@article{TAC_2005_14_a12,
     author = {Libor Barto and Petr Zima},
     title = {Every group is representable by all natural transformations of some 
set-functor},
     journal = {Theory and applications of categories},
     pages = {294--309},
     publisher = {mathdoc},
     volume = {14},
     year = {2005},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_2005_14_a12/}
}