# Relation between simple closed curve with curvature smaller than radius circle

Let $$\alpha :[0, l] \rightarrow \mathbb{R}^{2}$$ parameterize a simple closed curve by arc length. Suppose that there is a constant $$R$$ such that the curvature of $$\alpha$$ satisfies $$0<\kappa_{\sigma}(s) \leq R$$. Prove that $$\text{length}({\alpha}) \geq \frac{2 \pi}{R}.$$

Hint: Note that the hypothesis say that the curve is curved less than the circle of radius $$\frac{1}{R},$$ then its length is greater than the length of the circle.

That's the problem. I was thinking about using the theorem which says that the length of a curve C in $$\mathbb{R}^2$$ has length $$\frac{1}{4}\int_{0}^{2\pi}length(P_\sigma(C))d\sigma,$$ where $$P_\sigma(C)$$ is the projection of the curve. The problem is that I don't know how to get a relation between the curvature and the projection. Maybe I am completely wrong and there is another way to solve the problem. Any ideas?

• Hint: $\kappa_{\alpha}(s) = \left| \frac{d\theta}{ds}\right|$ where $\theta$ is the angle between the tangent vector at $\alpha(s)$ and $x$-axis. What is $\int_0^l \frac{d\theta}{ds} ds$? – achille hui Sep 14 '19 at 4:22
• Awesome, now it's done Thank you so much, really. I will post the completely answer just in case someone need it one day – vicase98 Sep 14 '19 at 13:59

Here is a different solution: By the Hopf Umlaufsatz, $$\int_C\kappa\,ds = 2\pi$$. Now use $$\kappa\le R$$ to get $$2\pi \le R\,\text{length}(\alpha)$$.