Hello,

I want to show that this function is smooth:

T is the Torus defined as: and R,r>0 constant.

I don't know how f could be differentiated. My Problem is, that f is not in the standard form f(x,y,z)=x. it is something like f((R+r.....))=(R+r*cosu)cosv.

I'm a little bit confused. Is here any trick or something

Thank you for your help

regards