Adaptive quadrature by expansion for layer potential evaluation in two dimensions

When solving partial differential equations using boundary integral equation methods, accurate evaluation of singular and nearly singular integrals in layer potentials is crucial. A recent scheme for this is quadrature by expansion (QBX), which solves the problem by locally approximating the potential using a local expansion centered at some distance from the source boundary. In this paper we introduce an extension of the QBX scheme in 2D denoted AQBX - adaptive quadrature by expansion - which combines QBX with an algorithm for automated selection of parameters, based on a target error tolerance. A key component in this algorithm is the ability to accurately estimate the numerical errors in the coefficients of the expansion. Combining previous results for flat panels with a procedure for taking the panel shape into account, we derive such error estimates for arbitrarily shaped boundaries in 2D that are discretized using panel-based Gauss-Legendre quadrature. Applying our scheme to numerical solutions of Dirichlet problems for the Laplace and Helmholtz equations, and also for solving these equations, we find that the scheme is able to satisfy a given target tolerance to within an order of magnitude, making it useful for practical applications. This represents a significant simplification over the original QBX algorithm, in which choosing a good set of parameters can be hard

A fast integral equation method for solid particles in viscous flow using quadrature by expansion

Boundary integral methods are advantageous when simulating viscous flow around rigid particles, due to the reduction in number of unknowns and straightforward handling of the geometry. In this work we present a fast and accurate framework for simulating spheroids in periodic Stokes flow, which is based on the completed double layer boundary integral formulation. The framework implements a new method known as quadrature by expansion (QBX), which uses surrogate local expansions of the layer potential to evaluate it to very high accuracy both on and off the particle surfaces. This quadrature method is accelerated through a newly developed precomputation scheme. The long range interactions are computed using the spectral Ewald (SE) fast summation method, which after integration with QBX allows the resulting system to be solved in M log M time, where M is the number of particles. This framework is suitable for simulations of large particle systems, and can be used for studying e.g. porous media models

An exploratory study of ten latency age delinquent children and their school problems

Thesis (M.S.)--Boston Universit

Fast Ewald summation for free-space Stokes potentials

We present a spectrally accurate method for the rapid evaluation of free-space Stokes potentials, i.e. sums involving a large number of free space Green's functions. We consider sums involving stokeslets, stresslets and rotlets that appear in boundary integral methods and potential methods for solving Stokes equations. The method combines the framework of the Spectral Ewald method for periodic problems, with a very recent approach to solving the free-space harmonic and biharmonic equations using fast Fourier transforms (FFTs) on a uniform grid. Convolution with a truncated Gaussian function is used to place point sources on a grid. With precomputation of a scalar grid quantity that does not depend on these sources, the amount of oversampling of the grids with Gaussians can be kept at a factor of two, the minimum for aperiodic convolutions by FFTs. The resulting algorithm has a computational complexity of O(N log N) for problems with N sources and targets. Comparison is made with a fast multipole method (FMM) to show that the performance of the new method is competitive.Comment: 35 pages, 15 figure

PRÉSENCES DE PROUST EN SUÈDE - L’Accueil critique de « Att läsa Proust » d’Olof Lagercrantz et de « Låt Proust förändra ditt liv » d’Alain de Botton

Marcel Prousts stora romansvit ”På spaning efter den tid som flytt” är en roman många känner till, åtminstone till namnet, men som få har läst i sin helhet. Nu, hundra år efter att delarna började publiceras i Frankrike, är det intressant att se vad en svensk läsare möter i fall man börjar närma sig verket. Denna uppsats fokuserar på vad som beskrivs i mer populärvetenskapliga sammanhang samt på kultursidor och i uppslagsverk. Uppsatsförfattaren har funnit att informationen fördelar sig i tre nivåer och även vilka ämnen inom dessa som är de mest frekvent behandlade, när verket beskrivs och omtalas. Den första nivån, som är mest frekvent förekommande, är frågor som har med romanens längd och svårläshet att göra, nästa nivå handlar om författaren och hans liv och först i tredje nivån omtalas verkets särdrag och inflytande på modern litteratur

Resilience and vulnerability among refugee children of traumatized and non-traumatized parents

BACKGROUND: The aim of the study was to explore resilience among refugee children whose parents had been traumatized and were suffering from Post-Traumatic Stress Disorder (PTSD). METHODS: The study comprised 80 refugee children (40 boys and 40 girls, age range 6–17 yrs), divided into two groups. The test group consisted of 40 refugee children whose parents had been tortured in Iraq before coming to Sweden. In accordance with DSM-IV criteria, these children were further divided in two sub-groups, those who were assessed as having PTSD-related symptoms (n = 31) and those who did not have PTSD-related symptoms (n = 9). The comparison group consisted of 40 children from Egypt, Syria and Morocco whose parents had not been tortured. Wechsler Intelligence Scale for Children, 3(rd )edn. (WISC-III), Diagnostic Interview for Children and Adolescents- Revised (DICA-R), Post-Traumatic Stress Symptoms checklist (PTSS), "I Think I am" (ITIA) and Strengths and Difficulties Questionnaire (SDQ) were used to assess IQ; PTSD-related symptoms; self-esteem; possible resilience and vulnerability. RESULTS: Children without PTSD/PTSS in the traumatized parents group had more favorable values (ITIA and SDQ) with respect to total scores, emotionality, relation to family, peer relations and prosocial behavior than the children in the same group with PTSD/PTSS and these values were similar to those the children in the comparison group (the non-traumatized parents group). The children in the non-traumatized parents group scored significantly higher on the IQ test than the children with traumatized parents, both the children with PTSD-related symptoms and those without PTSD-related symptoms. CONCLUSION: Adequate emotional expression, supportive family relations, good peer relations, and prosociality constituted the main indicators of resilience. Further investigation is needed to explore the possible effects of these factors and the effects of IQ. The findings of this study are useful for treatment design in a holistic perspective, especially in planning the treatment for refugee children, adolescents and their families

Observability-aware online multi-lidar extrinsic calibration

Accurate and robust extrinsic calibration is necessary for deploying autonomous systems which need multiple sensors for perception. In this paper, we present a robust system for real-time extrinsic calibration of multiple lidars in vehicle base frame without the need for any fiducial markers or features. We base our approach on matching absolute GNSS and estimated lidar poses in real-time. Comparing rotation components allows us to improve the robustness of the solution than traditional least-square approach comparing translation components only. Additionally, instead of comparing all corresponding poses, we select poses comprising maximum mutual information based on our novel observability criteria. This allows us to identify a subset of the poses helpful for real-time calibration. We also provide stopping criteria for ensuring calibration completion. To validate our approach extensive tests were carried out on data collected using Scania test vehicles (7 sequences for a total of ~ 6.5 Km). The results presented in this paper show that our approach is able to accurately determine the extrinsic calibration for various combinations of sensor setups.Comment: For associated video file, see https://youtu.be/aMWvWozBdr

In vivo absorption spectroscopy of tumor sensitizers with femtosecond white light

A system based on a femtosecond white-light continuum and a streak camera was used for recordings of the in vivo absorption spectra of the tumor-seeking agent disulphonated aluminum phthalocyanine. Measurements for different drug doses were performed on tumor tissue (muscle-implanted adenocarcinoma) and normal muscle tissue in rats. It was found that the shape of the spectrum is tissue dependent. The peak of the absorption spectrum is blueshifted in tumor tissue as compared with the muscle. Thus the contrast in the drug-related absorption can be altered by up to a factor of 2 from the primary drug molecular-concentration contrast between normal muscle and tumor by the proper selection of the illumination wavelength