### Abstract

The thesis consists of five chapters and is divided into two parts. The first one is devoted to the use of the so-called Harmonic Balance Method (HBM) as well as its theoretical basis. The second one deals with the quantitative and qualitative study of two families of polynomial differential equations in the plane. The HBM provides a method to obtain approximations of periodic solutions of differential equations and their period. In Chapters 1 and 2 we use the HBM to find approximations of the period function of certain families of differential equations in the plane. The main contribution of the thesis on this issue is the parallel analytical study of the period function and the verification that the approximations obtained via the HBM capture several local and global properties of this function. In Chapter 3, it is shown that near certain approximations obtained using the HBM are actual periodic solutions of the differential equation studied. For our results we rely on classical results of Urabe (1965) and Stokes (1972). The second part of the thesis addresses some quantitative problems within the Qualitative Theory of Differential Equations. More specifically, in both chapters we analytically determined the lower and upper bounds of the bifurcation values of two one-parameter families of polynomial differential equations. The main difference between the families studied in Chapter 4 and Chapter 5, is that while the first one is a rotated family, which implies that the bifurcations are more controlled, the second one is not, and hence the problem becomes more complicated. To establish the bounds discussed in the previous paragraph we introduce a method for the effective construction of algebraic curves without contact by the flow of the differential equation. These curves are good approximations of the separatrices of critical points, both finite or at the infinity. The verification that these curves are without contact essentially amounts to controlling the sign of one-parameter family of polynomials. To solve this problem, the concept of double discriminant is introduced in the thesis. Furthermore, the control of the number of limit cycles of differential equations is performed using the generalized Bendixson-Dulac Criterion. The last step to apply this approach also involves the control of the sign of a given polynomial, and again the double discriminant plays an important role. The methods developed in these two chapters allow one to calculate algebraic approximations of the separatrices of the critical points of differential equations in the plane, and to determine bounds for the parameter values that exist in the families of differential equations in such a way that they can have homoclinic or heteroclinic connections.Date of Award | 18 Jun 2010 |
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Original language | English |

Supervisor | Armengol Gasull Embid (Director) |