7,651 research outputs found
Extinction in lower Hessenberg branching processes with countably many types
We consider a class of branching processes with countably many types which we
refer to as Lower Hessenberg branching processes. These are multitype
Galton-Watson processes with typeset , in which
individuals of type may give birth to offspring of type only.
For this class of processes, we study the set of fixed points of the
progeny generating function. In particular, we highlight the existence of a
continuum of fixed points whose minimum is the global extinction probability
vector and whose maximum is the partial extinction probability
vector . In the case where
, we derive a global extinction
criterion which holds under second moment conditions, and when
we develop necessary and sufficient
conditions for
Symmetry & Controllability for Spin Networks with a Single-Node Control
We consider the relation of symmetries and subspace controllability for spin
networks with XXZ coupling subject to control of a single node by a local
potential (Z-control). Such networks decompose into excitation subspaces.
Focusing on the single excitation subspace it is shown that for single-node
Z-controls external symmetries are characterized by eigenstates of the system
Hamiltonian that have zero overlap with the control node, and there are no
internal symmetries. It is further shown that there are symmetries that persist
even in the presence of random perturbations. For uniformly coupled XXZ chains
a characterization of all possible symmetries is given, which shows a strong
dependence on the position of the node we control. Finally, it is shown
rigorously for uniform Heisenberg and XX chains subject to single-node
Z-control that the lack of symmetry is not only necessary but sufficient for
subspace controllability. The latter approach is then generalized to establish
controllability results for simple branched networks.Comment: 11 pages, some figures. 3 tables, minor revisio
Evidence for Partial Taylor Relaxation from Changes in Magnetic Geometry and Energy during a Solar Flare
Solar flares are powered by energy stored in the coronal magnetic field, a
portion of which is released when the field reconfigures into a lower energy
state. Investigation of sunspot magnetic field topology during flare activity
is useful to improve our understanding of flaring processes. Here we
investigate the deviation of the non-linear field configuration from that of
the linear and potential configurations, and study the free energy available
leading up to and after a flare. The evolution of the magnetic field in NOAA
region 10953 was examined using data from Hinode/SOT-SP, over a period of 12
hours leading up to and after a GOES B1.0 flare. Previous work on this region
found pre- and post-flare changes in photospheric vector magnetic field
parameters of flux elements outside the primary sunspot. 3D geometry was thus
investigated using potential, linear force-free, and non-linear force-free
field extrapolations in order to fully understand the evolution of the field
lines. Traced field line geometrical and footpoint orientation differences show
that the field does not completely relax to a fully potential or linear
force-free state after the flare. Magnetic and free magnetic energies increase
significantly ~ 6.5-2.5 hours before the flare by ~ 10^31 erg. After the flare,
the non-linear force-free magnetic energy and free magnetic energies decrease
but do not return to pre-flare 'quiet' values. The post-flare non-linear
force-free field configuration is closer (but not equal) to that of the linear
force-free field configuration than a potential one. However, the small degree
of similarity suggests that partial Taylor relaxation has occurred over a time
scale of ~ 3-4 hours.Comment: Accepted for Publication in Astronomy & Astrophysics. 11 pages, 11
figure
Resilience in middle-aged partners of patients diagnosed with incurable cancer : a thematic analysis
Background : Providing care for patients with advanced cancer is often the responsibility of the partner. Being confronted with an incurable cancer diagnosis can be highly disruptive for the patient's partner and can be considered a potentially traumatic event. However, most caregivers seem to adapt well during the process of providing care. This finding is in line with the concept of resilience in literature: a dynamic process of adapting well, resulting from the interplay between intrinsic and extrinsic resources and risks. Resilience is age-related, with the elderly population being higher in resilience as compared to the younger generation. However, resilience has been understudied in middle-aged caregivers.
Aim : To explore what intrinsic and extrinsic resources facilitate or hamper resilience in the middle-aged partner of a patient with incurable cancer.
Methods : Nine middle-aged partners of patients who died at home of cancer were selected and interviewed in depth within the first year following the death of their partner. A thematic analysis utilizing an inductive approach was conducted.
Findings : Resilience was challenged by the partner's diagnosis of incurable cancer. All participants made use of a set of interacting, caregiver-specific and context-related resources, facilitating a resilient process and leading to positive feelings and even personal growth. The partners demonstrated individual competences: adaptive flexibility, positivism, a sense of self-initiative and adaptive dependency. Furthermore, they relied on their context: cancer-related professionals and relatives. Context and situation interact continuously. The resulting dynamics were based on the context-availability, meaningful relationships and the patient's role.
Conclusion : A resilient trajectory results from an interplay between individual and contextual resources. To build resilience in middle-aged partners of patients with incurable cancer, health care professionals should address all available resources. Moreover, they should be aware of being part of the caregiver's context, a complex adaptive system that can be either resilience-supporting or -threatening
Lifetimes of atoms trapped in an optical lattice in proximity of a surface
We study the lifetime of an atom trapped in an optical vertical lattice in
proximity of a massive surface using a complex scaling approach. We analyze how
the presence of the surface modifies the known lifetimes of Wannier-Stark
states associated to Landau-Zener tunnelling. We also investigate how the
existence of a hypothetical short-distance deviation from Newton's
gravitational law could affect these lifetimes. Our study is relevant in order
to discuss the feasibility of any atomic- interferometry experiment performed
near a surface. Finally, the difficulties encountered in applying the
complex-scaling approach to the atom-surface Casimir-Polder interaction are
addressed.Comment: 10 pages, 8 figure
Automated Coronal Hole Identification via Multi-Thermal Intensity Segmentation
Coronal holes (CH) are regions of open magnetic fields that appear as dark
areas in the solar corona due to their low density and temperature compared to
the surrounding quiet corona. To date, accurate identification and segmentation
of CHs has been a difficult task due to their comparable intensity to local
quiet Sun regions. Current segmentation methods typically rely on the use of
single EUV passband and magnetogram images to extract CH information. Here, the
Coronal Hole Identification via Multi-thermal Emission Recognition Algorithm
(CHIMERA) is described, which analyses multi-thermal images from the
Atmospheric Image Assembly (AIA) onboard the Solar Dynamics Observatory (SDO)
to segment coronal hole boundaries by their intensity ratio across three
passbands (171 \AA, 193 \AA, and 211 \AA). The algorithm allows accurate
extraction of CH boundaries and many of their properties, such as area,
position, latitudinal and longitudinal width, and magnetic polarity of
segmented CHs. From these properties, a clear linear relationship was
identified between the duration of geomagnetic storms and coronal hole areas.
CHIMERA can therefore form the basis of more accurate forecasting of the start
and duration of geomagnetic storms
Towards a national web in a federated country : a Belgian case study
Although the .be domain was introduced in June 1988, the Belgian web is currently not
systematically archived. As of August 2016, 1.550.147 domains are registered by DNS Belgium.
Without a Belgian web archive, the content of these websites will not be preserved for future
generations and a significant portion of Belgian history will be lost forever. In this paper we present the initial findings of a research project exploring the policy, legal, technical and scientific issues around archiving the Belgian web. The aim of this project is to a) identify current best practices in web-archiving b) pilot a Belgian web archive and c) identify research use cases for the scientific study of the Belgian web. This case study is seen as a first step towards implementing a long-term web archiving strategy for Belgium
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