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@article{AL_1966_5_4_a2, author = {Yu. I. Merzlyakov}, title = {Positive formulae on free groups}, journal = {Algebra i logika}, pages = {25--42}, publisher = {mathdoc}, volume = {5}, number = {4}, year = {1966}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/AL_1966_5_4_a2/} }
Yu. I. Merzlyakov. Positive formulae on free groups. Algebra i logika, Tome 5 (1966) no. 4, pp. 25-42. http://geodesic.mathdoc.fr/item/AL_1966_5_4_a2/