1Department of Mathematics, University of Paris VIII, Saint-Denis, France
2University of Paris XIII, CNRS, LAGA UMR 7539, Sorbonne Paris Cité, Villetaneuse, France
3Telecom ParisTech, Palaiseau, France
4Institute of Mathematics, State Academy of Sciences, Pyongyang, Democratic People’s Republic of Korea
5PGItech Corp, Pyongyang, Democratic People’s Republic of Korea
6Master School, University of Natural Science, Pyongyang, Democratic People’s Republic of Korea