For citations:

Durnev V.G., Zetkina O.V., Zetkina A.I. On “simple” algorithmically undecidable fragments of elementary theory of an infinitely generated free semigroup. Chebyshevskii Sbornik. 2020;21(4):56-71. (In Russ.) https://doi.org/10.22405/2226-8383-2020-21-4-56-71