1. Department of Mathematics; K.N. Toosi University of Technology; P.O. Box 16315-1618 Tehran Iran
2. Faculty of Industrial Engineering; University of Douala; Douala Cameroon
3. Departmento de Matemática Aplicada II; Universidade de Vigo; E.E. Aeronáutica e do Espazo 32004 Ourense Spain