1. Instituto de Tecnologías Físicas y de la Información (ITEFI), Consejo Superior de Investigaciones Científicas (CSIC), Serrano 144, Madrid 28006, Spain
2. Department of Mathematics and Computer Science, Emanuel University of Oradea, 410597 Oradea, România
3. Romanian Institute of Science and Technology, Cireşilor 29, Cluj-Napoca, 400487, România