@article{AL_2012_51_5_a5,
     author = {D. R. Hirschfeldt and B. Khoussainov},
     title = {Finitely presented expansions of computably enumerable semigroups},
     journal = {Algebra i logika},
     pages = {652--667},
     year = {2012},
     volume = {51},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_2012_51_5_a5/}
}