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2 Negligible Sets 15 2.1 Porosity View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by idUS. Depósito de Investigación Universidad de Sevilla FACULTAD DE MATEMATICAS´ DEPARTAMENTO DE ANALISIS´ MATEMATICO´ GENERICITY OF THE FIXED POINT PROPERTY UNDER RENORMING Ph. D. Thesis presented by Supaluk Phothi UNIVERSIDAD DE SEVILLA FACULTAD DE MATEMATICAS´ DEPARTAMENTO DE ANALISIS´ MATEMATICO´ GENERICITY OF THE FIXED POINT PROPERTY UNDER RENORMING Memoria presentada por Supaluk Phothi para optar al grado de Doctora en Matemticas por la Universidad de Sevilla Fdo. Supaluk Phothi V◦.B◦.: Director del trabajo Fdo. Tom´asDom´ınguezBenavides Catedr´aticodel Departamento de An´alisisMatem´atico de la Universidad de Sevilla Sevilla, Julio de 2010 RESUMEN Una aplicaci´on T definida de un espacio m´etrico M en M se dice no expan- siva si d(T x; T y) ≤ d(x; y) para todo x; y 2 M. Diremos que un espacio de Banach X tiene la Propiedad D´ebildel Punto Fijo (w-FPP) si para toda aplicaci´onno expansiva T definida de un subconjunto d´ebilmente compacto convexo C de X en C tiene un punto fijo. En esta disertaci´on,estudiamos principalmente la w-FPP como una propiedad gen´ericaen el conjunto de todas las normas equivalentes de un espacio de Banach reflexivo dado. Una propiedad P se dice gen´ericaen un conjunto A si todos los elementos de A sat- isfacen P excepto aquellos pertenecientes a un conjunto de tama~nopeque~no. Con el fin de establecer los resultados de este trabajo, consideraremos varias nociones de conjuntos peque~nos,como por ejemplo los conjuntos de Baire de primera categor´ıa,conjuntos porosos, conjuntos nulos Gausianos o conjuntos direccionalmente porosos. M. Fabian, L. Zaj´ı^ceky V. Zizler probaron que casi todos los renor- mamientos de un espacio uniformemente convexo en cada direcci´on(UCED), en el sentido de la categor´ıade Baire, son tambi´enUCED. Debido al resultado de M.M. Day, R.C. James y S. Swaminathan, todo espacio de Banach sepa- rable admite una norma equivalente que es uniformemente convexa en cada direcci´on.Puesto que esta propiedad geom´etricaimplica la FPP, obtenemos la siguiente conclusi´on:Si X es un espacio de Banach reflexivo separable, en- tonces casi todos los renormamientos de X satisfacen la w-FPP. Este m´etodo no es v´alidopara el caso de los espacios reflexivos no separables. Sin em- bargo, recientemente T. Dom´ınguezBenavides ha probado que todo espacio de Banach que pueda ser sumergido en c0(Γ), donde Γ es un conjunto arbi- trario ( en particular, todo espacio reflexivo) puede ser renormado para tener la w-FPP. N´oteseque que el espacio c0(Γ) no es renormable UCED cuando Γ es no numerable, pero satisface la w-FPP porque R(c0(Γ)) < 2, donde R(·) es el coeficiente de Garc´ıa-Falset y todo espacio de Banach X con R(X) < 2 satisface la w-FPP. Usando la misma inmersi´on,obtenemos el siguiente re- sultado: Sea X un espacio de Banach tal que para alg´unconjunto Γ existe una aplicaci´oncontinua lineal uno a uno J : X ! c0(Γ). Entonces, casi todas las normas equivalentes q en X (en el sentido de la categor´ıade Baire) satisfacen la siguiente propiedad: Toda aplicaci´on q-no-expansiva, definida desde un subconjunto convexo d´ebilmente compacto C de X, en C, tiene un punto fijo. En particular, si X es reflexivo, entonces el espacio (X; q) satisface la FPP. Adem´as,extendemos este resultado a cualquier espacio de Banach que pueda ser sumergido en un espacio de Banach Y , m´asgeneral que c0(Γ) y que satisfaga R(Y ) < 2. Probamos que si X es un espacio de Banach satisfaciendo R(Y ) < 2 y X un espacio de Banach que pueda ser sumergido en Y de manera continua, entonces X puede ser renormado para satisfacer la w-FPP y el conjunto de todas las renormas en X, que no satisfacen la w-FPP, es de primera categor´ıa. En el caso del espacio C(K), donde K es un conjunto disperso tal que K(!) = ;, obtendremos que existe una norma j · j que es equivalente a la norma del supremo y R(C(K); j · j) < 2 (luego tiene la w-FPP). Adem´as,casi todas las normas equivalentes a la norma del supremo (en el sentido de la porosidad) tambi´ensatisfacen la w-FPP. Contents Introduction i 1 Notations and Preliminaries 1 1.1 Hyperbolic metric spaces . 1 1.2 Fixed points for non-expansive mappings . 2 1.3 Fixed points for non-expansive multi-valued mappings . 7 1.4 Weakly compactly generated spaces . 9 1.5 Cardinal numbers and Ordinal numbers . 10 1.6 G^ateauxand Fr´echet differentiability . 12 2 Negligible sets 15 2.1 Porosity . 