67,343 research outputs found
Submodular Optimization with Contention Resolution Extensions
This paper considers optimizing a submodular function subject to a set of downward closed constraints. Previous literature on this problem has often constructed solutions by (1) discovering a fractional solution to the multi-linear extension and (2) rounding this solution to an integral solution via a contention resolution scheme. This line of research has improved results by either optimizing (1) or (2).
Diverging from previous work, this paper introduces a principled method called contention resolution extensions of submodular functions. A contention resolution extension combines the contention resolution scheme into a continuous extension of a discrete submodular function. The contention resolution extension can be defined from effectively any contention resolution scheme. In the case where there is a loss in both (1) and (2), by optimizing them together, the losses can be combined resulting in an overall improvement. This paper showcases the concept by demonstrating that for the problem of optimizing a non-monotone submodular subject to the elements forming an independent set in an interval graph, the algorithm gives a .188-approximation. This improves upon the best known 1/(2e)~eq .1839 approximation
Out-of-sample generalizations for supervised manifold learning for classification
Supervised manifold learning methods for data classification map data samples
residing in a high-dimensional ambient space to a lower-dimensional domain in a
structure-preserving way, while enhancing the separation between different
classes in the learned embedding. Most nonlinear supervised manifold learning
methods compute the embedding of the manifolds only at the initially available
training points, while the generalization of the embedding to novel points,
known as the out-of-sample extension problem in manifold learning, becomes
especially important in classification applications. In this work, we propose a
semi-supervised method for building an interpolation function that provides an
out-of-sample extension for general supervised manifold learning algorithms
studied in the context of classification. The proposed algorithm computes a
radial basis function (RBF) interpolator that minimizes an objective function
consisting of the total embedding error of unlabeled test samples, defined as
their distance to the embeddings of the manifolds of their own class, as well
as a regularization term that controls the smoothness of the interpolation
function in a direction-dependent way. The class labels of test data and the
interpolation function parameters are estimated jointly with a progressive
procedure. Experimental results on face and object images demonstrate the
potential of the proposed out-of-sample extension algorithm for the
classification of manifold-modeled data sets
Efficient AUC Optimization for Information Ranking Applications
Adequate evaluation of an information retrieval system to estimate future
performance is a crucial task. Area under the ROC curve (AUC) is widely used to
evaluate the generalization of a retrieval system. However, the objective
function optimized in many retrieval systems is the error rate and not the AUC
value. This paper provides an efficient and effective non-linear approach to
optimize AUC using additive regression trees, with a special emphasis on the
use of multi-class AUC (MAUC) because multiple relevance levels are widely used
in many ranking applications. Compared to a conventional linear approach, the
performance of the non-linear approach is comparable on binary-relevance
benchmark datasets and is better on multi-relevance benchmark datasets.Comment: 12 page
Effects of electrostatic screening on the conformation of single DNA molecules confined in a nanochannel
Single T4-DNA molecules were confined in rectangular-shaped channels with a
depth of 300 nm and a width in the range 150-300 nm casted in a
poly(dimethylsiloxane) nanofluidic chip. The extensions of the DNA molecules
were measured with fluorescence microscopy as a function of the ionic strength
and composition of the buffer as well as the DNA intercalation level by the
YOYO-1 dye. The data were interpreted with scaling theory for a wormlike
polymer in good solvent, including the effects of confinement, charge, and
self-avoidance. It was found that the elongation of the DNA molecules with
decreasing ionic strength can be interpreted in terms of an increase of the
persistence length. Self-avoidance effects on the extension are moderate, due
to the small correlation length imposed by the channel cross-sectional
diameter. Intercalation of the dye results in an increase of the DNA contour
length and a partial neutralization of the DNA charge, but besides effects of
electrostatic origin it has no significant effect on the bare bending rigidity.
In the presence of divalent cations, the DNA molecules were observed to
contract, but they do not collapse into a condensed structure. It is proposed
that this contraction results from a divalent counterion mediated attractive
force between the segments of the DNA molecule.Comment: 38 pages, 10 figures, accepted for publication in The Journal of
Chemical Physic
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