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Search Results: 1 - 10 of 88845 matches for " Aaron W. Andersen "
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Flexible tooling for vibration-assisted microforming
W. Presz,B. Andersen
Journal of Achievements in Materials and Manufacturing Engineering , 2007,
Abstract: Purpose: Miniaturisation generates necessity of micro-parts production. Micro-scale means closer tolerances and better surface roughness. These requirements can be achieved with metal forming processes but under high pressure and sufficient relative sliding distance between tool and workpiece surface. It makes such a process proven to galling. This tendency increases with diminishing of component dimensions. It means that retarding undesirable surface phenomena with special regard to galling becomes a critical factor for microforming. Literature search and previous investigations shows that implementation of vibrations might be a solution for limitation of galling tendency in microforming.Design/methodology/approach: The group of so called “reference micro-components” has been chosen as an representation of micro-products. For these parts with FEM basic processes parameters were found. The tooling system for vibration assisted microforming of referenced parts has been designed. Vibrations are performed with vibrators based on stacked ceramic multilayer technology assuring accurate frequency and amplitude control. Findings: The method based on static and dynamic analytical and FEM calculations of proper design of vibration assisted flexible tooling with piezo-vibrators has been found.Research limitations/implications: Proposed reference micro-components and designed system can be used for investigations of technological parameters for utilisations of microforming.Practical implications: Flexible laboratory system is designed to manufacture a wide range of micro-components using tools vibrations for improving quality of products. After laboratory investigations it is attended to design industrial system working on same principles.Originality/value: Designed within this project flexible tooling for low frequency vibration assisted microforming seems to be original according to literature investigations.
Substrate effects in the magneto-optical second-harmonic generation from first principles: Fe/Cu(001)
Torsten Andersen,W. Huebner
Physics , 2002, DOI: 10.1103/PhysRevB.65.174409
Abstract: We compute the nonlinear optical response of an Fe monolayer placed on top of 1 to 4 monolayers of Cu(001). Our calculation is based on ab initio eigenstates of the slab, which are obtained within the full-potential linearized augmented plane-wave method. The ground-state spin-polarized electronic structure is converged self-consistently to an accuracy better than 0.1 mRy. Subsequently, we take the spin-orbit interaction into account within a second variational treatment. The new set of eigenstates allows us to calculate the magneto-optical transition matrix elements. The second-harmonic response is determined in the reflection geometry with magnetization perpendicular to the surface (the so-called polar configuration) using the surface-sheet model. Adding layers of a noble metal (Cu) to the Fe monolayer gives a new degree of freedom for the inclusion of nonmagnetic Cu d bands to the nonlinear magneto-optical response of the slab, and the energy bands show that such an addition converges essentially to an addition of d states and a small broadening of the d band with growing number of Cu layers. The screened nonlinear optical susceptibility is calculated and converges quite well with a growing number of Cu layers. Our first-principles results confirm that the magnetic tensor elements of the nonlinear optical response tensor are roughly of the same order of magnitude as the nonmagnetic ones (in contrast to linear optics, where the magnetic response is only a minor correction).
