Treballs Finals de Grau de Matemàtiques, Facultat de Matemàtiques, Universitat de Barcelona, Any: 2017, Director: Jordi Vitrià i Marca[en] I would like to thank my advisor, Jordi Vitrià Marca, for so much encouragement, support and feedback. Jordi was patient while I spent the first few months chasing half-baked ideas, and then gently suggested a series of notions which worked. It was wonderful working with someone who is always willing to help and whose dedication to science is inspiring.