[1] Bashkir State Pedag Univ, Fac Phys & Math, Dept Math Anal, Ufa 450000, Russia
[2] Univ Clermont Ferrand, CNRS, Math Lab, UMR 6620, F-63177 Aubiere, France
[3] Moscow MV Lomonosov State Univ, Fac Mech & Math, Dept Differential Equat, Moscow 119991, Russia
[4] Narvik Univ Coll, N-8505 Narvik, Norway