1. Faculty of Mathematics and Technology, University of Applied Sciences Koblenz, Remagen, Germany
2. Faculty of Mathematics, University of Duisburg-Essen, Essen, Germany
3. School of Data Science and Shenzhen Research Institute of Big Data, The Chinese University of Hong Kong, Shenzhen (CUHK-Shenzhen), Shenzhen, China
4. Université de Lorraine, Institut Élie Cartan de Lorraine, Nancy, France