# Math Help - Coordinate Charts and inverse of functions.

1. ## Coordinate Charts and inverse of functions.

Hello,

i have a question about this situation. Let denote $T^2=S^1 \times S^1$ the Torus.
And let $f:\mathbb{R}^2 \rightarrow T^2$ be the projection defined by
$f(x,y)=(e^{i2\pi x},e^{i2\pi y})$.

The claim is now, if $\phi$ is a chart for $\mathbb{R}^2$ , s.t. $f_{|dom\phi}$ is injective, then $\phi \circ (f_{|dom\phi})^{-1}$ is a chart for $T^2$.

i don't understand why the composition is a chart?

Can you help me?

Regards

2. As a chart means it's a 1-1 diffeomorphism in the domain that defined the map. Let V=dom(\phi), U=f(V), W=\phi^{-1}(V)
Since f is injective in V, f is a 1-1 diffeomorphism between V and f(V).
And \phi is defined to be a 1-1 diffeomorphism between W and \phi(W)=V.
Compose two 1-1 diffeomorphism we get a 1-1 diffeomorphism.