1. ISAG — European Business School and Research Center in Business Sciences and Tourism (CICET-FCVC), Campus de Salazares/Ramalde, R. de Salazares 842, 4100-442 Porto, Portugal
2. ISMAI — Universidade da Maia Av. Carlos de Oliveira Campos, 4475-690 Maia, Portugal
3. Department of Mathematics, Brooklyn College of the City University of New York, USA
4. Department of Mathematics, Graduate Center of the City University of New York, USA
5. Department of Mathematics and Computer Science, Queensborough Community College of the City University of New York, USA