1. "University Politehnica of Bucharest, Faculty of Applied Sciences, Department of Mathematics-Informatics, Splaiul Independentei 313, Bucharest 060042, Romania and Academy of Romanian Scientists, 3 Ilfov RO-050044, Bucharest, Romania e-mail: udriste@mathem.pub.ro, anet.udri@yahoo.com"
2. "University Politehnica of Bucharest, Faculty of Applied Sciences, Department of Mathematics-Informatics, Splaiul Independentei 313, Bucharest 060042, Romania e-mail: tevy@mathem.pub.ro, vascatevy@yahoo.fr"