Geodesic
On a Theorem of Cohen and Lyndon About Free Bases for Normal Subgroups
Canadian journal of mathematics, Tome 24 (1972) no. 6, pp. 1086-1091

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Let S(≠1) be a subgroup of a group G. We consider the question: when are the conjugates of S “as independent as possible“? Specifically, suppose SG (the normal subgroup generated by S in G) is the free product II*S0α where and gα ranges over a subset J of G. Then J must be part of a (left) coset representative system for G mod SG . N where N is the normalizer of S in G. (For, g ∊ SGgαN implies Sg is conjugate to Sgα in SG; however, distinct non-trivial free factors of a free product are never conjugate.)We say that SG is the free product of maximally many conjugates of S in G if SG = II*Sgα where gα ranges over a (complete) left coset representative system for G mod SGN (or equivalently, gα ranges over a double coset representative system for G mod (SG, N)); in this case we say briefly that S has the fpmmc property in G. 
Karrass, A.; Solitar, D. On a Theorem of Cohen and Lyndon About Free Bases for Normal Subgroups. Canadian journal of mathematics, Tome 24 (1972) no. 6, pp. 1086-1091. doi: 10.4153/CJM-1972-112-0
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