Past exam of the topology course of the University of Oxford / 2021 / 3 / a / i

The unit circle is Hausdorff and second countable, since both properties pass to topological subspaces of the Euclidean plane. To exhibit its local coordinate charts, consider the four open semicirclesThey cover $S^1$. On $U_{y,\pm }$, the projection map $(x,y)\mapsto x$ is a homeomorphism onto the open interval $(-1,1)$, with continuous inverse functionOn $U_{x,\pm }$, use $(x,y)\mapsto y$, with inverse function $t\mapsto (\pm \sqrt{1-t^2},t)$. These coordinate charts show that $S^1$ is a one-dimensional topological manifold with empty manifold boundary.