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 F. Let r and R denote the radii of the incircle and the circumcircle. Bonnesen proved the inequality

: L^2-4pi Fgeq pi^2 (R-r)^2. ,

The term pi^2 (R-r)^2 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.

References

* Bonnesen, T.: "Sur une amélioration de l'inégalité isopérimetrique du cercle et la démonstration d'une inégalité de Minkowski," "C. R. Acad. Sci. Paris" 172 (1921), 1087–1089.
* Yu. D. Burago and V. A. Zalgaller, "Geometric inequalities". Translated from the Russian by A. B. Sosinskiĭ. Springer-Verlag, Berlin, 1988. ISBN 3-540-13615-0.


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