In this work we study the following problem on constrained approximation. Let f be a continuous mapping defined on a bounded domain with piecewise smooth boundary in R^n and taking values in R^n. What are necessary and sufficient conditions for f to be uniformly approximable by C^1-smooth mappings with nonnegative Jacobian? When the dimension is equal to one, this is just approximation by monotone smooth functions. Hence, the necessary and sufficient condition is: the function is monotone. On the other hand, for higher dimensions the description is not as clear. We give a simple necessary condition in terms of the topological degree of continuous mapping. We also give some sufficient conditions for dimension 2. It also turns out that if the dimension is greater than one, then there exist real-analytic mappings with nonnegative Jacobian that cannot be approximated by smooth mappings with positive Jacobian. In our study of the above mentioned question we use topological degree theory, Schoenflies-type extension theorems, and Stoilow's topological characterization of complex analytic functions.

In this work we study the following problem on constrained approximation. Let f be a continuous mapping defined on a bounded domain with piecewise smooth boundary in R^n and taking values in R^n. What are necessary and sufficient conditions for f to be uniformly approximable by C^1-smooth mappings with nonnegative Jacobian? When the dimension is equal to one, this is just approximation by monotone smooth functions. Hence, the necessary and sufficient condition is: the function is monotone. On the other hand, for higher dimensions the description is not as clear. We give a simple necessary condition in terms of the topological degree of continuous mapping. We also give some sufficient conditions for dimension 2. It also turns out that if the dimension is greater than one, then there exist real-analytic mappings with nonnegative Jacobian that cannot be approximated by smooth mappings with positive Jacobian. In our study of the above mentioned question we use topological degree theory, Schoenflies-type extension theorems, and Stoilow's topological characterization of complex analytic functions.

published_or_final_version / Mathematics / Master / Master of Philosophy

Using topological degree methods, we give some existence and multiplicity results for nonlinear differential or difference equations. In Chapter 1 some continuation theorems are presented. Chapter 2 deal with nonlinear difference equations. Using Brouwer degree we obtain upper and lower solutions theorems, Ambrosetti and Prodi type results and sharp existence conditions for nonlinearities which are bounded from below or from above. In Chapter 3, using Leray-Schauder degree, we give various existence and multiplicity result for second order differential equations with $phi$-Laplacian. Such equations are in particular motivated by the one-dimensional mean curvature problems and by the acceleration of a relativistic particle of mass one at rest moving on a straight line. In Chapter 4, using Mawhin continuation theorem, sufficient conditions are obtained for the existence of positive periodic solutions for delay Lotka-Volterra systems. In the last chapter of this work we prove some results concerning the multiplicity of solutions for a class of superlinear planar systems. The results of Chapters 2 and 3 are joint work with Prof. Jean Mawhin. / En utilisant le degré topologique, nous obtenons quelques résultats d'existance et de multiplicité pour des équations non-linéaires différentielles ou aux différences. Quelques théorèmes de continuation sont présentés au Chapitre 1. Le Chapitre 2 concerne des équations aux différences non-linéaires. En utilisant le degré de Brouwer, nous obtenons des résultats de sur et sous-solutions, des résultats de type Ambrosetti-Prodi ainsi que des conditions optimales d'existence pour des non-linéarités bornées inférieurement ou supérieurement. En utilisant le degré de Leray-Schauder, nous donnons au Chapitre 3 des résultats d'existence et de multiplicité pour des équations différentielles du second ordre avec $phi$-Laplacien. De telles équations sont en particulier motivées par le problème de la courbure en dimension un et par l'accélération d'une particule relativisite de masse un sur une droite. Au Chapitre 4, en utilisant le théorème de continuation de Mawhin, des conditions suffisantes sont obtenues pour l'existence de solutions périodiques positives des systhèmes de Lotka-Volterra avec retard. Dans le dernier chapitre de ce travail, nous prouvons certains résultats concernant la multiplicité des solutions pour une classe de systhèmes superlinéaires planaires. Les résultats des Chapitre 2 et 3 sont faits en collaboration avec monsieur le Professeur Jean Mawhin.

