1,500 research outputs found
Tilting mutation of weakly symmetric algebras and stable equivalence
We consider tilting mutations of a weakly symmetric algebra at a subset of
simple modules, as recently introduced by T. Aihara. These mutations are
defined as the endomorphism rings of certain tilting complexes of length 1.
Starting from a weakly symmetric algebra A, presented by a quiver with
relations, we give a detailed description of the quiver and relations of the
algebra obtained by mutating at a single loopless vertex of the quiver of A. In
this form the mutation procedure appears similar to, although significantly
more complicated than, the mutation procedure of Derksen, Weyman and Zelevinsky
for quivers with potentials. By definition, weakly symmetric algebras connected
by a sequence of tilting mutations are derived equivalent, and hence stably
equivalent. The second aim of this article is to study these stable
equivalences via a result of Okuyama describing the images of the simple
modules. As an application we answer a question of Asashiba on the derived
Picard groups of a class of self-injective algebras of finite representation
type. We conclude by introducing a mutation procedure for maximal systems of
orthogonal bricks in a triangulated category, which is motivated by the effect
that a tilting mutation has on the set of simple modules in the stable
category.Comment: Description and proof of mutated algebra made more rigorous (Prop.
3.1 and 4.2). Okuyama's Lemma incorporated: Theorem 4.1 is now Corollary 5.1,
and proof is omitted. To appear in Algebras and Representation Theor
On Berenstein-Douglas-Seiberg Duality
I review the proposal of Berenstein-Douglas for a completely general
definition of Seiberg duality. To give evidence for their conjecture I present
the first example of a physical dual pair and explicitly check that it
satisfies the requirements. Then I explicitly show that a pair of toric dual
quivers is also dual according to their proposal. All these computations go
beyond tilting modules, and really work in the derived category. I introduce
all necessary mathematics where needed.Comment: 22 pages, LaTe
H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
Using high resolution infrared spectroscopy we have surveyed twenty
sightlines for H3+ absorption. H3+ is detected in eight diffuse cloud
sightlines with column densities varying from 0.6x10^14 cm^-2 to 3.9x10^14
cm^-2. This brings to fourteen the total number of diffuse cloud sightlines
where H3+ has been detected. These detections are mostly along sightlines
concentrated in the Galactic plane, but well dispersed in Galactic longitude.
The results imply that abundant H3+ is common in the diffuse interstellar
medium. Because of the simple chemistry associated with H3+ production and
destruction, these column density measurements can be used in concert with
various other data to infer the primary cosmic-ray ionization rate, zeta_p.
Values range from 0.5x10^-16 s^-1 to 3x10^-16 s^-1 with an average of 2x10^-16
s^-1. Where H3+ is not detected the upper limits on the ionization rate are
consistent with this range. The average value of zeta_p is about an order of
magnitude larger than both the canonical rate and rates previously reported by
other groups using measurements of OH and HD. The discrepancy is most likely
due to inaccurate measurements of rate constants and the omission of effects
which were unknown when those studies were performed. We believe that the
observed column density of H3+ is the most direct tracer for the cosmic-ray
ionization rate due to its simple chemistry. Recent models of diffuse cloud
chemistry require cosmic-ray ionization rates on the order of 10^-16 s^-1 to
reproduce observed abundances of various atomic and molecular species, in rough
accord with our observational findings.Comment: Accepted to ApJ, 35 pages, 5 figures, 5 table
Torsion pairs and simple-minded systems in triangulated categories
Let T be a Hom-finite triangulated Krull-Schmidt category over a field k.
Inspired by a definition of Koenig and Liu, we say that a family S of pairwise
orthogonal objects in T with trivial endomorphism rings is a simple-minded
system if its closure under extensions is all of T. We construct torsion pairs
in T associated to any subset X of a simple-minded system S, and use these to
define left and right mutations of S relative to X. When T has a Serre functor
\nu, and S and X are invariant under \nu[1], we show that these mutations are
again simple-minded systems. We are particularly interested in the case where T
is the stable module category of a self-injective algebra \Lambda. In this
case, our mutation procedure parallels that introduced by Koenig and Yang for
simple-minded collections in the derived category of \Lambda. It follows that
the mutation of the set of simple \Lambda-modules relative to X yields the
images of the simple \Gamma-modules under a stable equivalence between \Gamma\
and \Lambda, where \Gamma\ is the tilting mutation of \Lambda\ relative to X.Comment: Minor corrections. To appear in Applied Categorical Structures. The
final publication is available at springerlink.com:
http://link.springer.com/article/10.1007%2Fs10485-014-9365-
Seasonal dependence of peroxy radical concentrations at a Northern hemisphere marine boundary layer site during summer and winter: evidence for radical activity in winter
