Contributions to Operator Theory and Applications

Abstract This thesis consists of two parts. The first part deals with existerice and approximation techniques for finding solutions of operator equations or fixed points of operators belonging to certain classes of mappings. The classes of mappings studied include the K-posztz~~e definzte operators, the suppressive mappings and accretive-type rntippings. In particular, it is proved that for a real Banach space X, the equation Au = f, f E X, where A is a Kpd operator with the same domain as A', has a unique solution. An iteration process is constructed ant1 shown to converge strongly to the unique solution of this equation. Furtherniore, an asyrnptotzc version of Kpd operators is introduced and studied and a convergence result is proved. Drawing from the ideas of Alber [I], Alber and Guerre-Delabriere [2, 31, suppressive and accretive-type mappings are studied in more general settings. In particular, it is proved that if I< is a closed convex nonexpansive retract of a real uniformly smooth Banach space E, T : I< + E, a d-weakly contractive map such that a fixed point x* E intK of T exists then a descent-like approxirnation sequence converges strongly to z*. A related result deals with the approximation of a fixed point of T, when K is a subset of an arbitrary real Banach space &, aiacl.R(T) := {.c E E : Tx = z} # 0. Moreover, asymptotically d-weakly contractzve mappings are introtluc~ed and studied and convergence result,s are proved. The second part of the thesis deals with matliematical modelliiig of irlfectious diseases. Models for drug-resistant malaria parasites are presented both for single pol)ula.tions of hurnans and vectors and also for multigroup populations. Each of the models results in a system of nonlinear ordinary differential equations, which under suitable conditions leads to a locally stable equilibrium. The ecological significance of these ecluilibriunm point s emerges as a by-product. For the compartmental models, attention is devoted to the question of quailtitative agreement with published field observations by the application of new nonlinear least squares techniques. A time dependent immunity model is formulated arid used :is a baseline study to investigate parameter behaviour. Furthermore, the multigroup models are studied in Rn. The ultimate intention is to extend to infinite dimension, thereby providing a link between the analysis of these models and some well known and developed HilbertIBanach space theory.