2,469 research outputs found
Lensing of 21-cm Fluctuations by Primordial Gravitational Waves
Weak-gravitational-lensing distortions to the intensity pattern of 21-cm radiation from the dark ages can be decomposed geometrically into curl and curl-free components. Lensing by primordial gravitational waves induces a curl component, while the contribution from lensing by density fluctuations is strongly suppressed. Angular fluctuations in the 21-cm background extend to very small angular scales, and measurements at different frequencies probe different shells in redshift space. There is thus a huge trove of information with which to reconstruct the curl component of the lensing field, allowing tensor-to-scalar ratios conceivably as small as r∼10^(-9)—far smaller than those currently accessible—to be probed
Regularity of aperiodic minimal subshifts
At the turn of this century Durand, and Lagarias and Pleasants established
that key features of minimal subshifts (and their higher-dimensional analogues)
to be studied are linearly repetitive, repulsive and power free. Since then,
generalisations and extensions of these features, namely -repetitive,
-repulsive and -finite (), have been introduced
and studied. We establish the equivalence of -repulsive and
-finite for general subshifts over finite alphabets. Further, we
studied a family of aperiodic minimal subshifts stemming from Grigorchuk's
infinite -group . In particular, we show that these subshifts provide
examples that demonstrate -repulsive (and hence -finite) is not
equivalent to -repetitive, for . We also give necessary and
sufficient conditions for these subshifts to be -repetitive, and
-repulsive (and hence -finite). Moreover, we obtain an explicit
formula for their complexity functions from which we deduce that they are
uniquely ergodic.Comment: 15 page
Biodiesel via in situ wet microalgae biotransformation: Zwitter-type ionic liquid supported extraction and transesterification
The production of biodiesel derived from microalgae is among the most forthcoming technologies that provide an ecologic alternative to fossil fuels. Herein, a method was developed that enables the direct extraction and conversion of algal oil to biodiesel without prior isolation. The reaction occurs in aqueous media catalyzed by immobilized Candida antarctica lipase B (Novozyme 435). Zwitter-type ionic liquids were used as cocatalyst to improve the selectivity and reactivity of the enzyme. In a model reaction with sunflower oil, 64% biodiesel was obtained. Applying this method to a slurry of whole-cell Chlorella zof ingiensis in water resulted in 74.8% of lipid extraction, with 27.7% biotransformation products and up to 16% biodiesel. Factors that reduced the lipase activity with whole-cell algae were subsequently probed and discussed. This "in situ" method shows an improvement to existing methods, since it integrates the oil extraction and conversion into an one-pot procedure in aqueous conditions. The extraction is nondisruptive, and is a model for a greener algae to biodiesel process
Grading and metastable effects in admittance spectroscopy of CIGS-based solar cells
Cu(In, Ga)Se2-based (CIGS) solar cells have achieved efficiencies up to 20%. Despite these excellent results, the understanding of the underlying mechanisms and the influence of defects on their performance is still incomplete. The determination of the energetic position of the defects and of their density of states is important. Admittance spectroscopy is an adequate technique for this. By varying the external voltage during the measurement, the spatial position where the defect distribution is sensed can be varied. However, the application of external biases can lead to metastable effects in the absorber and therefore to defect relaxation and changes in the doping distribution. Hence, it is important to separate between the effects caused by metastable changes and the change in sensing position of the admittance spectroscopy measurement. This can be achieved by varying the applied voltage during the creation of the metastable state and the measurement itself independently or simultaneously. Admittance spectroscopy under different bias voltage conditions performed on a flexible CIGS-based solar cell are presented and assessed
List Defective Colorings: Distributed Algorithms and Applications
The distributed coloring problem is at the core of the area of distributed
graph algorithms and it is a problem that has seen tremendous progress over the
last few years. Much of the remarkable recent progress on deterministic
distributed coloring algorithms is based on two main tools: a) defective
colorings in which every node of a given color can have a limited number of
neighbors of the same color and b) list coloring, a natural generalization of
the standard coloring problem that naturally appears when colorings are
computed in different stages and one has to extend a previously computed
partial coloring to a full coloring.
In this paper, we introduce \emph{list defective colorings}, which can be
seen as a generalization of these two coloring variants. Essentially, in a list
defective coloring instance, each node is given a list of colors
together with a list of defects
such that if is colored with color , it is allowed to have at
most neighbors with color .
We highlight the important role of list defective colorings by showing that
faster list defective coloring algorithms would directly lead to faster
deterministic -coloring algorithms in the LOCAL model. Further, we
extend a recent distributed list coloring algorithm by Maus and Tonoyan [DISC
'20]. Slightly simplified, we show that if for each node it holds that
then
this list defective coloring instance can be solved in a
communication-efficient way in only communication rounds. This
leads to the first deterministic -coloring algorithm in the
standard CONGEST model with a time complexity of , matching the best time complexity in the LOCAL model up to a
factor
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