Affiliation: | a Instituto de Ciencia y Tecnologia de Materiales (IMRE), Universidad de La Habana, Habana, Cubab Department of Mathematics, University of Bologna, P.zza Porta S. Donato 5, 40126 Bologna, Italyc Department of Mathematical Sciences, Kent State University, Kent, OH 44242, USAd Department of Mathematics-CIRAM, University of Bologna, Via Saragozza 8, 40123 Bologna, Italy |