1,179 research outputs found
Maximum Likelihood Estimation in Gaussian Chain Graph Models under the Alternative Markov Property
The AMP Markov property is a recently proposed alternative Markov property
for chain graphs. In the case of continuous variables with a joint multivariate
Gaussian distribution, it is the AMP rather than the earlier introduced LWF
Markov property that is coherent with data-generation by natural
block-recursive regressions. In this paper, we show that maximum likelihood
estimates in Gaussian AMP chain graph models can be obtained by combining
generalized least squares and iterative proportional fitting to an iterative
algorithm. In an appendix, we give useful convergence results for iterative
partial maximization algorithms that apply in particular to the described
algorithm.Comment: 15 pages, article will appear in Scandinavian Journal of Statistic
Challenges for Efficient Query Evaluation on Structured Probabilistic Data
Query answering over probabilistic data is an important task but is generally
intractable. However, a new approach for this problem has recently been
proposed, based on structural decompositions of input databases, following,
e.g., tree decompositions. This paper presents a vision for a database
management system for probabilistic data built following this structural
approach. We review our existing and ongoing work on this topic and highlight
many theoretical and practical challenges that remain to be addressed.Comment: 9 pages, 1 figure, 23 references. Accepted for publication at SUM
201
Stable isotope records for the last 10 000 years from Okshola cave (Fauske, northern Norway) and regional comparisons
The sensitivity of terrestrial environments to past changes in heat transport is expected to be manifested in Holocene climate proxy records on millennial to seasonal timescales. Stalagmite formation in the Okshola cave near Fauske (northern Norway) began at about 10.4 ka, soon after the valley was deglaciated. Past monitoring of the cave and surface has revealed stable modern conditions with uniform drip rates, relative humidity and temperature. Stable isotope records from two stalagmites provide time-series spanning from c. 10 380 yr to AD 1997; a banded, multi-coloured stalagmite (Oks82) was formed between 10 380 yr and 5050 yr, whereas a pristine, white stalagmite (FM3) covers the period from ~7500 yr to the present. The stable oxygen isotope (&delta;<sup>18</sup>O<sub>c</sub>), stable carbon isotope (&delta;<sup>13</sup>C<sub>c</sub>), and growth rate records are interpreted as showing i) a negative correlation between cave/surface temperature and &delta;<sup>18</sup>O<sub>c</sub>, ii) a positive correlation between wetness and &delta;<sup>13</sup>C<sub>c</sub>, and iii) a positive correlation between temperature and growth rate. Following this, the data from Okshola show that the Holocene was characterised by high-variability climate in the early part, low-variability climate in the middle part, and high-variability climate and shifts between two distinct modes in the late part. <br><br> A total of nine Scandinavian stalagmite &delta;<sup>18</sup>O<sub>c</sub> records of comparable dating precision are now available for parts or most of the Holocene. None of them show a clear Holocene thermal optimum, suggesting that they are influenced by annual mean temperature (cave temperature) rather than seasonal temperature. For the last 1000 years, &delta;<sup>18</sup>O<sub>c</sub> values display a depletion-enrichment-depletion pattern commonly interpreted as reflecting the conventional view on climate development for the last millennium. Although the &delta;<sup>18</sup>O<sub>c</sub> records show similar patterns and amplitudes of change, the main challenges for utilising high-latitude stalagmites as palaeoclimate archives are i) the accuracy of the age models, ii) the ambiguity of the proxy signals, and iii) calibration with monitoring data
Effect of catalyst layer defects on local membrane degradation in polymer electrolyte fuel cells
