8,797 research outputs found
Simplicial cohomology of band semigroup algebras
We establish simplicial triviality of the convolution algebra ,
where is a band semigroup. This generalizes results of the first author
[Glasgow Math. J. 2005, Houston J. Math. 2010]. To do so, we show that the
cyclic cohomology of this algebra vanishes in all odd degrees, and is
isomorphic in even degrees to the space of continuous traces on .
Crucial to our approach is the use of the structure semilattice of , and the
associated grading of , together with an inductive normalization procedure
in cyclic cohomology; the latter technique appears to be new, and its
underlying strategy may be applicable to other convolution algebras of
interest.Comment: v1: AMS-LaTeX, 24 pages, 1 figure. v2: some typos corrected; a few
minor adjustments made for clarity; references updated. Accepted June 2011 by
Proc. Royal Soc. Edinburgh Sect.
A mathematical model of the maximum power density attainable in an alkaline hydrogen/oxygen fuel cell
A mathematical model of a hydrogen/oxygen alkaline fuel cell is presented that can be used to predict the polarization behavior under various power loads. The major limitations to achieving high power densities are indicated and methods to increase the maximum attainable power density are suggested. The alkaline fuel cell model describes the phenomena occurring in the solid, liquid, and gaseous phases of the anode, separator, and cathode regions based on porous electrode theory applied to three phases. Fundamental equations of chemical engineering that describe conservation of mass and charge, species transport, and kinetic phenomena are used to develop the model by treating all phases as a homogeneous continuum
Historical forest biomass dynamics modelled with Landsat spectral trajectories
Acknowledgements National Forest Inventory data are available online, provided by Ministerio de Agricultura, Alimentación y Medio Ambiente (España). Landsat images are available online, provided by the USGS.Peer reviewedPostprin
High redshift galaxies and the Lyman-alpha forest in a CDM universe
We use a cosmological hydrodynamic simulation of a cold dark matter universe
to investigate theoretically the relationship between high redshift galaxies
and the Lyman=alpha forest at redshift z=3. Galaxies in the simulation are
surrounded by halos of hot gas, which nevertheless contain enough neutral
hydrogen to cause a Ly-alpha flux decrement, its strength increasing with
galaxy mass. A comparison with recent observational data by Adelberger et. al
on the Ly-alpha forest around galaxies reveals that actual galaxies may have
systematically less Ly-alpha absorption within 1 Mpc of them than our simulated
galaxies. In order to investigate this possibility, we add several simple
prescriptions for galaxy feedback on the IGM to the evolved simulation. These
include the effect of photoionizing background radiation coming from galactic
sources, galactic winds whose only effect is to deposit thermal energy into the
IGM, and another, kinetic model for winds, which are assumed to evacuate
cavities in the IGM around galaxies. We find that only the latter is able to
produce a large effect, enough to match the tentative observational data, given
the energy available from star formation in the simulated galaxies. Another
intriguing possibility is that a selection effect is responsible, so that
galaxies with low
Ly-alpha absorption are preferentially included in the sample. This is also
viable, but predicts very different galaxy properties (including clustering)
than the other scenarios.Comment: Submitted to ApJ, 20 pages, 19 postscript figures, emulateapj.st
Target enrichment of ultraconserved elements from arthropods provides a genomic perspective on relationships among Hymenoptera
Gaining a genomic perspective on phylogeny requires the collection of data
from many putatively independent loci collected across the genome. Among
insects, an increasingly common approach to collecting this class of data
involves transcriptome sequencing, because few insects have high-quality genome
sequences available; assembling new genomes remains a limiting factor; the
transcribed portion of the genome is a reasonable, reduced subset of the genome
to target; and the data collected from transcribed portions of the genome are
similar in composition to the types of data with which biologists have
traditionally worked (e.g., exons). However, molecular techniques requiring RNA
as a template are limited to using very high quality source materials, which
are often unavailable from a large proportion of biologically important insect
samples. Recent research suggests that DNA-based target enrichment of conserved
genomic elements offers another path to collecting phylogenomic data across
insect taxa, provided that conserved elements are present in and can be
collected from insect genomes. Here, we identify a large set (n1510) of
ultraconserved elements (UCE) shared among the insect order Hymenoptera. We use
in silico analyses to show that these loci accurately reconstruct relationships
among genome-enabled Hymenoptera, and we design a set of baits for enriching
these loci that researchers can use with DNA templates extracted from a variety
of sources. We use our UCE bait set to enrich an average of 721 UCE loci from
30 hymenopteran taxa, and we use these UCE loci to reconstruct phylogenetic
relationships spanning very old (220 MYA) to very young (1 MYA)
divergences among hymenopteran lineages. In contrast to a recent study
addressing hymenopteran phylogeny using transcriptome data, we found ants to be
sister to all remaining aculeate lineages with complete support
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