@article{AL_1993_32_2_a3,
     author = {N. A. Peryazev},
     title = {Positive theories of free monoids},
     journal = {Algebra i logika},
     pages = {148--159},
     year = {1993},
     volume = {32},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1993_32_2_a3/}
}