1. CPM-TIP, University of Pavol Jozef Safarik, Tr. SNP 1, 040 11 Kosice, Slovakia
2. CITIMAC, University of Cantabria, Av. de los Castros s/n, 390 05 Santander, Spain
3. Faculty of Humanities and Natural Sciences, University of Presov, Ul. 17. novembra 1, 080 01 Presov, Slovakia
4. Department of Physics, National Institute of Technology, 620 015 Tiruchirappalli, India