[1] SP Timoshenko Inst Mech NAS Ukraine, Stabil Proc Dept, Nesterov Str 3, UA-03680 Kiev 57, Ukraine
[2] Yuzuncu Yil Univ, Fac Sci, Dept Math, TR-65080 Van Turkey, Turkey
[3] Bohdan Khmelnytsky Natl Univ Cherkasy, Dept Algebra & Math Anal, Cherkassy, Ukraine