PUBLICATIONS DE L'INSTITUT MATHÉMATIQUE (BEOGRAD) (N.S.) Vol. 33(47), pp. 123131 (1983) 

TWO RESULTS ON ASSOCIATIVITY OF COMPOSITE OPERATIONS IN GROUPSSava Krsti\'cMatematicki institut SANU, Beograd, YugoslaviaAbstract: The theorem of Hanna Neumann ([{\bf 4}]) states that all associative operations $w(x,y)$ in the case of a free $G$ are of one of the following forms: $$ a, x, y, xay, yax, $$ where a is an arbitrary element of $G$. In the first part of this article we generalize this result. Theorem 1 shows that operations of the forms listed above are the only possible (except trivial cases) when we require $w(x,y)$ to satisfy not the associativity law, but any consequence of it (any weakened associativity law). \par In the second part of the article we determine all associative operations $w(x,y)$ in the case of $G$ free nilpotent of class two. Classification (MSC2000): 20A99 Full text of the article:
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