Projectivities of free products
Rendiconti del Seminario Matematico della Università di Padova, Tome 42 (1969), pp. 341-387. 
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     pages = {341--387},
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     volume = {42},
     year = {1969},
     mrnumber = {268286},
     zbl = {0245.20031},
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     url = {http://archive.numdam.org/item/RSMUP_1969__42__341_0/}
}
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Holmes, Charles S. Projectivities of free products. Rendiconti del Seminario Matematico della Università di Padova, Tome 42 (1969), pp. 341-387. http://archive.numdam.org/item/RSMUP_1969__42__341_0/

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[3] Sadovsky, E.L. (ALSO SADOVSKI, SADOVSKII): a) Structural isomorphisms of free groups and free products, Mat. Sbornik 14 (56) (1944) 145-173. b) On structural isomorphism of free products of groups, Mat. Sbornik 21 (63) (1947), 63-82. | MR

[4] Suzuki, M.: Structure of a Group and the Structure of its Lattice of Subgroups, Springer Verlag, Berlin, 1956. | MR | Zbl