Affiliation: | a Simepar, Paraná State Power Company, Curitiba, PR 80420-170, Brazil b Dip, di Matematica–Fac. di Ingegneria, Universitá di Messina, Contrada Papardo, I-98166 Messina, Italy c Department of Electrical and Computer Engineering, Duke University, Durham, NC 27708-0291, USA |