Solving an investment optimization problem by an improved graphical approach / Evgeny R. Gafarov, Ecole Nationale Superieure des Mines, Saint-Etienne, France; Institute of Control Sciences of the Russian Academy of Sciences, Moscow, Russia; Alexandre Dolgui, Ecole Nationale Superieure des Mines, Saint-Etienne, France; Alexander A. Lazarev, Institute of Control Sciences of the Russian Academy of Sciences, Moscow, Russia; Frank Werner, Fakultät für Mathematik, Otto-von-Guericke-Universität Magdeburg, Magdeburg, Germany