**Author details**

Cristina Martinez Ramirez\* and Alberto Besana Department of Maths, UAB, Barcelona

\*Address all correspondence to: cristinamartine@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
