Thesis (doctoral)--Rheinische Friedrich-Wilhelms-Universität Bonn, 1995. / Includes bibliographical references (p. 112-119).

In this paper we consider an existence of a solution for a nonlinear nonmonotone wave equation in [0,π]xR and an existence of a positive solution for a non-positone Dirichlet problem in a bounded subset of R^n.

In this paper we consider the question of uniqueness of positive solutions for Dirichlet problems of the form - Δ u(x)= g(λ,u(x)) in B, u(x) = 0 on ϑB, where A is the Laplace operator, B is the unit ball in RˆN, and A>0. We show that if g(λ,u)=uˆ(N+2)/(N-2) + λ, that is g has "critical growth", then large positive solutions are unique. We also prove uniqueness of large solutions when g(λ,u)=A f(u) with f(0) < 0, f "superlinear" and monotone. We use a number of methods from nonlinear functional analysis such as variational identities, Sturm comparison theorems and methods of order. We also present a regularity result on linear elliptic equation where a coefficient has critical growth.

Orientadores: Francisco Julio Sobreira de Araujo Correa, Djairo Guedes de Figueiredo / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Matematica, Estatistica e Computação Cientifica / Made available in DSpace on 2018-08-09T23:52:44Z (GMT). No. of bitstreams: 1 Nascimento_RubiaGoncalves_D.pdf: 676237 bytes, checksum: b58f8b79d9e983ceccee3865787cfec0 (MD5) Previous issue date: 2008 / Resumo: Neste trabalho usaremos algumas técnicas da Análise Funcional não-linear para estudar a existência de soluções para a seguinte classe de problemas ... Observação: O resumo na íntegra, poderá ser visualizado no texto completo na tese digital / Abstract: In this work we will use some techniques of the Analysis Functional no linear to study the existence of solutions for the following class of problems ... Observation: The complete abstract in available with the full electronic digital thesis or dissertations / Doutorado / Matematica / Doutor em Matemática

<p>Using the finite element software FEMLAB® solutions are computed to Dirichlet problems for the Infinity-Laplace equation ∆∞(<i>u</i>) ≡ <i>u</i>²_x<i>u</i>_xx+ 2<i>u</i>_x<i>u</i>_y<i>u</i>_xy+<i>u</i>²_y<i>u</i>_yy= 0. For numerical reasons ∆<i>q</i>(<i>u</i>) = div (|▼<i>u</i>|<i>q</i>▼<i>u</i>)<i> = </i>0, which (formally) approaches as ∆∞(<i>u</i>) = 0 as <i>q</i> → ∞, is used in computation. A parametric nonlinear solver is used, which employs a variant of the damped Newton-Gauss method. The analysis of the experiments is done using the known theory of solutions to Dirichlet problems for ∆∞(<i>u</i>) = 0, which includes AMLEs (Absolutely Minimizing Lipschitz Extensions), sets of uniqueness, critical segments and lines of singularity. From the experiments one main conjecture is formulated: For Dirichlet problems, which have a non-constant boundary function, it is possible to predict the structure of the lines of singularity in solutions in the Infinity-Laplace case by examining the corresponding Laplace case.</p>

Using the finite element software FEMLAB® solutions are computed to Dirichlet problems for the Infinity-Laplace equation ∆∞(u) ≡ u2xuxx + 2uxuyuxy + u2yuyy = 0. For numerical reasons ∆q(u) = div (|▼u|q▼u) = 0, which (formally) approaches as ∆∞(u) = 0 as q → ∞, is used in computation. A parametric nonlinear solver is used, which employs a variant of the damped Newton-Gauss method. The analysis of the experiments is done using the known theory of solutions to Dirichlet problems for ∆∞(u) = 0, which includes AMLEs (Absolutely Minimizing Lipschitz Extensions), sets of uniqueness, critical segments and lines of singularity. From the experiments one main conjecture is formulated: For Dirichlet problems, which have a non-constant boundary function, it is possible to predict the structure of the lines of singularity in solutions in the Infinity-Laplace case by examining the corresponding Laplace case.

