1. Institute of Mathematics , P. J. Šafárik University in Košice , Slovakia ; and Institute of Discrete Mathematics and Geometry, TU Wien, Austria
2. Department of Mathematics and Statistics , Binghamton University , PO Box 6000 , Binghamton , NY 13902-6000 , USA
3. School of Mathematics and Statistics , University of St Andrews , St Andrews , United Kingdom
4. Department of Physics, Astronomy and Mathematics , University of Hertfordshire , Hatfield , Hertfordshire , United Kingdom