2,141,815 research outputs found
Counting independent sets in hypergraphs
Let be a triangle-free graph with vertices and average degree . We
show that contains at least independent sets. This improves a recent result of the
first and third authors \cite{countingind}. In particular, it implies that as
, every triangle-free graph on vertices has at least
independent sets, where . Further, we show that for all , there exists a triangle-free
graph with vertices which has at most
independent sets, where . This disproves a
conjecture from \cite{countingind}.
Let be a -uniform linear hypergraph with vertices and average
degree . We also show that there exists a constant such that the
number of independent sets in is at least This is tight apart from the constant
and generalizes a result of Duke, Lefmann, and R\"odl
\cite{uncrowdedrodl}, which guarantees the existence of an independent set of
size . Both of our lower bounds follow
from a more general statement, which applies to hereditary properties of
hypergraphs
Renormalization study of two-dimensional convergent solutions of the porous medium equation
In the focusing problem we study a solution of the porous medium equation
whose initial distribution is positive in the exterior of a
closed non-circular two dimensional region, and zero inside. We implement a
numerical scheme that renormalizes the solution each time that the average size
of the empty region reduces by a half. The initial condition is a function with
circular level sets distorted with a small sinusoidal perturbation of wave
number . We find that for nonlinearity exponents m smaller than a
critical value which depends on k, the solution tends to a self-similar regime,
characterized by rounded polygonal interfaces and similarity exponents that
depend on m and on the discrete rotational symmetry number k. For m greater
than the critical value, the final form of the interface is circular.Comment: 26 pages, Latex, 13 ps figure
Direct Estimation of Information Divergence Using Nearest Neighbor Ratios
We propose a direct estimation method for R\'{e}nyi and f-divergence measures
based on a new graph theoretical interpretation. Suppose that we are given two
sample sets and , respectively with and samples, where
is a constant value. Considering the -nearest neighbor (-NN)
graph of in the joint data set , we show that the average powered
ratio of the number of points to the number of points among all -NN
points is proportional to R\'{e}nyi divergence of and densities. A
similar method can also be used to estimate f-divergence measures. We derive
bias and variance rates, and show that for the class of -H\"{o}lder
smooth functions, the estimator achieves the MSE rate of
. Furthermore, by using a weighted ensemble
estimation technique, for density functions with continuous and bounded
derivatives of up to the order , and some extra conditions at the support
set boundary, we derive an ensemble estimator that achieves the parametric MSE
rate of . Our estimators are more computationally tractable than other
competing estimators, which makes them appealing in many practical
applications.Comment: 2017 IEEE International Symposium on Information Theory (ISIT
Comparing the Ca II H and K Emission Lines in Red Giant Stars
Measurements of the asymmetry of the emission peaks in the core of the Ca II
H line for 105 giant stars are reported. The asymmetry is quantified with the
parameter V/R, defined as the ratio between the maximum number of counts in the
blueward peak and the redward peak of the emission profile. The Ca II H and K
emission lines probe the differential motion of certain chromospheric layers in
the stellar atmosphere. Data on V/R for the Ca II K line are drawn from
previous papers and compared to the analogous H line ratio, the H and K spectra
being from the same sets of observations. It is found that the H line V/R value
is +0.04 larger, on average, than the equivalent K line ratio, however, the
difference varies with B-V color. Red giants cooler than B-V = 1.2 are more
likely to have the H line V/R larger than the K line V/R, whereas the opposite
is true for giants hotter than B-V = 1.2. The differences between the Ca II H
and K line asymmetries could be caused by the layers of chromospheric material
from which these emission features arise moving with different velocities in an
expanding outflow.Comment: 36 pages, 12 figures, 2 tables. Accepted to PASP. Corrected a typo in
Table
On Multiple Pattern Avoiding Set Partitions
We study classes of set partitions determined by the avoidance of multiple
patterns, applying a natural notion of partition containment that has been
introduced by Sagan. We say that two sets S and T of patterns are equivalent if
for each n, the number of partitions of size n avoiding all the members of S is
the same as the number of those that avoid all the members of T.
Our goal is to classify the equivalence classes among two-element pattern
sets of several general types. First, we focus on pairs of patterns
{\sigma,\tau}, where \sigma\ is a pattern of size three with at least two
distinct symbols and \tau\ is an arbitrary pattern of size k that avoids
\sigma. We show that pattern-pairs of this type determine a small number of
equivalence classes; in particular, the classes have on average exponential
size in k. We provide a (sub-exponential) upper bound for the number of
equivalence classes, and provide an explicit formula for the generating
function of all such avoidance classes, showing that in all cases this
generating function is rational.
Next, we study partitions avoiding a pair of patterns of the form
{1212,\tau}, where \tau\ is an arbitrary pattern. Note that partitions avoiding
1212 are exactly the non-crossing partitions. We provide several general
equivalence criteria for pattern pairs of this type, and show that these
criteria account for all the equivalences observed when \tau\ has size at most
six.
In the last part of the paper, we perform a full classification of the
equivalence classes of all the pairs {\sigma,\tau}, where \sigma\ and \tau\
have size four.Comment: 37 pages. Corrected a typ
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