Bonnesen's inequality
# Bonnesen's inequality

Bonnesen's inequality is an inequality relating the length, the area, the radius of the incircle and the radius of the circumcircle of a Jordan curve. It is a strengthening of the classical isoperimetric inequality.

More precisely, consider a planar simple closed curve of length $L$ bounding a domain of area $A$. Let $r$ and $R$ denote the radii of the incircle and the circumcircle. Bonnesen proved the inequality

$\pi ^{2}(R-r)^{2}\leq L^{2}-4\pi A.$

The term $L^{2}-4\pi A$ in the right hand side is known as the isoperimetric defect.

Loewner's torus inequality with isosystolic defect is a systolic analogue of Bonnesen's inequality.

