1. Institute of Mathematics, Ukrainian National Academy of Sciences, 3 Tereshchenkivs’ka Str., Kyiv 01601, Ukraine
2. Pershotravnevyi Prosp., Poltava 36011, Ukraine
3. Division of Physics, Poltava National Technical Yuri Kondratyuk University, 24 Pershotravnevyi Prosp., Poltava 36011, Ukraine