Affiliation: | 1. Department of Mathematics, Federal University of Paraíba, 58051-900, Jo?o Pessoa, PB, Brazil 2. Department of Mathematics, Federal University of Juiz de Fora, 36036-330, Juiz de Fora, Minas Gerais, Brazil 3. Dipartimento di Informatica, Università degli Studi di Verona, Cá Vignal 2, Strada Le Grazie 15, 37134, Verona, Italy
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