1. Instituto de Tecnologías Físicas y de la Información (ITEFI), Consejo Superior de Investigaciones Científicas (CSIC), Serrano 144, Madrid 28006, Spain
2. Dept. of Mathematics and Computer Science, "Avram Iancu" University, Ilie Măcelaru 1A, Cluj-Napoca 400380, România
3. Romanian Institute of Science and Technology, Cireşilor 29, Cluj-Napoca 400487, România