Nevanlinna Theory is a powerful quantitative tool used to study the growth and behaviour of meromorphic functions on the complex plane. It plays an important role in value distribution theory, including generalising Picard's theorem that an entire function which omits two finite values is constant. The Nevanlinna Characteristic T(r,f) is a measure of a function's growth, and its associated counting function estimates how often certain values are taken. Using these tools, as well as other forms of modern complex analysis, we investigate several problems relating to differential polynomials in meromorphic functions. We also present a result relating to integer-valued meromorphic functions.

In this thesis, we mainly worked in the following areas: value distributions of meromorphic functions, normal families, Bank-Laine functions and complex oscillation theory. In the first chapter we will give an introduction to those areas and some related topics that are needed. In Chapter 2 we will prove that for a meromorphic function f and a positive integer k, the function af(f(k))n -1, n ≥ 2, has infinitely many zeros and then we will prove that it is still true when we replace f(k) by a differential polynomial. In Chapter 3 we will prove that for a merornorphic function f and a positive integer k, the function af f(k) -1 with N1(r, 1/f^((k)) ) = S(r, f) has infinitely many zeros and then we will prove that it is still true when we replace f(k) by a differential polynomial. In Chapter 4 we will apply Bloch's Principle to prove that a family of functions meromorphic on the unit disc B(0, 1), such that f(f1)m≠ 1, m ≠ 2, is normal. Also we will prove that a family of functions meromorphic on B(0,1), such that each f ≠ 0 and f(f(k))m ,k, m ∈N omits the value 1, is normal. In the fifth chapter we will generalise Theorem 5.1.1 for a sequence of distinct complex numbers instead of a sequence of real numbers. Also, we will get very nice new results on Bank-Laine functions and Bank-Laine sequences. In the last chapter we will work on the relationship between the order of growth of A and the exponent of convergence of the solutions y(k) +Ay =0, where A is a transcendental entire function with ρ(A) < 1/2.

Dans cette thèse, nous nous sommes intéressés à divers problèmes mettant en oeuvre des structures algébriques de certains anneaux fonctionnels, en particulier dans l'espace H infini des fonctions holomorphes bornées dans le disque unité, dans l'algèbre de Sarason H infini + C et dans C(X,t)={fEC(X) : fot=f}, avec X un espace compact séparé et t une involution topologique sur X. Plus précisément, nous avons caractérisé les idéaux radicaux finiment engendrés dans H infini + C. En second lieu, nous avons démontré que le rang stable absolu de C(X,t) coïncide avec le rang stable Bass et topologique de cette dernière. En dernier lieu, nous nous sommes intéressés au problème de la couronne généralisé dans H infini / In this thesis, we are interested in various problems of algebraic structures of some functional rings, in particular in the space H infinity of bounded analytic functions in the unit disc, in the Sarason algebra H infinity + C and in C(X,t)={fEC(X) : fot=f} with X compact Hausdorff space and t a topological involution on X. More precisely, we have characterized the finitely generated radical ideals in H infinity + C. Secondly, we have demonstrated that the absolute stable rank of C (X, t) coincides with Bass stable rank and topological stable rank. Finally, we are interested in the generalized corona problem in H infinity

This thesis is concerned with a conjecture of Zilber: that the complex field expanded with the exponential function should be `quasi-minimal'; that is, all its definable subsets should be countable or have countable complement. Our purpose is to study the geometry of this structure and other expansions by holomorphic functions of the complex field without having first to settle any number-theoretic problems, by treating all countable sets on an equal footing. We present axioms, modelled on those for a Zariski geometry, defining a non-first-order class of ``quasi-Zariski'' structures endowed with a dimension theory and a topology in which all countable sets are of dimension zero. We derive a quantifier elimination theorem, implying that members of the class are quasi-minimal. We look for analytic structures in this class. To an expansion of the complex field by entire holomorphic functions $\mathcal{R}$ we associate a sheaf $\mathcal{O}^{\scriptscriptstyle{\mathcal{R}}}$ of analytic germs which is closed under application of the implicit function theorem. We prove that $\mathcal{O}^{\scriptscriptstyle{\mathcal{R}}}$ is also closed under partial differentiation and that it admits Weierstrass preparation. The sheaf defines a subclass of the analytic sets which we call $\mathcal{R}$-analytic. We develop analytic geometry for this class proving a Nullstellensatz and other classical properties. We isolate a condition on the asymptotes of the varieties of certain functions in $\mathcal{R}$. If this condition is satisfied then the $\mathcal{R}$-analytic sets induce a quasi-Zariski structure under countable union. In the motivating case of the complex exponential we prove a low-dimensional case of the condition, towards the original conjecture.