First Author:   Marina Nunez Department of Economic, Financial and Actuarial Mathematics. University of Barcelona mnunez@ub.edu   Av. Diagonal, 690 Barcelona, 08034, Spain       Second Author: Carles Rafels Department of Economic, Fianncial and Actuarial Mathematics. University of Barcelona crafels@ub.edu   Av. Diagonal, 690 Barcelona, 08034, Spain