Donika Valcheva
ORCID iD St. Cyril and St. Methodius University
Bulgaria

Department of Mathematics and Informatics, St. Cyril and St. Methodius University, Veliko Tarnovo, Bulgaria.

d.valcheva@ts.uni-vt.bg

https://orcid.org/0000-0003-2107-0190 

Teodor Kalushkov
ORCID iD St. Cyril and St. Methodius University
Bulgaria

Department of Mathematics and Informatics, St. Cyril and St. Methodius University, Veliko Tarnovo, Bulgaria.

t.kalushkov@ts.uni-vt.bg

https://orcid.org/0000-0002-5748-6238 

Georgi Shipkovenski
ORCID iD St. Cyril and St. Methodius University
Bulgaria

Department of Mathematics and Informatics, St. Cyril and St. Methodius University, Veliko Tarnovo, Bulgaria.

g.shipkovenski@ts.uni-vt.bg

https://orcid.org/0000-0002-7627-0635