29,248 research outputs found
Non-Euclidean geometry in nature
I describe the manifestation of the non-Euclidean geometry in the behavior of
collective observables of some complex physical systems. Specifically, I
consider the formation of equilibrium shapes of plants and statistics of sparse
random graphs. For these systems I discuss the following interlinked questions:
(i) the optimal embedding of plants leaves in the three-dimensional space, (ii)
the spectral statistics of sparse random matrix ensembles.Comment: 52 pages, 21 figures, last section is rewritten, a reference to
chaotic Hamiltonian systems is adde
Percolation on a product of two trees
We show that critical percolation on a product of two regular trees of degree
3 satisfies the triangle condition. The proof does not examine the
degrees of vertices and is not "perturbative" in any sense. It relies on an
unpublished lemma of Oded Schramm.Comment: Published in at http://dx.doi.org/10.1214/10-AOP618 the Annals of
Probability (http://www.imstat.org/aop/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Persistence of the Jordan center in Random Growing Trees
The Jordan center of a graph is defined as a vertex whose maximum distance to
other nodes in the graph is minimal, and it finds applications in facility
location and source detection problems. We study properties of the Jordan
Center in the case of random growing trees. In particular, we consider a
regular tree graph on which an infection starts from a root node and then
spreads along the edges of the graph according to various random spread models.
For the Independent Cascade (IC) model and the discrete Susceptible Infected
(SI) model, both of which are discrete time models, we show that as the
infected subgraph grows with time, the Jordan center persists on a single
vertex after a finite number of timesteps. Finally, we also study the
continuous time version of the SI model and bound the maximum distance between
the Jordan center and the root node at any time.Comment: 28 pages, 14 figure
Query Learning with Exponential Query Costs
In query learning, the goal is to identify an unknown object while minimizing
the number of "yes" or "no" questions (queries) posed about that object. A
well-studied algorithm for query learning is known as generalized binary search
(GBS). We show that GBS is a greedy algorithm to optimize the expected number
of queries needed to identify the unknown object. We also generalize GBS in two
ways. First, we consider the case where the cost of querying grows
exponentially in the number of queries and the goal is to minimize the expected
exponential cost. Then, we consider the case where the objects are partitioned
into groups, and the objective is to identify only the group to which the
object belongs. We derive algorithms to address these issues in a common,
information-theoretic framework. In particular, we present an exact formula for
the objective function in each case involving Shannon or Renyi entropy, and
develop a greedy algorithm for minimizing it. Our algorithms are demonstrated
on two applications of query learning, active learning and emergency response.Comment: 15 page
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