Geodesic
An estimate for the rank of the intersection of subgroups in free amalgamated products of two groups with normal finite amalgamated subgroup
Sbornik. Mathematics, Tome 204 (2013) no. 2, pp. 223-236

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We generalize the estimate for the rank of intersection of subgroups in free products of groups, proved earlier by Ivanov and Dicks (which is analogous to the Hanna Neumann inequality in free groups) to the case of free amalgamated products of groups with normal finite amalgamated subgroup. We also prove that the estimate obtained is sharp and cannot be further improved when the amalgamated product contains an involution. Bibliography: 11 titles.
Keywords: amalgamated free products, Hanna Neumann inequality. 
A. O. Zakharov. An estimate for the rank of the intersection of subgroups in free amalgamated products of two groups with normal finite amalgamated subgroup. Sbornik. Mathematics, Tome 204 (2013) no. 2, pp. 223-236. http://geodesic.mathdoc.fr/item/SM_2013_204_2_a3/
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