@incollection{MEM_2020_49_a0,
     author = {Kolev, Emil},
     title = {Preface},
     booktitle = {},
     series = {Mathematics and Education in Mathematics},
     pages = {5--6},
     year = {2020},
     volume = {49},
     language = {bg},
     url = {http://geodesic.mathdoc.fr/item/MEM_2020_49_a0/}
}