@incollection{MEM_2012_41_1_a20,
     author = {Stefanov, Simeon and Dragieva, Velika},
     title = {Evolution of {Sets} {Systems} and {Homotopy} {Groups} of {Spheres}},
     booktitle = {},
     series = {Mathematics and Education in Mathematics},
     pages = {202--206},
     year = {2012},
     volume = {41},
     number = {1},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/MEM_2012_41_1_a20/}
}