3,050 research outputs found
New lower bounds for the topological complexity of aspherical spaces
Date of Acceptance: 5/04/2015 15 pages, 4 figuresPeer reviewedPostprin
A mapping theorem for topological complexity
Peer reviewedPublisher PD
Bredon cohomology and robot motion planning
In this paper we study the topological invariant reflecting
the complexity of algorithms for autonomous robot motion. Here, stands for
the configuration space of a system and is, roughly, the
minimal number of continuous rules which are needed to construct a motion
planning algorithm in . We focus on the case when the space is
aspherical; then the number depends only on the fundamental group
and we denote it . We prove that
can be characterised as the smallest integer such that the canonical
-equivariant map of classifying spaces can be equivariantly deformed into the
-dimensional skeleton of . The symbol
denotes the classifying space for free actions and
denotes the classifying space for actions with
isotropy in a certain family of subgroups of . Using
this result we show how one can estimate in terms of the
equivariant Bredon cohomology theory. We prove that where denotes the cohomological dimension of with
respect to the family of subgroups . We also introduce a Bredon
cohomology refinement of the canonical class and prove its universality.
Finally we show that for a large class of principal groups (which includes all
torsion free hyperbolic groups as well as all torsion free nilpotent groups)
the essential cohomology classes in the sense of Farber and Mescher are exactly
the classes having Bredon cohomology extensions with respect to the family
.Comment: This revision contains a few additional comments, among them is
Corollary 3.5.
Helium-3 in the Guaymas Basin: Evidence for injection of mantle volatiles in the Gulf of California
Helium isotope measurements in six major basins in the Gulf of California show that the deep Guaymas Basin has 3He/4He 65–70% higher than atmospheric helium, clear evidence of mantle helium injection. Smaller 3He excesses observed in the Carmen and Farallon basins may be derived from this Guaymas Basin anomaly. The 3He concentrations in the Mazatlan Basin in the mouth of the Gulf of California are similar to average eastern Pacific values, indicating that the Gulf does not provide a significant flux of 3He into the general Pacific circulation. On the basis of temperature and salinity measurements an upper limit of 0.28°C can be placed on the amount of geothermal heating observed in any of the basins. The isotopic ratio of the injected Guaymas Basin helium is found to be 3He/4He = (1.10±0.06) × 10−5, almost identical to the helium signature observed at the Galapagos Rift but somewhat lower than the average ratio in oceanic basalt glasses
Frequency up-conversion as a temperature probe of organic opto-electronic devices
Journal ArticleFrequency up-conversion is demonstrated in a polyfluorene-based conjugated polymer. Up-converted emission is observed upon excitation to the red of the 0-0 luminescence band. The emission intensity depends strongly on temperature and provides an accurate probe of the operating temperature of organic light-emitting diodes. Temperature rises of up to 30 K are observed at standard operating current densities of 225 mA/cm2. Due to the low thermal conductivity and heat capacity of the polymer film, the temperature of the film is found to rise slowly on a time-scale of seconds. Upon termination of the current, the temperature decreases again on a time-scale of minutes
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