179 research outputs found
Minimizing the Continuous Diameter when Augmenting Paths and Cycles with Shortcuts
We seek to augment a geometric network in the Euclidean plane with shortcuts
to minimize its continuous diameter, i.e., the largest network distance between
any two points on the augmented network. Unlike in the discrete setting where a
shortcut connects two vertices and the diameter is measured between vertices,
we take all points along the edges of the network into account when placing a
shortcut and when measuring distances in the augmented network.
We study this network augmentation problem for paths and cycles. For paths,
we determine an optimal shortcut in linear time. For cycles, we show that a
single shortcut never decreases the continuous diameter and that two shortcuts
always suffice to reduce the continuous diameter. Furthermore, we characterize
optimal pairs of shortcuts for convex and non-convex cycles. Finally, we
develop a linear time algorithm that produces an optimal pair of shortcuts for
convex cycles. Apart from the algorithms, our results extend to rectifiable
curves.
Our work reveals some of the underlying challenges that must be overcome when
addressing the discrete version of this network augmentation problem, where we
minimize the discrete diameter of a network with shortcuts that connect only
vertices
A Note on the Unsolvability of the Weighted Region Shortest Path Problem
Let S be a subdivision of the plane into polygonal regions, where each region
has an associated positive weight. The weighted region shortest path problem is
to determine a shortest path in S between two points s, t in R^2, where the
distances are measured according to the weighted Euclidean metric-the length of
a path is defined to be the weighted sum of (Euclidean) lengths of the
sub-paths within each region. We show that this problem cannot be solved in the
Algebraic Computation Model over the Rational Numbers (ACMQ). In the ACMQ, one
can compute exactly any number that can be obtained from the rationals Q by
applying a finite number of operations from +, -, \times, \div, \sqrt[k]{}, for
any integer k >= 2. Our proof uses Galois theory and is based on Bajaj's
technique.Comment: 6 pages, 1 figur
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Bioactive coatings of glass-ceramics on metals
To combine the mechanical properties of high-strength base metals with the biological properties of bioactive materials, coatings of BIOVERITŸ glasses and glass-ceramics on CoCr alloys and titanium have been produced. Different kinds of coating processes have been used: dipping, sputtering, plasma spraying, sintering and pasting. Dipping and pasting seem not to be suitable to produce layers because of cracks and low adhesion strength of the coatings (dipping) and the limitations in relation to the implant shape and the thickness of the layers (pasting). Using sputter techniques it is possible to produce dense layers ( < 10”im) with a high adhesion strength. Plasma spraying resuhed in layers with a thickness of 50 to 300”im, but during the plasma spraying process there is a partial evaporation of the highly volatile glass components. Sintering processes are very suitable to produce layers with a high adhesion strength; these layers are long term stable in simulated body fluid
Wavelength dependent ac-Stark shift of the 1S0 - 3P1 transition at 657 nm in Ca
We have measured the ac-Stark shift of the 4s2 1S0 - 4s4p 3P1 line in 40Ca
for perturbing laser wavelengths between 780 nm and 1064 nm with a time domain
Ramsey-Borde atom interferometer. We found a zero crossing of the shift for the
mS = 0 - mP = 0 transition and \sigma polarized perturbation at 800.8(22) nm.
The data was analyzed by a model deriving the energy shift from known
transition wavelengths and strengths. To fit our data, we adjusted the Einstein
A coefficients of the 4s3d 3D - 4s4p 3P and 4s5s 3S - 4s4p 3P fine structure
multiplets. With these we can predict vanishing ac-Stark shifts for the 1S0 m =
0 - 3P1 m = 1 transition and \sigma- light at 983(12) nm and at 735.5(20) nm
for the transition to the 3P0 level.Comment: 8 pages, 5 figures, 2 table
Effectiveness of a lowâdose mindfulnessâbased intervention for alleviating distress in young unemployed adults
While the effectiveness of mindfulnessâbased interventions (MBIs) with respect to distress has been widely researched, unemployed individuals, who often suffer from high levels of distress, have largely been neglected in MBI research. The present study aimed to investigate the effects of a lowâdose MBI on distress in a sample of young unemployed adults. The sample included 239 young unemployed adults enrolled for a 6âweek long employabilityârelated training camp. Participants were allocated into an intervention group that received weekly 1âhour mindfulness training over 4 weeks, and a control group. Dispositional mindfulness, distress and wellâbeing were assessed in the entire sample prior to the start and upon completion of the mindfulness training. A mixed model ANCOVA showed that distress was inversely and significantly predicted by baseline levels of mindfulness and wellâbeing. After accounting for the baseline levels of mindfulness and wellâbeing, a significant effect of the mindfulness intervention was evident. This result shows that a lowâdose MBI can decrease distress in a sample of young unemployed adults and its effectiveness is positively associated with initial levels of dispositional mindfulness and wellâbeing
Network Farthest-Point Diagrams
Consider the continuum of points along the edges of a network, i.e., an
undirected graph with positive edge weights. We measure distance between these
points in terms of the shortest path distance along the network, known as the
network distance. Within this metric space, we study farthest points.
We introduce network farthest-point diagrams, which capture how the farthest
points---and the distance to them---change as we traverse the network. We
preprocess a network G such that, when given a query point q on G, we can
quickly determine the farthest point(s) from q in G as well as the farthest
distance from q in G. Furthermore, we introduce a data structure supporting
queries for the parts of the network that are farther away from q than some
threshold R > 0, where R is part of the query.
We also introduce the minimum eccentricity feed-link problem defined as
follows. Given a network G with geometric edge weights and a point p that is
not on G, connect p to a point q on G with a straight line segment pq, called a
feed-link, such that the largest network distance from p to any point in the
resulting network is minimized. We solve the minimum eccentricity feed-link
problem using eccentricity diagrams. In addition, we provide a data structure
for the query version, where the network G is fixed and a query consists of the
point p.Comment: A preliminary version of this work was presented at the 24th Canadian
Conference on Computational Geometr
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