1. 1Department of Mathematics, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara — 390 002 (Gujarat), India e-mail: bhikhu_ghodadra@yahoo.com
2. 2Bolyai Institute, University of Szeged, Aradi Vértanúk tere 1, Szeged 6720, Hungary e-mail: fulopv@math.u-szeged.hu