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This thesis is devoted to show various applications of fixed point theorems on dif- ferential equations. In the beginning we use a notion of topological degree to derive several fixed points theorems, primarily Brouwer, Schauder and Kakutani-Ky Fan the- orem. Then we apply them on a wide range of relatively simple problems from ordinary and partial differential equations (ode and pde). Finally, we take a look on a few more complex problems. First is an existence of a solution to the model of mechanical os- cillator with non-monotone dependence of both displacement and velocity. Second is a solution to so called Gause predator-prey model with a refuge. The last one is cer- tain partial differential equation with a constraint which determines maximal monotone graph. 1

Submitted by Marlene Santos (marlene.bc.ufg@gmail.com) on 2014-09-01T19:21:42Z No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Dassael Fabricio dos Reis Santos - Dissertação de Mestrado.pdf: 2389476 bytes, checksum: 8ca3d9cabd2862c5e82bc4db0cec4071 (MD5) / Made available in DSpace on 2014-09-01T19:21:42Z (GMT). No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Dassael Fabricio dos Reis Santos - Dissertação de Mestrado.pdf: 2389476 bytes, checksum: 8ca3d9cabd2862c5e82bc4db0cec4071 (MD5) Previous issue date: 2014-03-28 / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / In this work we study existence and multiplicity of non-negative solutions of the nonlinear elliptic problem −div(A(x,∇u)) = λf(x,u) in Ω, u = 0 in ∂Ω where Ω⊂IRN is a bounded domain with smooth boundary∂Ω,λ≥ 0 is a parameter, f :Ω×[0,∞)−→ IR and A :Ω×IRN−→ IRN satisfy the Carathéodory conditions, A is monotone and f satisfies a growth condition. To this end we use the method of Sub and Supersolutions, Topological Degree Theory, simmetry arguments and variational methods. / Neste trabalho estudaremos existência e multiplicidade de soluções não-negativas do problema elíptico não-linear −div(A(x,∇u)) = λf(x,u) em Ω, u = 0 em ∂Ω, Onde Ω ⊂ IRN é um domínio limitado com fronteira∂Ω suave,λ≥ 0 é um parâmetro, f :Ω×[0,∞)−→ IR e A :Ω×IRN−→ IRN satisfazem as condições de Carathéodory, A é monotônico e f satisfaz uma condição de crescimento. Para este fim utilizaremos o método de Sub e Super Soluções, Teoria do Grau Topológico, argumentos de simetria e métodos variacionais.

Submitted by DIANA YOVANI RODRÍGUEZ VILLENA null (dayaniss_23@hotmail.com) on 2017-10-09T19:16:47Z No. of bitstreams: 1 Dissertação Diana.pdf: 1051753 bytes, checksum: df5c2679c43a774ec3d6809c69271fd4 (MD5) / Approved for entry into archive by Monique Sasaki (sayumi_sasaki@hotmail.com) on 2017-10-09T19:43:34Z (GMT) No. of bitstreams: 1 rodriguezvillena_dy_me_sjrp.pdf: 1051753 bytes, checksum: df5c2679c43a774ec3d6809c69271fd4 (MD5) / Made available in DSpace on 2017-10-09T19:43:34Z (GMT). No. of bitstreams: 1 rodriguezvillena_dy_me_sjrp.pdf: 1051753 bytes, checksum: df5c2679c43a774ec3d6809c69271fd4 (MD5) Previous issue date: 2017-08-08 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Neste trabalho, estudamos a teoria de bifurcação e algumas das suas aplicações. Apresentamos alguns resultados básicos e definimos o conceito de ponto de bifurcação. Logo, estudamos a teoria do grau topológico. Em seguida, enunciamos dois teoremas importantes que são os teoremas de Krasnoselski e de Rabinowitz. Finalmente apresentamos um exemplo e duas aplicações do teorema de Rabinowitz nas quais os valores característicos com que lidamos são simples, no exemplo se consegue provar que a segunda alternativa do teorema ocorre, a primeira aplicação é um problema de autovalores não lineares de Sturm-Liouville para uma E.D.O de segunda ordem na qual se prova que a primeira alternativa do teorema de Rabinowitz é válida e a segunda aplicação é um problema de autovalores para uma equação diferencial parcial quase-linear a qual se prova que também ocorre a primeira alternativa do teorema. / In this work, we study bifurcation theory and its applications. We present some basic results and define the concept of bifurcation point. Then we study the theory of topological degree. Next we state two important theorems that are Krasnoselski's theorem and Rabinowitz's theorem. Finally we present an example and two applications of Rabinowitz theorem in which the characteristic values we deal with are simple, in an example we can prove that the second item of theorem occurs and the first application is a nonlinear Sturm-Liouville eigenvalue problem for a second order ordinary differential equation were we prove that the first alternative of Rabinowitz's theorem holds and the second application is an eigenvalue problem for a quasilinear elliptic partial differential equation where we prove that the first alternative of the theorem also holds.