16 2.2 Gaussian null sets . 24 2.3 Aronszajn null sets . 27 2.4 Directional porosity . 28 3 Generic fixed point results in a classic sense 35 3.1 Generic fixed point results on the set of non-expansive mappings 35 3.2 Generic non-expansive mappings with another metric . 38 3.3 Generic multi-valued non-expansive mappings . 43 4 Generic fixed point property in separable reflexive spaces 49 4.1 Generic fixed point results on renormings of a Banach space . 50 4.2 Equivalent metrics on the set of renormings of a Banach space 56 6 4.3 Porous Fabian-Zaj´ıˇcek-Zizler'sresult on normalized renormings 61 5 Generic fixed point results on nonseparable reflexive Banach spaces 67 5.1 Generic w-FPP on spaces with R(X) less than 2 . 68 5.2 Generic w-FPP on a Banach space embedded into c0(Γ) . 74 5.3 On a Banach space embedded into Y satisfying R(Y ) < 2 . 86 5.4 Generic fixed point results on the space C(K) . 104 6 Generic multi-valued fixed point property on renormings of a Banach space 111 6.1 Generic MFPP on renormings of a reflexive space . 111 6.2 Generic MFPP concerning with the Szlenk index . 119 Bibliography 123 Introduction Introduction Assume that A is a set and P a property which can be either satisfied or not by the elements of A. The property P is said to be generic in A if \almost all" elements of A satisfy P. When speaking about almost all elements we mean all of them except those in a \negligible set". There are different ways to define the notion of negligible set, according to the setting where we are interested. For instance, in measure theory, a set with null measure can be considered as negligible and many generic results are well known in this theory. Consider, for instance, the following example: Let f and g be Bochner Z Z integrable functions. If fdµ = gdµ for every µ-measurable set E, then E E f = g µ-almost everywhere, i.e. the set fx : f(x) 6= g(x)g is negligible. In topological space, a Baire first category set can be considered as a negligible set. The interest of this notion depends on the size of the whole set, because if the whole set were of Baire first category, then all subsets would be negligible. Thus, this notion is only interesting in second category topological spaces, for instance, in complete metric spaces, according to Baire Theorem. It must be noted that negligibility in the sense of null measure and in the sense of Baire category can be different in spaces where both notions can be simultaneously considered. For instance, the real line R is the disjoint union of a set of first category and a set of Lebesgue measure zero. To avoid this problem, we can use the concept of \porosity" as a refined notion of Baire first category. Every σ-porous set is of first category and in a finite dimensional space, it has Lebesgue measure zero. Many generic results have appeared concerning different subjects. One of the first generic result was obtained by W. Orlicz [62], who proved that the uniqueness of solution of the Cauchy problem for an ordinary differen- tial equations is a generic property in the space of all bounded continuous n+1 n functions mapping from R into R . Later, this result was extended to i Introduction the generic uniqueness of solutions of different equations in infinite dimen- sional spaces by A. Lasota and J.A. Yorke [53]. In this dissertation, we study generic property concerning Metric Fixed Point Theory. Fixed point theory has been usually used to study the existence of solu- tions of differential equations and also has been applied in many branches of mathematics.The most well-known fixed point theorem is the Contraction Mapping Principle, due to S. Banach [5]. The statement is the following: Theorem. (Banach Fixed Point Theorem) Let X be a complete metric space and T : X ! X a contraction, i.e. there exists k 2 [0; 1) such that d(T x; T y) ≤ kd(x; y) for every x; y 2 X. Then T has a (unique) fixed point n x0. Furthermore x0 = lim T x for every x 2 X. n However, Banach Contraction Principle fails for non-expansive mapping, i.e. a mapping T : M ! M, where M is a metric space, such that d(T x; T y) ≤ d(x; y) for every x; y 2 M. But in 1965, F. E. Browder, D. G¨ohdeand W. A. Kirk proved the existence of fixed points for non-expansive mappings in Banach spaces which satisfy some geometrical properties. Browder [8] proved that every non-expansive mapping defined from a convex closed bounded sub- set C of a Hilbert space into C, has a fixed point.
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