1. Institute of Applied Mathematics and Mechanics of NAS of Ukraine, Sloviansk, Ukraine
2. Institute of Applied Mathematics and Mechanics of the NAS of Ukraine, Slavyansk, Ukraine
3. Institute of Mathematics of the NAS of Ukraine, Kyiv, Ukraine
4. Zhytomyr Ivan Franko State University, Zhytomyr, Ukraine, Institute of Applied Mathematics and Mechanics of the NAS of Ukraine, Slavyansk, Ukraine