1. SP Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, ul.Nesterova 3, Kiev 03057, Ukraine e-mail:
2. The Chair of Higher Mathematics and IT, Surgut State Pedagogical University, Artema St. 9, Surgut 628400, Russia e-mail:
3. Department of Mathematics, The University of Auckland, Private Bag 92019, Auckland 1012, New Zealand e-mail: