1. a Math. Institute B/3, University Stuttgart , Pfaffenwaldring 57 (Vaihingen), Stuttgart , D-70550 , Germany
2. b Faculty of Mechanics and Mathematics , Kiev Taras Shevchenko University , Vladimirskaya Str., 64, Kiev , 01033 , Ukraine
3. c Glushkov Institute of Cybernetics NAS Ukraine , Glushkov Av., 40, Kiev , 03680 , Ukraine