Submitted by Izabel Franco (izabel-franco@ufscar.br) on 2016-09-23T20:10:37Z No. of bitstreams: 1 TeseWSR.pdf: 754474 bytes, checksum: f94a1d7e509de51f9a78d4e27289b7aa (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-26T20:46:07Z (GMT) No. of bitstreams: 1 TeseWSR.pdf: 754474 bytes, checksum: f94a1d7e509de51f9a78d4e27289b7aa (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-26T20:46:12Z (GMT) No. of bitstreams: 1 TeseWSR.pdf: 754474 bytes, checksum: f94a1d7e509de51f9a78d4e27289b7aa (MD5) / Made available in DSpace on 2016-09-26T20:46:19Z (GMT). No. of bitstreams: 1 TeseWSR.pdf: 754474 bytes, checksum: f94a1d7e509de51f9a78d4e27289b7aa (MD5) Previous issue date: 2015-03-06 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / The aim of this work is to present results of existence of solutions for a class of nonlinear asymmetryc elliptic problems. The asymmetry that we consider here has linear behavior on - infnity and superlinear on + infnity. To obtain these results we apply variational methods as linking theorems and topological methods like topological degree theory. / Neste trabalho apresentamos resultados de existência de soluções para uma classe de problemas elípticos não lineares assimétricos. A assimetria que consideramos aqui tem comportamento linear em - infinito e superlinear em + infinito. Para obter tais resultados aplicamos métodos variacionais como teoremas de linking e métodos topológicos como a teoria do grau topológico.

Made available in DSpace on 2014-07-29T16:02:19Z (GMT). No. of bitstreams: 1 Dissertacao Kaye O da Silva.pdf: 935849 bytes, checksum: 3342ffadf63161660c1795053815a170 (MD5) Previous issue date: 2012-02-24 / In this work, we employ topological methods in order to study existence and multiplicity of solutions, of nonlinear boundary value problems of the fourth order. More precisely, we make use of results on connected components of fixed points, as well as global bifurcation, to show existence and multiplicity of weak solutions of Partial Differential Equations, involving the Biharmonic operator under Navier boundary conditions. Proofs of the abstract results used, are presented in detail. / Neste trabalho, utilizamos métodos topológicos para estudar existência e multiplicidade de soluções de Problemas de Contorno Elípticos Não Lineares de 4a ordem. Mais precisamente, utilizamos resultados sobre componentes conexas de pontos fixos e tambem bifurcação global, para provar existência e multiplicidade de soluções fracas de Equações Diferenciais Parciais, envolvendo o Operador Binarmônico, sob condições de fronteira de Navier. As demonstrações dos resultados abstratos que utilizamos, são apresentadas em detalhes.