1) Nikos Tzanakis, University of Crete, Crete-GREECE (September 2014)

2) László Szalay, University of Sopron, Institute of Mathematics, Sopron-HUNGARY (January 2017)

3) Paul Voutier,  London-UK (October 2017)

4) Mohammad Sadek, Sabancı University, Department of Mathematics, İstanbul (November 2018)

5) Andrej Dujella, University of Zagreb, Department of Mathematics, Zagreb-CROATIA (November 2019)

6) Andrew Bremner, Arizona State University, School of Mathematical and Statistical Sciences, Tempe-Arizona-USA (September 2022)

7) László Nemeth, University of Sopron, Institute of Mathematics and Informatics, Sopron-HUNGARY (May 2023)

8) Salah Eddine Rihane, The National Higher School of Mathematics, Algiers-ALGERIA (February 2023, February 2024)