1) Pedro Jose Cazorla Garcia, Universidad Pontificia Comillas, Departamento de Matema'tica Aplicada, Madrid-SPAIN (May 2026)

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

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

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

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

6) Mohammad Sadek, Sabancı University, Department of Mathematics, İstanbul (November 2018, April 2026)

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

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

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