[1] Ovidius Univ, Fac Math & Comp Sci, Constanta 900527, Romania
[2] Univ Duisburg Essen, Fachbereich Math, D-45117 Essen, Germany
[3] Osaka Univ, Grad Sch Informat Sci & Technol, Dept Pure & Appl Math, Toyonaka, Osaka 5600043, Japan
[4] Math Sci Res Inst, Berkeley, CA 94720 USA