137,604 research outputs found
Viral processes by random walks on random regular graphs
We study the SIR epidemic model with infections carried by particles
making independent random walks on a random regular graph. Here we assume
, where is the number of vertices in the random graph,
and is some sufficiently small constant. We give an edge-weighted
graph reduction of the dynamics of the process that allows us to apply standard
results of Erd\H{o}s-R\'{e}nyi random graphs on the particle set. In
particular, we show how the parameters of the model give two thresholds: In the
subcritical regime, particles are infected. In the supercritical
regime, for a constant determined by the parameters of the
model, get infected with probability , and get
infected with probability . Finally, there is a regime in which all
particles are infected. Furthermore, the edge weights give information
about when a particle becomes infected. We exploit this to give a completion
time of the process for the SI case.Comment: Published in at http://dx.doi.org/10.1214/13-AAP1000 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
The cavity approach for Steiner trees packing problems
The Belief Propagation approximation, or cavity method, has been recently
applied to several combinatorial optimization problems in its zero-temperature
implementation, the max-sum algorithm. In particular, recent developments to
solve the edge-disjoint paths problem and the prize-collecting Steiner tree
problem on graphs have shown remarkable results for several classes of graphs
and for benchmark instances. Here we propose a generalization of these
techniques for two variants of the Steiner trees packing problem where multiple
"interacting" trees have to be sought within a given graph. Depending on the
interaction among trees we distinguish the vertex-disjoint Steiner trees
problem, where trees cannot share nodes, from the edge-disjoint Steiner trees
problem, where edges cannot be shared by trees but nodes can be members of
multiple trees. Several practical problems of huge interest in network design
can be mapped into these two variants, for instance, the physical design of
Very Large Scale Integration (VLSI) chips. The formalism described here relies
on two components edge-variables that allows us to formulate a massage-passing
algorithm for the V-DStP and two algorithms for the E-DStP differing in the
scaling of the computational time with respect to some relevant parameters. We
will show that one of the two formalisms used for the edge-disjoint variant
allow us to map the max-sum update equations into a weighted maximum matching
problem over proper bipartite graphs. We developed a heuristic procedure based
on the max-sum equations that shows excellent performance in synthetic networks
(in particular outperforming standard multi-step greedy procedures by large
margins) and on large benchmark instances of VLSI for which the optimal
solution is known, on which the algorithm found the optimum in two cases and
the gap to optimality was never larger than 4 %
Limits of dense graph sequences
We show that if a sequence of dense graphs has the property that for every
fixed graph F, the density of copies of F in these graphs tends to a limit,
then there is a natural ``limit object'', namely a symmetric measurable
2-variable function on [0,1]. This limit object determines all the limits of
subgraph densities. We also show that the graph parameters obtained as limits
of subgraph densities can be characterized by ``reflection positivity'',
semidefiniteness of an associated matrix. Conversely, every such function
arises as a limit object. Along the lines we introduce a rather general model
of random graphs, which seems to be interesting on its own right.Comment: 27 pages; added extension of result (Sept 22, 2004
On the exact learnability of graph parameters: The case of partition functions
We study the exact learnability of real valued graph parameters which are
known to be representable as partition functions which count the number of
weighted homomorphisms into a graph with vertex weights and edge
weights . M. Freedman, L. Lov\'asz and A. Schrijver have given a
characterization of these graph parameters in terms of the -connection
matrices of . Our model of learnability is based on D. Angluin's
model of exact learning using membership and equivalence queries. Given such a
graph parameter , the learner can ask for the values of for graphs of
their choice, and they can formulate hypotheses in terms of the connection
matrices of . The teacher can accept the hypothesis as correct, or
provide a counterexample consisting of a graph. Our main result shows that in
this scenario, a very large class of partition functions, the rigid partition
functions, can be learned in time polynomial in the size of and the size of
the largest counterexample in the Blum-Shub-Smale model of computation over the
reals with unit cost.Comment: 14 pages, full version of the MFCS 2016 conference pape
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