70,740 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
.
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
Magnitude cohomology
Magnitude homology was introduced by Hepworth and Willerton in the case of
graphs, and was later extended by Leinster and Shulman to metric spaces and
enriched categories. Here we introduce the dual theory, magnitude cohomology,
which we equip with the structure of an associative unital graded ring. Our
first main result is a 'recovery theorem' showing that the magnitude cohomology
ring of a finite metric space completely determines the space itself. The
magnitude cohomology ring is non-commutative in general, for example when
applied to finite metric spaces, but in some settings it is commutative, for
example when applied to ordinary categories. Our second main result explains
this situation by proving that the magnitude cohomology ring of an enriched
category is graded-commutative whenever the enriching category is cartesian. We
end the paper by giving complete computations of magnitude cohomology rings for
several large classes of graphs.Comment: 27 page
Rectilinear Link Diameter and Radius in a Rectilinear Polygonal Domain
We study the computation of the diameter and radius under the rectilinear
link distance within a rectilinear polygonal domain of vertices and
holes. We introduce a \emph{graph of oriented distances} to encode the distance
between pairs of points of the domain. This helps us transform the problem so
that we can search through the candidates more efficiently. Our algorithm
computes both the diameter and the radius in time, where denotes the matrix
multiplication exponent and is the number of
edges of the graph of oriented distances. We also provide a faster algorithm
for computing the diameter that runs in time
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
- …