1. Poltava National Technical Yuri Kondratyuk University, 24 Pershotravnevyi Ave., 36011 Poltava, Ukraine
2. Institute of Mathematics, National Academy of Sciences of Ukraine, 3 Tereshchenkivs’ka Str., 01004 Kyiv-4, Ukraine
3. National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute,” 37 Peremohy Ave., 03056 Kyiv, Ukraine