Ph.D. Thesis 1 21 This is the final thesis for the Ph.D. program in Mathematics and Physics -Section: Mathematics. The final defence has been held on June 4, 2009.2 Sinceri ringraziamenti al Prof. Zanolin per l'infinita disponibilità ed il prezioso e costante aiuto.The N-dimensional Euclidean space R N is endowed with the usual scalar product • , • , norm • and distance dist(• , •). In the case of R, the norm• will be replaced with the absolute value |In a normed space (X, • X ), we denote by B(x 0 , r) and B[x 0 , r] the open and closed balls centered in x 0 ∈ X with radius r > 0, i.e. B(x 0 , r) := {x ∈ X : x − x 0 X < r} and B[x 0 , r] := {x ∈ X : x − x 0 X ≤ r}. For M ⊆ X, we set B(M, r) := {x ∈ X : ∃ w ∈ M with x − w X < r}. The set B[M, r] is defined accordingly. If we wish to specify the dimension m of the ball, when it is contained in R m , we write B m (x 0 , r) and B m [x 0 , r] in place of B(x 0 , r) and B[x 0 , r]. We denote byGiven an open bounded subset Ω of R N , an element p of R N and a continuous function f : Ω → R N , we indicate with deg(f, Ω, p) the topological degree of the map f with respect to Ω and p. We say that degWe denote by C(D, R N ) the set of the continuous maps f : D → R N on a compact set D, endowed with the infinity norm |f | ∞ := max x∈D f (x) .