Affiliation: | (1) Combinatorics and Optimization, University of Waterloo, N2L 3G1 Waterloo, Ontario, Canada;(2) Mathematics and Statistics, McMaster University, L8S 4K1 Hamilton, Ontario, Canada;(3) Katedra algebry a teórie ísel, Univerzita Komenského, 84215 Bratislava, Czechoslovakia |