual adaptive adversaries. For a
-oblivious adversary, we show that even for any (for any constant ) and for any , global broadcast in -interval -connected networks requires at least
time. Further, for a oblivious adversary,
broadcast cannot be solved in -interval -connected networks as long as
.Comment: 17 pages, conference version appeared in OPODIS 201
The effects of hemodynamic lag on functional connectivity and behavior after stroke
Stroke disrupts the brain's vascular supply, not only within but also outside areas of infarction. We investigated temporal delays (lag) in resting state functional magnetic resonance imaging signals in 130 stroke patients scanned two weeks, three months and 12 months post stroke onset. Thirty controls were scanned twice at an interval of three months. Hemodynamic lag was determined using cross-correlation with the global gray matter signal. Behavioral performance in multiple domains was assessed in all patients. Regional cerebral blood flow and carotid patency were assessed in subsets of the cohort using arterial spin labeling and carotid Doppler ultrasonography. Significant hemodynamic lag was observed in 30% of stroke patients sub-acutely. Approximately 10% of patients showed lag at one-year post-stroke. Hemodynamic lag corresponded to gross aberrancy in functional connectivity measures, performance deficits in multiple domains and local and global perfusion deficits. Correcting for lag partially normalized abnormalities in measured functional connectivity. Yet post-stroke FC-behavior relationships in the motor and attention systems persisted even after hemodynamic delays were corrected. Resting state fMRI can reliably identify areas of hemodynamic delay following stroke. Our data reveal that hemodynamic delay is common sub-acutely, alters functional connectivity, and may be of clinical importance
Absolutely Continuous Spectrum for Random Schroedinger Operators on the Bethe Strip
The Bethe Strip of width is the cartesian product \B\times\{1,...,m\},
where \B is the Bethe lattice (Cayley tree). We prove that Anderson models on
the Bethe strip have "extended states" for small disorder. More precisely, we
consider Anderson-like Hamiltonians \;H_\lambda=\frac12 \Delta \otimes 1 + 1
\otimes A + \lambda \Vv on a Bethe strip with connectivity , where
is an symmetric matrix, \Vv is a random matrix potential, and
is the disorder parameter. Given any closed interval , where
and are the smallest and largest
eigenvalues of the matrix , we prove that for small the random
Schr\"odinger operator has purely absolutely continuous spectrum
in with probability one and its integrated density of states is
continuously differentiable on the interval
Redividing the Cake
A heterogeneous resource, such as a land-estate, is already divided among
several agents in an unfair way. It should be re-divided among the agents in a
way that balances fairness with ownership rights. We present re-division
protocols that attain various trade-off points between fairness and ownership
rights, in various settings differing in the geometric constraints on the
allotments: (a) no geometric constraints; (b) connectivity --- the cake is a
one-dimensional interval and each piece must be a contiguous interval; (c)
rectangularity --- the cake is a two-dimensional rectangle or rectilinear
polygon and the pieces should be rectangles; (d) convexity --- the cake is a
two-dimensional convex polygon and the pieces should be convex.
Our re-division protocols have implications on another problem: the
price-of-fairness --- the loss of social welfare caused by fairness
requirements. Each protocol implies an upper bound on the price-of-fairness
with the respective geometric constraints.Comment: Extended IJCAI 2018 version. Previous name: "How to Re-Divide a Cake
Fairly
Spatiotemporal Regularity in Networks with Stochastically Varying Links
In this work we investigate time varying networks with complex dynamics at
the nodes. We consider two scenarios of network change in an interval of time:
first, we have the case where each link can change with probability pt, i.e.
the network changes occur locally and independently at each node. Secondly we
consider the case where the entire connectivity matrix changes with probability
pt, i.e. the change is global. We show that network changes, occurring both
locally and globally, yield an enhanced range of synchronization. When the
connections are changed slowly (i.e. pt is low) the nodes display nearly
synchronized intervals interrupted by intermittent unsynchronized chaotic
bursts. However when the connections are switched quickly (i.e. pt is large),
the intermittent behavior quickly settles down to a steady synchronized state.
Furthermore we find that the mean time taken to reach synchronization from
generic random initial states is significantly reduced when the underlying
links change more rapidly. We also analyze the probabilistic dynamics of the
system with changing connectivity and the stable synchronized range thus
obtained is in broad agreement with those observed numerically.Comment: 15 pages, 8 figures, Keywords: Complex Networks, Temporal Networks,
Synchronization, Coupled Map Lattic
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