Minimizing total weighted completion time approximately for the paraellel machine problem with a single server / Keramat Hasani (Islamic Azad University, Malayer Branch, Malayer, Iran), Svetlana A. Kravchenko (United Institute of Informatics Problems, Minsk, Belarus), Frank Werner (Otto-von-Guericke-Universität, Fakultät für Mathematik, Magdeburg, Germany)