[1] Imperial Coll London, Dept Math, Huxley Bldg,180 Queens Gate, London SW7 2AZ, England
[2] Univ Paul Sabatier, Inst Math Toulouse, 118 Route Narbonne, F-31062 Toulouse, France
[3] Univ Ghent, Dept Math Anal Log & Discrete Math, 281 Krijgslaan,Bldg S8, Ghent, Belgium
[4] Suleyman Demirel Univ, Kaskelen, Kazakhstan
[5] Inst Math & Math Modeling, 125 Pushkin Str, Alma Ata 050010, Kazakhstan