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On the isoperimetric deficit upper limit. (English) Zbl 1266.52014

In this paper, reverse Bonnesen style inequalities are discussed. Namely, given a convex domain \(K\) in the Euclidean plane \(E^2\), let \(P\), \(A\), \(r_I\), \(r_E\), \(d\) stand for the perimeter, the area, the in-radius, the circum-radius and the diameter of \(K\) respectively. Then the following inequalities hold true for the isoperimetric deficit \(\Delta_2(K) = P^2-4\pi A\) of \(K\): \[ \Delta_2(K) \leq 2\pi P\left(r_E-r_I\right), \quad \Delta_2(K) \leq 2\pi P\left(\frac{d}{2}-r_I\right), \]
\[ \Delta_2(K) \leq \frac{\pi P^2}{A}\left(r_E^2-r_I^2\right),\quad \Delta_2(K) \leq 4\pi^2\left(r_E^2-r_I^2\right), \]
\[ \Delta_2(K) \leq \frac{\pi P^2}{A}\left(\frac{d^2}{4}-r_I^2\right),\quad \Delta_2(K) \leq 4\pi^2\left(\frac{d^2}{4}-r_I^2\right). \] Moreover, the equalities hold true if and only if \(K\) is a disc.

MSC:

52A40 Inequalities and extremum problems involving convexity in convex geometry
52A10 Convex sets in \(2\) dimensions (including convex curves)
52A22 Random convex sets and integral geometry (aspects of convex geometry)
52A38 Length, area, volume and convex sets (aspects of convex geometry)