We say a knot in the 3-sphere has {\it Property } if the infinite cyclic cover of the knot exterior embeds into . Clearly all fibred knots have Property . There are infinitely many non-fibred knots with Property and infinitely many non-fibred knots without property . Both kinds of examples are established here for the first time. Indeed we show that if a genus 1 non-fibred knot has Property , then its Alexander polynomial must be either 1 or , and we give two infinite families of non-fibred genus 1 knots with Property and having and respectively. Hence among genus one non-fibred knots, no alternating knot has Property , and there is only one knot with Property up to ten crossings. We also give an obstruction to embedding infinite cyclic covers of a compact 3-manifold into any compact 3-manifold.

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