**A. Pereira do Vale & M. R. Pinto (ed.),
**

Proceedings of the 1st International Meeting on Geometry and Topology.

Proceedings of the conference held in Braga, September 11-13, 1997.

Universidade do Minho, Portugal

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The parallel group of a plane curve

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F.J.Craveiro de Carvalho and S.A.Robertson

Departamento de Matematica, Universidade de Coimbra, 3000 Coimbra, Portugal fjcc@mat.uc.pt
Faculty of Mathematical Studies, University of Southampton, Southampton SO17 1BJ, United Kingdom sar@maths.soton.ac.uk

**Abstract:** For any smooth immersion $f$ of the circle in the plane, the parallel group $P(f)$ consists of all self-diffeomorphisms of the circle such that the normal lines at points of each orbit are parallel. The action of $P(f)$ on $S^1$ cannot be transitive. Thus, for example, $P(f)\neq SO(2)$. We construct examples where $P(f)$ contains a subgroup isomorphic to the group of self-diffeomorphisms of a closed interval (fixing the end-points), is isomorphic to the cyclic group $** Z**_n$ for any $n\epsilon ** N**$, and to the dihedral group $D_{n}$, for any $n\epsilon ** N**$. If the curvature of $f$ is nowhere zero, however, then $P(f)$ is cyclic of even order.

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