
doi: 10.4036/iis.2003.23
Let \(F\) be a closed, orientable surface of genus \(g\), endowed with a metric of constant curvature and denote by \({\mathcal G}_g\) the set of closed geodesics on \(F\). Closed geodesics are considered as images of periodic maps from \(\mathbb{R}\) into \(F\). When \(F=S^2\), then \({\mathcal G}_0\) inherits a topology by the natural bijection between the set of the great circles on \(S^2\) and \(\mathbb{R} P^2\). This topology can be described as the one having the set \({\mathcal U}_\varepsilon(c)= \{c'\in{\mathcal G}_0\mid \delta_{c,\varepsilon} (c')\geq\text{length} (c)\}\), \(\varepsilon>0\), as fundamental system of neighborhoods of any \(c\in{\mathcal G}_0\). Here, \(\delta_{c,\varepsilon} (c')\) is defined as the length of an arc obtained as intersection of \(c'\in{\mathcal G}_0\) with the \(\varepsilon\)-neighborhood of \(c\) on \(S^2\). If \(F\) has genus \(g\geq 1\), the author describes a topology \({\mathcal T}\) on \({\mathcal G}_g\), slightly modifying the previous description passing through the universal covering of \(F\). He proves that the so obtained topology is Hausdorff when \(g=1\), \(F\) being the torus \(T^2\) with a flat metric, whereas it is not Hausdorff when \(g>1\), \(F\) being a surface with a fixed hyperbolic metric. Moreover, in the last case, one can consider the set \({\mathcal G}^0_g\) of all geodesics on \(F\), dropping the closing condition, and topologize it as follows: The preimage in \(\mathbb{H}^2\) of a geodesic on \(F\) is a set of geodesics which is invariant with respect to the covering transformation group \(\Gamma\). Then \({\mathcal G}^0_g\) is identified with the quotient space \(((S^1_\infty\times S^1_\infty -\Delta)/ \mathbb{Z}_2)/\Gamma\), where \(S^1_\infty\) is the sphere at infinity of \(\mathbb{H}^2\) and \((S^1_\infty\times S^1_\infty-\Delta)/ \mathbb{Z}_2\) is homeomorphic to an open Möbius strip. The author proves that the relative topology induced from \({\mathcal G}^0_g\) on \({\mathcal G}_g\) is equivalent to \({\mathcal T}\) and this implies that the space \({\mathcal G}^0_g= ((S^1_\infty\times S^1_\infty-\Delta)/ \mathbb{Z}_2)/ \Gamma\) is not a Hausdorff space for \(g>1\).
Manifolds of mappings, Hausdorff space, Function spaces in general topology, Geodesics in global differential geometry, closed geodesic, hyperbolic surface
Manifolds of mappings, Hausdorff space, Function spaces in general topology, Geodesics in global differential geometry, closed geodesic, hyperbolic surface
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