© 2016 Elsevier B.V. All rights reserved. Aiming at durability issues of fuel cells, this research is dedicated to a novel experimental approach in the analysis of local membrane degradation phenomena in polymer electrolyte fuel cells, shedding light on the potential effects of manufacturing imperfections on this process. With a comprehensive review on historical failure analysis data from field operated fuel cells, local sources of iron oxide contaminants, catalyst layer cracks, and catalyst layer delamination are considered as potential candidates for initiating or accelerating the local membrane degradation phenomena. Customized membrane electrode assemblies with artificial defects are designed, fabricated, and subjected to membrane accelerated stress tests followed by extensive post-mortem analysis. The results reveal a significant accelerating effect of iron oxide contamination on the global chemical degradation of the membrane, but dismiss local traces of iron oxide as a potential stressor for local membrane degradation. Anode and cathode catalyst layer cracks are observed to have negligible impact on the membrane degradation phenomena. Notably however, distinct evidence is found that anode catalyst layer delamination can accelerate local membrane thinning, while cathode delamination has no apparent effect. Moreover, a substantial mitigating effect for platinum residuals on the site of delamination is observed
Steinberg modules and Donkin pairs
We prove that in positive characteristic a module with good filtration for a
group of type E6 restricts to a module with good filtration for a subgroup of
type F4. (Recall that a filtration of a module for a semisimple algebraic group
is called good if its layers are dual Weyl modules.) Our result confirms a
conjecture of Brundan for one more case. The method relies on the canonical
Frobenius splittings of Mathieu. Next we settle the remaining cases, in
characteristic not 2, with a computer-aided variation on the old method of
Donkin.Comment: 16 pages; proof of Brundan's conjecture adde
Collisional Damping of Nuclear Collective Vibrations in a Non-Markovian Transport Approach
A detailed derivation of the collisional widths of collective vibrations is
presented in both quantal and semi-classical frameworks by considering the
linearized limits of the extended TDHF and the BUU model with a non-Markovian
binary collision term. Damping widths of giant dipole and giant quadrupole
excitations are calculated by employing an effective Skyrme force, and the
results are compared with GDR measurements in Lead and Tin nuclei at finite
temperature.Comment: 23 pages, 6 Figure
Multi-proxy speleothem-based reconstruction of mid-MIS 3 climate in South Africa
The southern coast of South Africa displays a highly
dynamical climate as it is at the convergence of the Atlantic and
Indian oceans, and it is located near the subtropical/temperate zone boundary
with seasonal influence of easterlies and westerlies. The region hosts some key
archeological sites with records of significant cognitive, technological and
social developments. Reconstructions of the state and variability of past
climate and environmental conditions around sites of archeological
significance can provide crucial context for understanding the evolution of
early humans. Here we present a short but high-resolution record of
hydroclimate and temperature in South Africa. Our reconstructions are based
on trace elements, calcite and fluid inclusion stable isotopes, as well as fluid
inclusion microthermometry, from a speleothem collected in Bloukrantz cave,
in the De Hoop Nature Reserve in the southern Cape region of South Africa.
Our record covers the time period from 48.3 to 45.2 ka during marine isotope
stage 3. Both δ18Oc and δ13Cc show strong variability
and covary with Sr/Ca. This correlation suggests that the control on these
proxies originates from internal cave processes such as prior carbonate
precipitation, which we infer to be related to precipitation amount. The
hydroclimate indicators furthermore suggest a shift towards overall drier
conditions after 46 ka, coincident with cooling in Antarctica and drier
conditions in the eastern part of South Africa corresponding to the summer
rainfall zone (SRZ).
Fluid inclusion-based temperature reconstructions show good agreement
between the oxygen isotope and microthermometry methods, and results from
the latter display little variation throughout the record, with
reconstructed temperatures close to the present-day cave temperature of 17.5 ∘C. Overall, the BL3 speleothem record thus suggests relatively stable temperature from
48.3 to 45.2 ka, whereas precipitation was variable with marked drier
episodes on sub-millennial timescales.</p
Microscopic Origin of Quantum Chaos in Rotational Damping
The rotational spectrum of Yb is calculated diagonalizing different
effective interactions within the basis of unperturbed rotational bands
provided by the cranked shell model. A transition between order and chaos
taking place in the energy region between 1 and 2 MeV above the yrast line is
observed, associated with the onset of rotational damping. It can be related to
the higher multipole components of the force acting among the unperturbed
rotational bands.Comment: 7 pages, plain TEX, YITP/K-99
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