36,074 research outputs found
On the Wiener Index of Orientations of Graphs
The Wiener index of a strong digraph is defined as the sum of the
distances between all ordered pairs of vertices. This definition has been
extended to digraphs that are not necessarily strong by defining the distance
from a vertex to a vertex as if there is no path from to in
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Knor, \u{S}krekovski and Tepeh [Some remarks on Wiener index of oriented
graphs. Appl.\ Math.\ Comput.\ {\bf 273}] considered orientations of graphs
with maximum Wiener index. The authors conjectured that for a given tree ,
an orientation of of maximum Wiener index always contains a vertex
such that for every vertex , there is either a -path or a
-path in . In this paper we disprove the conjecture.
We also show that the problem of finding an orientation of maximum Wiener
index of a given graph is NP-complete, thus answering a question by Knor,
\u{S}krekovski and Tepeh [Orientations of graphs with maximum Wiener index.
Discrete Appl.\ Math.\ 211].
We briefly discuss the corresponding problem of finding an orientation of
minimum Wiener index of a given graph, and show that the special case of
deciding if a given graph on edges has an orientation of Wiener index
can be solved in time quadratic in
Fast and Compact Exact Distance Oracle for Planar Graphs
For a given a graph, a distance oracle is a data structure that answers
distance queries between pairs of vertices. We introduce an -space
distance oracle which answers exact distance queries in time for
-vertex planar edge-weighted digraphs. All previous distance oracles for
planar graphs with truly subquadratic space i.e., space
for some constant ) either required query time polynomial in
or could only answer approximate distance queries.
Furthermore, we show how to trade-off time and space: for any , we show how to obtain an -space distance oracle that answers
queries in time . This is a polynomial
improvement over the previous planar distance oracles with query
time
Maximized Posteriori Attributes Selection from Facial Salient Landmarks for Face Recognition
This paper presents a robust and dynamic face recognition technique based on
the extraction and matching of devised probabilistic graphs drawn on SIFT
features related to independent face areas. The face matching strategy is based
on matching individual salient facial graph characterized by SIFT features as
connected to facial landmarks such as the eyes and the mouth. In order to
reduce the face matching errors, the Dempster-Shafer decision theory is applied
to fuse the individual matching scores obtained from each pair of salient
facial features. The proposed algorithm is evaluated with the ORL and the IITK
face databases. The experimental results demonstrate the effectiveness and
potential of the proposed face recognition technique also in case of partially
occluded faces.Comment: 8 pages, 2 figure
Unsupervised cryo-EM data clustering through adaptively constrained K-means algorithm
In single-particle cryo-electron microscopy (cryo-EM), K-means clustering
algorithm is widely used in unsupervised 2D classification of projection images
of biological macromolecules. 3D ab initio reconstruction requires accurate
unsupervised classification in order to separate molecular projections of
distinct orientations. Due to background noise in single-particle images and
uncertainty of molecular orientations, traditional K-means clustering algorithm
may classify images into wrong classes and produce classes with a large
variation in membership. Overcoming these limitations requires further
development on clustering algorithms for cryo-EM data analysis. We propose a
novel unsupervised data clustering method building upon the traditional K-means
algorithm. By introducing an adaptive constraint term in the objective
function, our algorithm not only avoids a large variation in class sizes but
also produces more accurate data clustering. Applications of this approach to
both simulated and experimental cryo-EM data demonstrate that our algorithm is
a significantly improved alterative to the traditional K-means algorithm in
single-particle cryo-EM analysis.Comment: 35 pages, 14 figure
Revisiting the Impact of Atmospheric Dispersion and Differential Refraction on Widefield Multiobject Spectroscopic Observations. From VLT/VIMOS to Next Generation Instruments
(Abridged) Atmospheric dispersion and field differential refraction impose
severe constraints on widefield MOS observations. Flux reduction and spectral
distortions must be minimised by a careful planning of the observations --
which is especially true for instruments that use slits instead of fibres. This
is the case of VIMOS at the VLT, where MOS observations have been restricted,
since the start of operations, to a narrow two-hour range from the meridian to
minimise slit losses. We revisit in detail the impact of atmospheric effects on
the quality of VIMOS-MOS spectra. We model slit losses across the entire VIMOS
FOV as a function of target declination. We explore two different slit
orientations at the meridian: along the parallactic angle (North-South), and
perpendicular to it (East-West). We show that, for fields culminating at zenith
distances larger than 20 deg, slit losses are minimised with slits oriented
along the parallactic angle at the meridian. The two-hour angle rule holds for
these observations using N-S orientations. Conversely, for fields with zenith
angles smaller than 20 deg at culmination, losses are minimised with slits
oriented perpendicular to the parallactic angle at the meridian. MOS
observations can be effectively extended to plus/minus three hours from the
meridian in these cases. In general, night-long observations of a single field
will benefit from using the E-W orientation. All-sky or service mode
observations, however, require a more elaborate planning that depends on the
target declination, and the hour angle of the observations. We establish
general rules for the alignment of slits in MOS observations that will increase
target observability, enhance the efficiency of operations, and speed up the
completion of programmes -- a particularly relevant aspect for the forthcoming
spectroscopic public surveys with VIMOS.Comment: Accepted to A&A. 11 pages, 15 figures. This paper presents the new
recommendations for optimal slit alignment in VLT/VIMOS observation
Switching Reconstruction of Digraphs
Switching about a vertex in a digraph means to reverse the direction of every
edge incident with that vertex. Bondy and Mercier introduced the problem of
whether a digraph can be reconstructed up to isomorphism from the multiset of
isomorphism types of digraphs obtained by switching about each vertex. Since
the largest known non-reconstructible oriented graphs have 8 vertices, it is
natural to ask whether there are any larger non-reconstructible graphs. In this
paper we continue the investigation of this question. We find that there are
exactly 44 non-reconstructible oriented graphs whose underlying undirected
graphs have maximum degree at most 2. We also determine the full set of
switching-stable oriented graphs, which are those graphs for which all
switchings return a digraph isomorphic to the original
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