612 research outputs found
On Topological Minors in Random Simplicial Complexes
For random graphs, the containment problem considers the probability that a
binomial random graph contains a given graph as a substructure. When
asking for the graph as a topological minor, i.e., for a copy of a subdivision
of the given graph, it is well-known that the (sharp) threshold is at .
We consider a natural analogue of this question for higher-dimensional random
complexes , first studied by Cohen, Costa, Farber and Kappeler for
.
Improving previous results, we show that is the
(coarse) threshold for containing a subdivision of any fixed complete
-complex. For higher dimensions , we get that is an
upper bound for the threshold probability of containing a subdivision of a
fixed -dimensional complex.Comment: 15 page
Higher Dimensional Discrete Cheeger Inequalities
For graphs there exists a strong connection between spectral and
combinatorial expansion properties. This is expressed, e.g., by the discrete
Cheeger inequality, the lower bound of which states that , where is the second smallest eigenvalue of the Laplacian of
a graph and is the Cheeger constant measuring the edge expansion of
. We are interested in generalizations of expansion properties to finite
simplicial complexes of higher dimension (or uniform hypergraphs).
Whereas higher dimensional Laplacians were introduced already in 1945 by
Eckmann, the generalization of edge expansion to simplicial complexes is not
straightforward. Recently, a topologically motivated notion analogous to edge
expansion that is based on -cohomology was introduced by Gromov
and independently by Linial, Meshulam and Wallach. It is known that for this
generalization there is no higher dimensional analogue of the lower bound of
the Cheeger inequality. A different, combinatorially motivated generalization
of the Cheeger constant, denoted by , was studied by Parzanchevski,
Rosenthal and Tessler. They showed that indeed , where
is the smallest non-trivial eigenvalue of the (-dimensional
upper) Laplacian, for the case of -dimensional simplicial complexes with
complete -skeleton.
Whether this inequality also holds for -dimensional complexes with
non-complete -skeleton has been an open question. We give two proofs of
the inequality for arbitrary complexes. The proofs differ strongly in the
methods and structures employed, and each allows for a different kind of
additional strengthening of the original result.Comment: 14 pages, 2 figure
Not All Saturated 3-Forests Are Tight
A basic statement in graph theory is that every inclusion-maximal forest is
connected, i.e. a tree. Using a definiton for higher dimensional forests by
Graham and Lovasz and the connectivity-related notion of tightness for
hypergraphs introduced by Arocha, Bracho and Neumann-Lara in, we provide an
example of a saturated, i.e. inclusion-maximal 3-forest that is not tight. This
resolves an open problem posed by Strausz
Improving Cardiovascular Stent Design Using Patient-Specific Models and Shape Optimization
Stent geometry influences local hemodynamic alterations (i.e. the forces moving blood through the cardiovascular system) associated with adverse clinical outcomes. Computational fluid dynamics (CFD) is frequently used to quantify stent-induced hemodynamic disturbances, but previous CFD studies have relied on simplified device or vascular representations. Additionally, efforts to minimize stent-induced hemodynamic disturbances using CFD models often only compare a small number of possible stent geometries. This thesis describes methods for modeling commercial stents in patient-specific vessels along with computational techniques for determining optimal stent geometries that address the limitations of previous studies.
An efficient and robust method was developed for virtually implanting stent models into patient-specific vascular geometries derived from medical imaging data. Models of commercial stent designs were parameterized to allow easy control over design features. Stent models were then virtually implanted into vessel geometries using a series of Boolean operations. This approach allowed stented vessel models to be automatically regenerated for rapid analysis of the contribution of design features to resulting hemodynamic alterations. The applicability of the method was demonstrated with patient-specific models of a stented coronary artery bifurcation and basilar trunk aneurysm to reveal how it can be used to investigate differences in hemodynamic performance in complex vascular beds for a variety of clinical scenarios.
To identify hemodynamically optimal stents designs, a computational framework was constructed to couple CFD with a derivative-free optimization algorithm. The optimization algorithm was fully-automated such that solid model construction, mesh generation, CFD simulation and time-averaged wall shear stress (TAWSS) quantification did not require user intervention. The method was applied to determine the optimal number of circumferentially repeating stent cells (NC) for a slotted-tube stents and various commercial stents. Optimal stent designs were defined as those minimizing the area of low TAWSS. It was determined the optimal value of NC is dependent on the intrastrut angle with respect to the primary flow direction. Additionally, the geometries of current commercial stents were found to generally incorporate a greater NC than is hemodynamically optimal.
The application of the virtual stent implantation and optimization methods may lead to stents with superior hemodynamic performance and the potential for improved clinical outcomes. Future in vivo studies are needed to validate the findings of the computational results obtained from the methods developed in this thesis
Leiharbeit und befristete BeschÀftigung: Soziale Teilhabe ist eine Frage von stabilen Jobs
Die Integration in den Arbeitsmarkt gilt als zentral fĂŒr soziale Teilhabe und gesellschaftliche Integration. Dass die Zahl der Arbeitslosen Ende 2010 auf unter drei Millionen gesunken ist, wĂ€re demnach ein gutes Zeichen fĂŒr eine Verbesserung des Zusammenhalts in der Gesellschaft. Allerdings wird Personal immer hĂ€ufiger ĂŒber befristete ArbeitsvertrĂ€ge eingestellt oder ĂŒber Zeitarbeitsfirmen ausgeliehen. Hier wird untersucht, wie sich temporĂ€re BeschĂ€ftigung auf die subjektive Wahrnehmung der Betroffenen auswirkt: FĂŒhlen sie sich sozial integriert oder von der Gesellschaft ausgeschlossen
Changes in Income Poverty Risks at the Transition from Unemployment to Employment : Comparing the Short-Term and Medium-Term Effects of Fixed-Term and Permanent Jobs
Unemployment is a major risk factor of poverty and employment is regarded key to overcoming it. The present study examines how the income poverty risk of unemployed individuals changes in the short and medium term, when they take up work, and whether the effects differ according to the type of employment. The focus is on permanent and fixed-term job contracts, as the political promotion of fixed-term employment has often been framed as an effort to reduce long-term unemployment and poverty. Drawing on longitudinal data from the German panel study âLabour Market and Social Securityâ (PASS) 2010â18, we apply a first difference estimator with asymmetric effects to examine the effect of starting a job out of unemployment on income poverty risks in the subsequent four years. Strikingly, starting in a fixed-term and permanent contract have similarly strong and lasting poverty-reducing effects in the short and medium term. Thus, with regard to risks of income poverty, starting a permanent job does not appear more beneficial than starting a fixed-term job for unemployed persons. We discuss the reasons for this finding and also explore how the poverty-reducing effects of transitions from unemployment to fixed-term versus permanent employment vary by household type, occupation, working time and firm size
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