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