1. Dept. of Mathematics, Faculty of Science, Birzeit University, Birzeit, Palestine
2. Dept. of Mathematics, Brooklyn College of The City University of New York, Brooklyn, NY, USA
3. Dept. of Mathematics, Graduate Center of the City University of New York, New York, NY, USA
4. Dept. of Mathematics and Computer Science, Queensborough Community College of The City University of New York, Bayside, NY, USA
5. LIAAD – INESC TEC, Faculty of Science, University of Porto, Porto, Portugal
6. Dept. of Mathematics, Faculty of Science, University of Porto, Porto, Portugal