Let $\gamma :I \subset \mathbb{R} \rightarrow \mathbb{R} ^ 3 $an arc length parameterization of a smooth curve C.
If $\tau (s)= 0$ for all $s \in I $, C is contained in a plane?
And if also, the curvature is constant and none zero for all $s \in I $. Is C (or is contained in) a circle?
I don´t know how to prove it, Do I have to use the Frenet-Serret equations?