**Author details**

Sergio Venturini Dipartimento di Matematica, Università di Bologna, Bologna, Italy

\*Address all correspondence to: sergio.venturini@unibo.it

© 2021 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.
