Affiliation: | UFR de Mathématiques et d'Informatique, Université de Bordeaux I, 351, Cours de la Libération, 33405 Talence, France ; Department of Mathematics, College of Natural Science, Kyungpook National University, Daegu 702-701, Korea ; Department of Mathematics, Ewha Women's University, Seoul 120-750, Korea ; Department of Mathematics, Texas A&M University, College Station, Texas 77843 |