11,780 research outputs found
Learning to Understand by Evolving Theories
In this paper, we describe an approach that enables an autonomous system to
infer the semantics of a command (i.e. a symbol sequence representing an
action) in terms of the relations between changes in the observations and the
action instances. We present a method of how to induce a theory (i.e. a
semantic description) of the meaning of a command in terms of a minimal set of
background knowledge. The only thing we have is a sequence of observations from
which we extract what kinds of effects were caused by performing the command.
This way, we yield a description of the semantics of the action and, hence, a
definition.Comment: KRR Workshop at ICLP 201
Hartmann's Procedure or Primary Anastomosis?
Perforation following acute diverticulitis is a typical scenario during the first attack. Different classification systems exist to classify acute perforated diverticulitis. While the Hinchey classification, which is based on intraoperative findings, is internationally best known, the German Hansen-Stock classification which is based on CT scan is widely accepted within Germany. When surgery is necessary, sigmoid colectomy is the standard of care. An important question is whether patients should receive primary anastomosis or a Hartmann procedure subsequently. A priori there are several arguments for both procedures. Hartmann's operation is extremely safe and, therefore, represents the best option in severely ill patients and/or extensive peritonitis. However, this operation carries a high risk of stoma nonreversal, or, when reversal is attempted, a high risk in terms of morbidity and mortality. In contrast, primary anastomosis with or without loop ileostoma is a slightly more lengthy procedure as normally the splenic flexure needs to be mobilized and construction of the anastomosis may consume more time than the Hartmann operation. The big advantage of primary anastomosis, however, is that there is no need for the potentially risky stoma reversal operation. The most interesting question is when to do the Hartmann operation or primary anastomosis. Several comparative case series were published showing that primary anastomosis is feasible in many patients. However, no randomized trial is available to date. It is of note, that all non-randomized case series are biased, i.e. that patients in better condition received anastomosis and those with severe peritonitis underwent Hartmann's operation. This bias is undoubtedly likely to be present, even if not obvious, in the published papers! Our own data suggest that this decision should not be based on the extent of peritonitis but rather on patient condition and comorbidity. In conclusion, sigmoid colectomy and primary anastomosis is feasible and safe in many patients who need surgery for perforated diverticulitis, particularly when combined with loop ileostomy. Based on our own published analysis, however, we recommend performing Hartmann's operation in severely ill patients who carry substantial comorbidity, while the extent of peritonitis appears not to be of predominant importance. Copyright (C) 2012 S. Karger AG, Base
Unlocking Complex Soil Systems as Carbon Sinks: Multi-pool Management as the Key
Much research focuses on increasing carbon storage in mineral-associated organic matter (MAOM), in which carbon may persist for centuries to millennia. However, MAOM-targeted management is insufficient because the formation pathways of persistent soil organic matter are diverse and vary with environmental conditions. Effective management must also consider particulate organic matter (POM). In many soils, there is potential for enlarging POM pools, POM can persist over long time scales, and POM can be a direct precursor of MAOM. We present a framework for context-dependent management strategies that recognizes soils as complex systems in which environmental conditions constrain POM and MAOM formation
A Survey for Massive Giant Planets in Debris Disks with Evacuated Inner Cavities
The commonality of collisionally replenished debris around main sequence
stars suggests that minor bodies are frequent around Sun-like stars. Whether or
not debris disks in general are accompanied by planets is yet unknown, but
debris disks with large inner cavities - perhaps dynamically cleared - are
considered to be prime candidates for hosting large-separation massive giant
planets. We present here a high-contrast VLT/NACO angular differential imaging
survey for eight such cold debris disks. We investigated the presence of
massive giant planets in the range of orbital radii where the inner edge of the
dust debris is expected. Our observations are sensitive to planets and brown
dwarfs with masses >3 to 7 Jupiter mass, depending on the age and distance of
the target star. Our observations did not identify any planet candidates. We
compare the derived planet mass upper limits to the minimum planet mass
required to dynamically clear the inner disks. While we cannot exclude that
single giant planets are responsible for clearing out the inner debris disks,
our observations constrain the parameter space available for such planets. The
non-detection of massive planets in these evacuated debris disks further
reinforces the notion that the giant planet population is confined to the inner
disk (<15 AU).Comment: Accepted for publication in Ap
Saturation and BFKL dynamics in the HERA data at small x
We show that the HERA data for the inclusive structure function F_2(x,Q^2)
for x < 0.01 and 0.045 < Q^2 < 45 GeV^2 can be well described within the color
dipole picture, with a simple analytic expression for the dipole-proton
scattering amplitude, which is an approximate solution to the non-linear
evolution equations in QCD. For dipole sizes less than the inverse saturation
momentum 1/Q_s(x), the scattering amplitude is the solution to the BFKL
equation in the vicinity of the saturation line. It exhibits geometric scaling
and scaling violations by the diffusion term. For dipole sizes larger than
1/Q_s(x), the scattering amplitude saturates to one. The fit involves three
parameters: the proton radius R, the value x_0 of x at which the saturation
scale Q_s equals 1GeV, and the logarithmic derivative of the saturation
momentum \lambda. The value of \lambda extracted from the fit turns out to be
consistent with a recent calculation using the next-to-leading order BFKL
formalism.Comment: 15 pages, 3 figures, 3 table
Towards a New Global QCD Analysis: Solution to the Non-Linear Equation at Arbitrary Impact Parameter
A numerical solution is presented for the non-linear evolution equation that
governs the dynamics of high parton density QCD. It is shown that thesolution
falls off as at large values of the impact parameter . The
power-like tail of the amplitude appears in impact parameter distributions only
after the inclusion of dipoles of size larger than the target, a configuration
for which the non-linear equation is not valid. The value, energy and impact
parameterof the saturation scale ) are calculated both for
fixed and running QCD coupling cases. It is shown that the solution exhibits
geometrical scaling behaviour. The radius of interaction increases as the
rapidity in accordance with the Froissart theorem. The solution we obtain
differs from previous attempts, where an anzatz for behaviour was made. The
solutions for running and fixed \as differ. For running \as we obtain a
larger radius of interaction (approximately twice as large), a steeper rapidity
dependence, and a larger value of the saturation scale.Comment: 26 pages with 19 figures in eps.file
High Energy Bounds on Soft N=4 SYM Amplitudes from AdS/CFT
Using the AdS/CFT correspondence, we study the high-energy behavior of
colorless dipole elastic scattering amplitudes in N=4 SYM gauge theory through
the Wilson loop correlator formalism and Euclidean to Minkowskian analytic
continuation. The purely elastic behavior obtained at large impact-parameter L,
through duality from disconnected AdS_5 minimal surfaces beyond the
Gross-Ooguri transition point, is combined with unitarity and analyticity
constraints in the central region. In this way we obtain an absolute bound on
the high-energy behavior of the forward scattering amplitude due to the
graviton interaction between minimal surfaces in the bulk. The dominant
"Pomeron" intercept is bounded by alpha less than or equal to 11/7 using the
AdS/CFT constraint of a weak gravitational field in the bulk. Assuming the
elastic eikonal approximation in a larger impact-parameter range gives alpha
between 4/3 and 11/7. The actual intercept becomes 4/3 if one assumes the
elastic eikonal approximation within its maximally allowed range L larger than
exp{Y/3}, where Y is the total rapidity. Subleading AdS/CFT contributions at
large impact-parameter due to the other d=10 supergravity fields are obtained.
A divergence in the real part of the tachyonic KK scalar is cured by
analyticity but signals the need for a theoretical completion of the AdS/CFT
scheme.Comment: 25 pages, 3 eps figure
Renormalon Model Predictions for Power-Corrections to Flavour Singlet Deep Inelastic Structure Functions
We analyze power corrections to flavour singlet deep inelastic scattering
structure functions in the framework of the infrared renormalon model. Our
calculations, together with previous results for the non-singlet contribution,
allow to model the x-dependence of higher twist corrections to F_2, F_L and g_1
in the whole x domain.Comment: LaTeX, 25 pages, 8 eps figures included, one figure was added. Final
version for publication in Nucl.Phys.
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