Thyago c. C. Nepomuceno Universidade Federal de Pernambuco   Av. Prof. Moraes Rego, 1235 Recife, Pernambuco 50740-550 Brasil   nepomuceno@diag.uniroma1.it             JEL Areas: C6 : Mathematical Methods and Programming: General C4 : Econometric and Statistical Methods: Special Topics