# Chart coordinate maps of topological manifolds

## Main Question or Discussion Point

Hello every one .
first of all consider the 2-dim. topological manifold case
My Question : is there any difference between
$$f \times g : R \times R \to R \times R$$
$$(x,y) \to (f(x),g(y))$$
and $$F : R^2 \to R^2$$
$$(x,y) \to (f(x,y),g(x,y))$$
Consider two topological manifolds the torus $T^2$ and the euclidean plane $R^2$ such that both of them for any point (p) that lies on each manifold can be represented by an ordered pair $p=(a,b)$
Since $R^2 = R \times R$
and $T^2 = S^1 \times S^1$
and consider chart maps $X(p)$ for the two manifolds separately such that
$X : U \to D$
for $M=R^2 , U \subset R^2 and D \subset R^2$ same for $T^2$
for the case of $R^2$
$$M=R^2={ (a,b) \in R^2 }$$
choose $U \subset R^2$ and $D \subset R^2$
and say that $$X : U \to D$$
$$(a,b) \to (f(a,b),g(a,b))$$
but for the torus $T^2$ we consider the notion of PRODUCT MANIFOLD
$$M=T^2={(a,b) \in T^2 : T^2 = S^1 \times S^1}$$
choose $U \subset S^1 , D \subset R$
such that $$X : U_1 \to D$$
$$a \to X(a)$$ same for the other circle with $Y$ chart map
now consider the Cartesian product of the two chart maps $$X \times Y$$
$$X \times Y : U_1 \times U_2 \to D \times D$$
$$(a,b) \to (f(a),g(b))$$
for the manifold being the euclidean plane $(R^2)$we used the chart map such that each coordinate is a function of the point which is function of two variables $(a,b)$ and for the manifold being the torus $(T^2)$ we used the chart map such that each coordinate is function of each point which is function of single variable .
The question is , can we use the method or the notion of PRODUCT MANIFOLD for the Euclidean Plane (Since $R^2 = R \times R$ ) same as it was used for the Torus $T^2$ ?
Example for some chart maps for both the manifolds
$M=R^2$
$$(a,b) \to (f(a,b),g(a,b))$$
$$(a,b) \to (\sqrt {a^2 + b^2} , \tan^{-1}(\frac a b))$$
for $U=\{(a.b) \in R^2 : a \gt 0 \}$ and $D=\{(f(a,b),g(a,b)) \in R^2 :f(a,b) \gt 0 ,0 \gt g(a,b) \lt \pi \}$
and for $M=T^2$ if we consider the configuration space of the double pendulum being the Torus $T^2$ such that $$(a,b) \mapsto (f(a),g(b))$$
$$(a,b) \mapsto (\sin(a) , \sin(b))$$
for $U=\{(a,b) \in T^2 : 0 \gt a \lt \frac{\pi }{2} , 0 \gt b \lt \frac{\pi }{2} \}$

Thanks.

<Moderation note: fixed some minor LaTex errors to improve readability>

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