@article{CHEB_2020_21_4_a6,
     author = {V. G. Durnev and O. V. Zetkina and A. I. Zetkina},
     title = {On {\textquotedblleft}simple{\textquotedblright} algorithmically undecidable fragments of elementary theory of an infinitely generated free semigroup},
     journal = {\v{C}eby\v{s}evskij sbornik},
     pages = {56--71},
     year = {2020},
     volume = {21},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/CHEB_2020_21_4_a6/}
}