1. Risk Assessment Division (7403), Office of Pollution Prevention and Toxics, U.S. Environmental Protection Agency, 1200 Pennsylvania Avenue, N.W., Washington, DC 20460
2. Department of Mathematics, University of Maribor, Koroška cesta 160, SI-2000 Maribor, Slovenia
3. Department of Mathematics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia