1. Department of Mathematics, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711 103, India
2. Doctoral School of Economic and Regional Sciences, Hungarian University of Agriculture and Life Sciences, 2100 Godollo, Hungary
3. Hungarian National Bank—Research Center, John Von Neumann University, 6000 Kecskemet, Hungary
4. Department of Mathematics, Sonarpur Mahavidyalaya, Rajpur, Kolkata 700 149, India