1. Institute of Mechanics, National Academy of Sciences of the Republic of Armenia, Yerevan 0019, Armenia
2. Faculty of Mathematics, Physics and Informatics, Department of Mathematics and its Teaching Methods, Armenian State Pedagogical University after Khachatur Abovyan, Abovyan 0010, Armenia
3. School of Computer Science and Mathematics, Keele University, Keele, Staffordshire ST5 5BG, UK