Review of Signaling Pathways Governing MSC Osteogenic and Adipogenic Differentiation
Aaron W. James
Scientifica , 2013, DOI: 10.1155/2013/684736
Abstract: Mesenchymal stem cells (MSC) are multipotent cells, functioning as precursors to a variety of cell types including adipocytes, osteoblasts, and chondrocytes. Between osteogenic and adipogenic lineage commitment and differentiation, a theoretical inverse relationship exists, such that differentiation towards an osteoblast phenotype occurs at the expense of an adipocytic phenotype. This balance is regulated by numerous, intersecting signaling pathways that converge on the regulation of two main transcription factors: peroxisome proliferator-activated receptor-γ (PPARγ) and Runt-related transcription factor 2 (Runx2). These two transcription factors, PPARγ and Runx2, are generally regarded as the master regulators of adipogenesis and osteogenesis. This review will summarize signaling pathways that govern MSC fate towards osteogenic or adipocytic differentiation. A number of signaling pathways follow the inverse balance between osteogenic and adipogenic differentiation and are generally proosteogenic/antiadipogenic stimuli. These include β-catenin dependent Wnt signaling, Hedgehog signaling, and NELL-1 signaling. However, other signaling pathways exhibit more context-dependent effects on adipogenic and osteogenic differentiation. These include bone morphogenic protein (BMP) signaling and insulin growth factor (IGF) signaling, which display both proosteogenic and proadipogenic effects. In summary, understanding those factors that govern osteogenic versus adipogenic MSC differentiation has significant implications in diverse areas of human health, from obesity to osteoporosis to regenerative medicine. 1. Introduction Mesenchymal stem cells (MSC) are multipotent stromal cells capable of self-renewal and capable of multilineage mesenchymal differentiation [1]. These nonhematopoietic cells can differentiate down multiple mesenchymal lineages, including osteogenic, chondrogenic, adipogenic, myogenic, and neurogenic lineages [2] (Figure 1). Originally identified in the bone marrow, MSC are readily obtained from numerous mesenchymal tissue types, including skeletal muscle and adipose depots. In particular, adipose tissue is an attractive source for MSC isolation, as it is readily accessible with minimal morbidity by routine liposuction procedures [3–5]. Indeed, human adipose-derived stromal cells (or hASC) have been demonstrated to have significant potential for use in tissue engineering applications, as shown in preclinical animal models [6]. However, the uncultured stromal vascular fraction of adipose tissue represents a heterogeneous cell population that is not
Constraints On Dynamics Preserving Certain Hyperbolic Sets
Aaron W. Brown
Mathematics , 2009, DOI: 10.1017/S014338571000009X
Abstract: We establish two results under which the topology of a hyperbolic set constrains ambient dynamics. First if a set is a compact, transitive, expanding hyperbolic attractor of codimension 1 for some diffeomorphism, then it is a union of transitive, expanding attractors (or contracting repellers) of codimension 1 for any diffeomorphism such that it is hyperbolic. Secondly, if a set is a nonwandering, locally maximal, compact hyperbolic set for a surface diffeomorphism, then it is locally maximal for any diffeomorphism for which it is hyperbolic.
Nonexpanding Attractors: Conjugacy to Algebraic Models and Classification in 3-Manifolds
Aaron W. Brown
Mathematics , 2010,
Abstract: We prove a result motivated by Williams's classification of expanding attractors and the Franks-Newhouse Theorem on codimension-1 Anosov diffeomorphisms: If a mixing hyperbolic attractor has 1-dimensional unstable manifolds then it is either is expanding or is homeomorphic to a compact abelian group (a toral solenoid); in the latter case the dynamics is conjugate to a group automorphism. As a corollary we obtain a classification of all 2-dimensional basic sets in 3-manifolds. Furthermore we classify all hyperbolic attractors in 3-manifolds in terms of the classically studied examples, answering a question of Bonatti.
Rigidity of measures on the torus: smooth stabilizers and entropy
Aaron W. Brown
Mathematics , 2010,
Abstract: For a nonlinear Anosov diffeomorphism of the 2-torus, we present examples of measures so that the group of $\mu$-preserving diffeomorphisms is, up to zero-entropy transformations, cyclic. For families of equilibrium states $\mu$, we strengthen this to show that the group of $\mu$-preserving diffeomorphism is virtually cyclic.
Discovery of protein-protein interactions using a combination of linguistic, statistical and graphical information
James W Cooper, Aaron Kershenbaum
BMC Bioinformatics , 2005, DOI: 10.1186/1471-2105-6-143
Abstract: This paper reports a scalable method for the discovery of protein-protein interactions in Medline abstracts, using a combination of text analytics, statistical and graphical analysis, and a set of easily implemented rules. Applying these techniques to 12,300 abstracts, a precision of 0.61 and a recall of 0.97 were obtained, (f = 0.74) and when allowing for two-hop and three-hop relations discovered by graphical analysis, the precision was 0.74 (f = 0.83).This combination of linguistic and statistical approaches appears to provide the highest precision and recall thus far reported in detecting protein-protein relations using text analytic approaches.Scientists in molecular biology find that a significant technique for studying protein function is through the study of protein-protein interactions. While the actual experimental study of such interactions remains the most important manner of obtaining these data, the number of protein-protein interactions reported in the literature is substantial and growing rapidly. There are a number of tabulations of these interactions, such as that provided by the Munich Institute for Protein Sequence (MIPS); these tabulations are of necessity incomplete.To address this problem, we have been developing a group of biology-specific computational annotators that work in conjunction with our group's text analytic software, for the discovery of protein-protein relations in text.In this paper, we undertook a study that utilizes a combination of computational linguistics, statistics and domain-specific rules to detect protein-protein interactions in a set of Medline abstracts.The system we describe here is particularly appealing because it can be used both to find known interactions and to find interactions not yet tabulated. According to the National Library of Medicine, Medline contains over 11 million abstracts, with about 40,000 being added each month. Thus, having a scalable, robust system for protein interaction discovery provides a
The Metabolic and Ecological Interactions of Oxalate-Degrading Bacteria in the Mammalian Gut
Aaron W. Miller,Denise Dearing
Pathogens , 2013, DOI: 10.3390/pathogens2040636
Abstract: Oxalate-degrading bacteria comprise a functional group of microorganisms, commonly found in the gastrointestinal tract of mammals. Oxalate is a plant secondary compound (PSC) widely produced by all major taxa of plants and as a terminal metabolite by the mammalian liver. As a toxin, oxalate can have a significant impact on the health of mammals, including humans. Mammals do not have the enzymes required to metabolize oxalate and rely on their gut microbiota for this function. Thus, significant metabolic interactions between the mammalian host and a complex gut microbiota maintain the balance of oxalate in the body. Over a dozen species of gut bacteria are now known to degrade oxalate. This review focuses on the host-microbe and microbe-microbe interactions that regulate the degradation of oxalate by the gut microbiota. We discuss the pathways of oxalate throughout the body and the mammalian gut as a series of differentiated ecosystems that facilitate oxalate degradation. We also explore the mechanisms and functions of microbial oxalate degradation along with the implications for the ecological and evolutionary interactions within the microbiota and for mammalian hosts. Throughout, we consider questions that remain, as well as recent technological advances that can be employed to answer them.
A generalized aggregation-disintegration model for the frequency of severe terrorist attacks
Aaron Clauset,Frederik W. Wiegel
Physics , 2009, DOI: 10.1177/0022002709352452
Abstract: We present and analyze a model of the frequency of severe terrorist attacks, which generalizes the recently proposed model of Johnson et al. This model, which is based on the notion of self-organized criticality and which describes how terrorist cells might aggregate and disintegrate over time, predicts that the distribution of attack severities should follow a power-law form with an exponent of alpha=5/2. This prediction is in good agreement with current empirical estimates for terrorist attacks worldwide, which give alpha=2.4 \pm 0.2, and which we show is independent of certain details of the model. We close by discussing the utility of this model for understanding terrorism and the behavior of terrorist organizations, and mention several productive ways it could be extended mathematically or tested empirically.
A Lower Bound for the Number of Group Actions on a Compact Riemann Surface
James W. Anderson,Aaron Wootton
Mathematics , 2011, DOI: 10.2140/agt.2012.12.19
Abstract: We prove that the number of distinct group actions on compact Riemann surfaces of a fixed genus $\sigma \geq 2$ is at least quadratic in $\sigma$. We do this through the introduction of a coarse signature space, the space $\mathcal{K}_\sigma$ of {\em skeletal signatures} of group actions on compact Riemann surfaces of genus $\sigma$. We discuss the basic properties of $\mathcal{K}_\sigma$ and present a full conjectural description.
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