Fenchel's theorem

In differential geometry, Fenchel's theorem (Werner Fenchel, 1929) states that the average curvature of any closed convex plane curve is

$\frac{2 \pi}{P},$

where P is the perimeter. More generally, for an arbitrary closed curve in space the average curvature is $\ge \frac{2 \pi}{P}$ with equality holding only for convex plane curves.

References

W. Fenchel, Über Krümmung und Windung geschlossener Raumkurven, Math. Ann. 101 (1929), 238-252. [1]