1. Institute of Mathematics, National Academy of Sciences of Ukraine, 3 Tereshchenkivs’ka Street, 01601 Kyiv, Ukraine
2. Bakul Institute for Superhard Materials, National Academy of Sciences of Ukraine, 2 Avtozavods’ka Street, 04074 Kyiv, Ukraine
3. School of Mathematical Sciences, University of Nottingham, University Park, Nottingham NG7 2RD, UK