L'équation de Weinstein est une équation régissant les Potentiels à Symétrie Axiale (PSA) qui est $L_m[u]=Delta u+(m/x)partial_x u=0$, $minmathbb{C}$. On généralise des résultats connus pour $min mathbb{R}$ au cas $minmathbb{C}$. On donne des expressions de solutions fondamentales des opérateurs $L_m[u]$ et leurs estimations, on démontre une formule de Green pour les PSA dans le demi-plan droit $mathbb{H}^+$ pour Re $m< 1$. On prouve un nouveau théorème de décomposition des PSA dans des anneaux quelconques pour $minmathbb{C}$ et dans une géométrie annulaire particulière utilisant les coordonnées bipolaires, on prouve qu'une famille de solutions des PSA en termes de fonctions de Legendre Associées de 1re et 2de espèce est complète, on montre lorsque $min mathbb{R}$ que celle-ci est une base de Riesz.Dans la 2e partie, par une méthode qui est due à A. S. Fokas, on donne des formules des PSA dans un disque de $mathbb{H}^+$, avec $minmathbb{Z}$. Ces représentations sont obtenues par la résolution d'un problème de Riemann-Hilbert sur $mathbb{C}$ ou sur une surface de Riemann à deux feuillets.Dans la 3e partie, on étudie les fonctions pseudo-holomorphes, {it i. e.} les solutions de l'équation $overline{partial} w=alphaoverline{w}$, $alphain L^r$, $2leq r<infty$. Une nouvelle extension de la régularité du principe de similarité et une réciproque de celui-ci qui conduit à un paramétrage analytique de ces fonctions dans le cas critique $r=2$ ont été obtenues. On résoud un problème de Dirichlet à données $L^p$ pondérées sur des domaines lisses pour des équations du type conductivité à coefficient dont le log appartient à l'espace de Sobolev $W^{1,2}$. / The Weinstein equation with complex coefficients is the equation governing axisymmetric potentials (PSA) which can be written as $L_m[u]=Delta u+left(m/xright)partial_x u =0$, where $minmathbb{C}$. We generalize results known for $minmathbb{R}$ to $minmathbb{C}$. We give explicit expressions of fundamental solutions for Weinstein operators and their estimates near singularities, then we prove a Green's formula for PSA in the right half-plane $mathbb{H}^+$ for Re $m<1$. We establish a new decomposition theorem for the PSA in any annular domains for $minmathbb{C}$. In particular, using bipolar coordinates, we prove for annuli that a family of solutions for PSA equation in terms of associated Legendre functions of first and second kind is complete. For $minmathbb{R}$, we show that this family is even a Riesz basis in some non-concentric circular annulus. In the second part, basing on a method due to A. S. Fokas, we give formulas for PSA in a circular domain of $mathbb{H}^+$ when $m$ is an integer. These representations are obtained by solving a Riemann-Hilbert problem on the complex plane or on a Riemann surface with two sheets according to the parity of $m$.In the last part, we study the pseudo-holomorphic functions, i.e. solutions of the complex equation $overline{partial} w=alpha overline{w}$, with $alphain L^r$, $2leq r<infty$. We extend the Bers similarity principle and a converse of this principle to the critical regularity case $r=2$. We establish well-posedness of Dirichlet problem in smooth domains with weighted $L^p$ boundary data for 2-D isotropic conductivity equations whose coefficients have logarithm in the Sobolev space $W^{1,2}$.

In this thesis, we propose many developments in the context of Structural Similarity. We address both node (local) similarity and graph (global) similarity. Concerning node similarity, we focus on improving the diffusive process leading to compute this similarity (e.g. Commute Times) by means of modifying or rewiring the structure of the graph (Graph Densification), although some advances in Laplacian-based ranking are also included in this document. Graph Densification is a particular case of what we call graph rewiring, i.e. a novel field (similar to image processing) where input graphs are rewired to be better conditioned for the subsequent pattern recognition tasks (e.g. clustering). In the thesis, we contribute with an scalable an effective method driven by Dirichlet processes. We propose both a completely unsupervised and a semi-supervised approach for Dirichlet densification. We also contribute with new random walkers (Return Random Walks) that are useful structural filters as well as asymmetry detectors in directed brain networks used to make early predictions of Alzheimer's disease (AD). Graph similarity is addressed by means of designing structural information channels as a means of measuring the Mutual Information between graphs. To this end, we first embed the graphs by means of Commute Times. Commute times embeddings have good properties for Delaunay triangulations (the typical representation for Graph Matching in computer vision). This means that these embeddings can act as encoders in the channel as well as decoders (since they are invertible). Consequently, structural noise can be modelled by the deformation introduced in one of the manifolds to fit the other one. This methodology leads to a very high discriminative similarity measure, since the Mutual Information is measured on the manifolds (vectorial domain) through copulas and bypass entropy estimators. This is consistent with the methodology of decoupling the measurement of graph similarity in two steps: a) linearizing the Quadratic Assignment Problem (QAP) by means of the embedding trick, and b) measuring similarity in vector spaces. The QAP problem is also investigated in this thesis. More precisely, we analyze the behaviour of $m$-best Graph Matching methods. These methods usually start by a couple of best solutions and then expand locally the search space by excluding previous clamped variables. The next variable to clamp is usually selected randomly, but we show that this reduces the performance when structural noise arises (outliers). Alternatively, we propose several heuristics for spanning the search space and evaluate all of them, showing that they are usually better than random selection. These heuristics are particularly interesting because they exploit the structure of the affinity matrix. Efficiency is improved as well. Concerning the application domains explored in this thesis we focus on object recognition (graph similarity), clustering (rewiring), compression/decompression of graphs (links with Extremal Graph Theory), 3D shape simplification (sparsification) and early prediction of AD. / Ministerio de Economía, Industria y Competitividad (Referencia TIN2012-32839 BES-2013-064482)