O intuito da presente dissertação é o estudo do chamado problema de Kakeya. Mais precisamente, depois introduzir todos os pré-requisitos necessários, demostraremos a conjectura de Kakeya no caso de R2, destacando que o caso geral dessa conjectura, que afirma que cada conjunto de Kakeya de Rn possui dimensão de Hausdorff igual a n, é ainda um problema aberto. / In this dissertation we study the so called Kakeya problem. More precisely, after introducing all the necessary prerequisites, our main goal will be to give a proof of the the Kakeya conjecture in the case of R2, whose general case, stating that every Kakeya set in Rn has Hausdorff dimension equal to n, is still an open problem.

O intuito da presente dissertação é o estudo do chamado problema de Kakeya. Mais precisamente, depois introduzir todos os pré-requisitos necessários, demostraremos a conjectura de Kakeya no caso de R2, destacando que o caso geral dessa conjectura, que afirma que cada conjunto de Kakeya de Rn possui dimensão de Hausdorff igual a n, é ainda um problema aberto. / In this dissertation we study the so called Kakeya problem. More precisely, after introducing all the necessary prerequisites, our main goal will be to give a proof of the the Kakeya conjecture in the case of R2, whose general case, stating that every Kakeya set in Rn has Hausdorff dimension equal to n, is still an open problem.

This thesis is concerned with two different problems in harmonic analysis: the multilinear Kakeya theorem, and Wolff-type inequalities for paraboloids. Chapter 1 gives an overview of both of these problems. In Chapter 2 we investigate an important special case of the multilinear Kakeya theorem, the so-called “bush example”. While the endpoint case of the multilinear Kakeya theorem was recently proved by Guth, the proof is highly abstract; our aim is to provide a more elementary proof in this special case. This is achieved for a significant part of the three-dimensional case in the main result of the chapter. Chapter 3 is a study of the endpoint case of a mixed-norm Wolff-type inequality for the paraboloid. The main result adapts an example of Bourgain to show that the endpoint inequality cannot hold with an absolute constant; there must be a dependence on the thickening of the paraboloid. The remainder of the chapter is a series of case studies, through which we establish positive endpoint results for certain classes of function, as well as indicating specific examples which need to be better understood in order to obtain the full endpoint result.

This thesis investigates two problems that are discrete analogues of two harmonic analytic problems which lie in the heart of research in the field. More specifically, we consider discrete analogues of the maximal Kakeya operator conjecture and of the recently solved endpoint multilinear Kakeya problem, by effectively shrinking the tubes involved in these problems to lines, thus giving rise to the problems of counting joints and multijoints with multiplicities. In fact, we effectively show that, in R3, what we expect to hold due to the maximal Kakeya operator conjecture, as well as what we know in the continuous case due to the endpoint multilinear Kakeya theorem by Guth, still hold in the discrete case. In particular, let L be a collection of L lines in R3 and J the set of joints formed by L, that is, the set of points each of which lies in at least three non-coplanar lines of L. It is known that |J| = O(L3/2) ( first proved by Guth and Katz). For each joint x ∈ J, let the multiplicity N(x) of x be the number of triples of non-coplanar lines through x. We prove here that X x2J N(x)1=2 = O(L3=2); while we also extend this result to real algebraic curves in R3 of uniformly bounded degree, as well as to curves in R3 parametrized by real univariate polynomials of uniformly bounded degree. The multijoints problem is a variant of the joints problem, involving three finite collections of lines in R3; a multijoint formed by them is a point that lies in (at least) three non-coplanar lines, one from each collection. We finally present some results regarding the joints problem in different field settings and higher dimensions.

Consider the class of polynomials P (z) = (Formula Presented) with 0 ≤ a0 ≤ a1 ≤ · · · ≤ an. The classical Eneström–Kakeya Theorem states that any polynomial in this class has all its zeros in the unit disk |z| ≤ 1 in the complex plane. We introduce new classes of polynomials by imposing a monotonicity-type condition on the coefficients with all indices congruent modulo m for some given m ≤ n. We give the inner and outer radii of an annulus containing all zeros of such polynomials. We also give an upper bound on the number of zeros in a disk for polynomials in these classes.

The well-known Eneström–Kakeya Theorem states that a polynomial with real, nonnegative, monotone increasing coefficients has all its complex zeros in the closed unit disk in the complex plane. In this paper, we extend this result by showing that all quaternionic zeros of such a polynomial lie in the unit sphere in the quaternions. We also extend related results from the complex to quaternionic setting.