# The knowledge of $n=n(s)$ can be used to determine the curvature $k(s)$ and the torsion $\tau (s)$

Question:

Show that the knowledge of the vector function $n=n(s)$ of a curve $\alpha:I\rightarrow \mathbb{R^3}$ with nonzero torsion everywhere, determines the curvature $k(s)$ and the torsion $\tau (s)$ of $\alpha$.

Notes: $n$ is the normal versor to $\alpha$.

Attempt: I tried using Frenet-Serret formulas, and then using the vector product between $n$ and $n'$, but it seems like I can't get to any result.

The Question as posed here is not solvable (as is indicated by the comments in the answer above). One needs the knowledge of the function $$\frac{\kappa}{\tau}$$ at one point $$t_0$$.

Lets give a counterexample:

Consider the Helix

$$c(s) := (a \cdot \cos(s) , a \cdot \sin(s) , b \cdot s) \text{ for } s \in \mathbb{R}$$

with $$a^2 +b^2 = 1$$, $$a,b>0$$. Then $$c(s)$$ ist parametrized by arclength.

The normal vector is given by

$$n(s) = (- \cos(s) , -\sin(s), 0) \text{ for all } s\in \mathbb{R}.$$

One has in general $$\kappa=a, \tau= -b$$ and $$\frac{\kappa}{\tau}=-a/b$$.

The choices $$a_1=b_1= 1/\sqrt{2}$$ and $$a_2= 1/2 ,b_2= \sqrt{3}/2$$ give two different curves parametrized by arclength. Both curves have the same normal vector for all times and non-vanishing torsion. And both curves have different curvature and torsion.

• Nice counterexample. Sep 26, 2021 at 5:35

Hint: This is rather a long calculation. If you need any more steps feel free to ask.

The idea though is to show that $$\frac{(n \wedge n') \cdot n''}{|n'|^2} = \frac{(\frac{\kappa}{\tau})'}{(\frac{\kappa}{\tau})^2 + 1} : = a (s)$$

Then,

$$\int a(s) ds = \arctan \left(\frac{\kappa}{\tau}\right)$$

Thus $\kappa/\tau$ can be determined, plus we have that $\kappa > 0$ then we may get the sign of $\tau$. Finally, use

$$|n'|^2 = |- \kappa t - \tau b|^2 = \kappa ^2 + \tau^2$$

to determine $\kappa^2$ and $\tau^2$.

• ok everything is clear, but aren't you ignoring the costant when you integrate $a(s)$ ? Feb 12, 2016 at 19:09
• This is a skecth of the solution. What do you think? Feb 12, 2016 at 19:12
• It seems to work for me, the only problem it's I can't understand why you can just ignore the integration constant. Feb 12, 2016 at 19:17
• It's a constant, you will still be able to find $\frac{\kappa}{\tau}$. Plus, you may choose a "chunk" of the curve to integrate (definite integral) over. The main idea though is to find $\kappa$ and $\tau$ in terms of $n(s)$. Feb 12, 2016 at 19:22
• @AaronMaroja I won't be able to find $\frac{\kappa}{\tau}$ with the constant, i.e; estimate $\frac{\kappa}{\tau}$ in the equation $\int_{a}^{t} a(s)ds= \arctan\left( \frac{\kappa(t)}{\tau(t)}\right) −\arctan \left( \frac{\kappa (a)}{\tau (a)} \right)$. Can you explain how to do this manipulation? Aug 13, 2017 at 23:00