1987
DOI: 10.1017/s0004972700026393
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A note on groups with separable finitely generated subgroups

Abstract: An example is given of an infinite cyclic extension of a free group of finite rank in which not every finitely generated subgroup is finitely separable. This answers negatively the question of Peter Scott as to whether in all finitely generated 3-manifold groups the finitely generated subgroups are finitely separable. In the positive direction it is shown that in knot groups and one-relator groups with centre, the finitely generated normal subgroups are finitely separable.

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Cited by 57 publications
(74 citation statements)
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“…One is derived from the recent work of Rubinstein and Wang, [6], and we consider it in Theorem 1. The other was the first known example of a 3-manifold group which failed to be subgroup separable and was introduced in [1] and further studied in [4] and [5]. Our proof that it fails to satisfy the engulfing property is more elementary than the original proof that it fails subgroup separability, and we hope that it sheds some light on this fact.…”
Section: Introductionmentioning
confidence: 87%
“…One is derived from the recent work of Rubinstein and Wang, [6], and we consider it in Theorem 1. The other was the first known example of a 3-manifold group which failed to be subgroup separable and was introduced in [1] and further studied in [4] and [5]. Our proof that it fails to satisfy the engulfing property is more elementary than the original proof that it fails subgroup separability, and we hope that it sheds some light on this fact.…”
Section: Introductionmentioning
confidence: 87%
“…This raises the question of which 3-manifolds have LERF fundamental group. There are examples of certain graph 3-manifolds M for which π 1 (M ) is not LERF [10]. But it is conjectured that the fundamental group of every closed hyperbolic 3-manifold is LERF.…”
Section: Subgroup Separability Special Cube Complexes and Virtual Fimentioning
confidence: 99%
“…The first example is ht; a; b j tat 1 D ab; tbt 1 D bi due to Burns, Karrass and Solitar in [24] and is also famous for being the first 3-manifold group known not to be LERF. Also Corollary 3 of [55] gives further examples.…”
Section: Theorem 31 If G Is Lerf and Of Deficiency 1 Then Either G mentioning
confidence: 99%