1. Department of Algebra, Stochastics and Knowledge-Based Mathematical Systems, Johannes Kepler University, 4040 Linz, Austria
2. Department of Mathematics and Descriptive Geometry, Faculty of Civil Engineering, Slovak Technical University, 81 368 Bratislava, Slovakia
3. Institute of the Theory of Information and Automation, Czech Academy of Sciences, Prague, Republic of Czech
4. Institute of Mathematics, University of Novi Sad, 21000 Novi Sad, Yugoslavia