Peroxy radicals (HO2+Σ RO2) were measured at the Weybourne Atmospheric Observatory (52° N, 1° E), Norfolk using a PEroxy Radical Chemical Amplifier (PERCA) during the winter and summer of 2002. The peroxy radical diurnal cycles showed a marked difference between the winter and summer campaigns with maximum concentrations of 12 pptv at midday in the summer and maximum concentrations as high as 30 pptv (10 min averages) in winter at night. The corresponding nighttime peroxy radical concentrations were not as high in summer (3 pptv). The peroxy radical concentration shows a distinct anti-correlation with increasing NOx during the daylight hours. At night, peroxy radicals increase with increasing NOx indicative of the role of NO3 chemistry. The average diurnal cycles for net ozone production, N(O3) show a large variability in ozone production, P(O3), and a large ozone loss, L(O3) in summer relative to winter. For a daylight average, net ozone production in summer was higher than winter (1.51±0.5 ppbv h−1 and 1.11±0.47 ppbv h−1, respectively). The variability in NO concentration has a much larger effect on N(O3) than the peroxy radical concentrations. Photostationary state (PSS) calculations show an NO2 lifetime of 5 min in summer and 21 minutes in the winter, implying that steady-state NO-NO2 ratios are not always attained during the winter months. The results show an active peroxy radical chemistry at night and that significant oxidant levels are sustained in winter. The net effect of this with respect to production of ozone in winter is unclear owing to the breakdown in the photostationary state
The atmospheric impacts of monoterpene ozonolysis on global stabilised Criegee intermediate budgets and SO2 oxidation : experiment, theory and modelling
The gas-phase reaction of alkenes with ozone is known to produce stabilised
Criegee intermediates (SCIs). These biradical/zwitterionic species have the
potential to act as atmospheric oxidants for trace pollutants such as
SO<sub>2</sub>, enhancing the formation of sulfate aerosol with impacts on air
quality and health, radiative transfer and climate. However, the importance
of this chemistry is uncertain as a consequence of limited understanding of
the abundance and atmospheric fate of SCIs. In this work we apply
experimental, theoretical and numerical modelling methods to quantify the
atmospheric impacts, abundance and fate of the structurally diverse SCIs
derived from the ozonolysis of monoterpenes, the second most abundant group
of unsaturated hydrocarbons in the atmosphere. We have investigated the
removal of SO<sub>2</sub> by SCIs formed from the ozonolysis of three
atmospherically important monoterpenes (<i>α</i>-pinene, <i>β</i>-pinene and
limonene) in the presence of varying amounts of water vapour in large-scale
simulation chamber experiments that are representative of boundary layer conditions.
The SO<sub>2</sub> removal displays a clear dependence on water vapour
concentration, but this dependence is not linear across the range of
[H<sub>2</sub>O] explored. At low [H<sub>2</sub>O] a strong dependence of SO<sub>2</sub>
removal on [H<sub>2</sub>O] is observed, while at higher [H<sub>2</sub>O] this dependence
becomes much weaker. This is interpreted as being caused by the production of
a variety of structurally (and hence chemically) different SCIs in each of the
systems studied, which displayed different rates of reaction with water and
of unimolecular rearrangement or decomposition. The determined rate constants,
<i>k</i>(SCI+H<sub>2</sub>O), for those SCIs that react primarily with H<sub>2</sub>O range
from 4 to 310  ×  10<sup>−15</sup> cm<sup>3</sup> s<sup>−1</sup>. For those SCIs that
predominantly react unimolecularly, determined rates range from
130 to 240 s<sup>−1</sup>. These values are in line with previous results for the
(analogous) stereo-specific SCI system of <i>syn</i>-/<i>anti</i>-CH<sub>3</sub>CHOO. The experimental results are
interpreted through theoretical studies of the SCI unimolecular reactions and
bimolecular reactions with H<sub>2</sub>O, characterised for <i>α</i>-pinene and
<i>β</i>-pinene at the M06-2X/aug-cc-pVTZ level of theory. The theoretically
derived rates agree with the experimental results within the uncertainties. A
global modelling study, applying the experimental results within the
GEOS-Chem chemical transport model, suggests that > 97 % of
the total monoterpene-derived global SCI burden is comprised of SCIs with a
structure that determines that they react slowly with water and that their atmospheric
fate is dominated by unimolecular reactions. Seasonally averaged boundary
layer concentrations of monoterpene-derived SCIs reach up to
1.4  ×  10<sup>4</sup> cm<sup>−3</sup> in regions of elevated monoterpene
emissions in the tropics. Reactions of monoterpene-derived SCIs with SO<sub>2</sub>
account for < 1 % globally but may account for up to 60 % of
the gas-phase SO<sub>2</sub> removal over areas of tropical forests, with
significant localised impacts on the formation of sulfate aerosol and hence
the lifetime and distribution of SO<sub>2</sub>
On Morita and derived equivalences for cohomological Mackey algebras
By results of the second author, a source algebra equivalence between two p-blocks of finite groups induces an equivalence between the categories of cohomological Mackey functors associated with these blocks, and a splendid derived equivalence between two blocks induces a derived equivalence between the corresponding categories ofcohomological Mackey functors. The main result of this paper proves a partial converse: an equivalence (resp. Rickard equivalence) between the categories of cohomological Mackey functors of two blocks of finite groups induces a permeable Morita (resp. derived) equivalence between the two block algebras
The LWA1 Radio Telescope
LWA1 is a new radio telescope operating in the frequency range 10-88 MHz,
located in central New Mexico. The telescope consists of 258 pairs of
dipole-type antennas whose outputs are individually digitized and formed into
beams. Simultaneously, signals from all dipoles can be recorded using one of
the instrument's "all dipoles" modes, facilitating all-sky imaging. Notable
features of the instrument include high intrinsic sensitivity (about 6 kJy
zenith system equivalent flux density), large instantaneous bandwidth (up to 78
MHz), and 4 independently-steerable beams utilizing digital "true time delay"
beamforming. This paper summarizes the design of LWA1 and its performance as
determined in commissioning experiments. We describe the method currently in
use for array calibration, and report on measurements of sensitivity and
beamwidth.Comment: 9 pages, 14 figures, accepted by IEEE Trans. Antennas & Propagation.
Various minor changes from previous versio
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