Faculty of Economics and Business Administration, Sofia University “St. Kl. Ohridski,” 125 Tsarigradsko Shosse Boulevard, bl.3, 1113 Sofia, Bulgaria
Vasile Dragan
Institute of Mathematics “Simion Stoilow” of the Romanian Academy, Calea Grivitei No. 21, P.O. Box 1-764, 014700 Bucharest, Romania
Oswaldo Luiz do Valle Costa
Departamento de Engenharia de Telecomunicações de Controle, Institute Escola Politécnica da Universidade de São Paulo (EPUSP), 05508-900 São Paulo, SP, Brazil
