# Defining $S^1$ with four charts

Hello,

I am trying to implement the construction of the manifold $S^1$ using the four charts, as described at en.wikipedia.org/wiki/Manifold#Circle . It's not clear to me how to define the charts in terms of the projection operators within SageMath. I'm also not sure how to isolate open sets that I would like to define the charts over. The circle is parameterized using polar coordinates, and the four charts have the form:

$
\begin{align*}
\varphi_1(r, \theta) &= \theta && \theta \in (-\frac{\pi}{3}, \frac{\pi}{3}) \
\tag{1}
\end{align*}
$

$
\begin{align*}
\varphi_2(r, \theta) &= \theta && \theta \in (\frac{\pi}{6}, \frac{5\pi}{6}) \
\tag{2}
\end{align*}
$

$
\begin{align*}
\varphi_3(r, \theta) &= \theta && \theta \in (\frac{2\pi}{3}, \frac{4\pi}{3}) \
\tag{3}
\end{align*}
$

$
\begin{align*}
\varphi_4(r, \theta) &= \theta && \theta \in (\frac{7\pi}{6}, \frac{11\pi}{6}) \
\tag{4}
\end{align*}
$

Once we have these charts, how would I define the four transition functions to complete the description?

Thank you for your help!

Note: also discussed on the sage-manifolds list.