Popchev, Dimitar
Staykov, Kalin V.
Doneva, Daniela D.
Yazadjiev, Stoytcho S.
Funding for this research was provided by:
European Social Fund
Baden-Württemberg Stiftung
Sofia University (3258/2017)
Ministerium für Wissenschaft, Forschung und Kunst Baden-Württemberg
Bulgarian National Science Fund (DCOST 01/6)
Article History
Received: 9 December 2018
Accepted: 15 February 2019
First Online: 26 February 2019