# A note on Hurwitz's inequality

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7 Citations (Scopus)

## Abstract

© 2017 Elsevier Inc. Given a simple closed plane curve Γ of length L enclosing a compact convex set K of area F, Hurwitz found an upper bound for the isoperimetric deficit, namely L2−4πF≤π|Fe|, where Fe is the algebraic area enclosed by the evolute of Γ. In this note we improve this inequality finding strictly positive lower bounds for the deficit π|Fe|−Δ, where Δ=L2−4πF. These bounds involve either the visual angle of Γ or the pedal curve associated to K with respect to the Steiner point of K or the L2 distance between K and the Steiner disk of K. For compact convex sets of constant width Hurwitz's inequality can be improved to L2−4πF≤[Formula presented]π|Fe|. In this case we also get strictly positive lower bounds for the deficit [Formula presented]π|Fe|−Δ. For each established inequality we study when equality holds. This occurs for those compact convex sets being bounded by a curve parallel to an hypocycloid of 3, 4 or 5 cusps or the Minkowski sum of this kind of sets.
Original language English 436-451 Journal of Mathematical Analysis and Applications 458 1 https://doi.org/10.1016/j.jmaa.2017.09.017 Published - 1 Feb 2018

## Keywords

• Convex set
• Evolute
• Hypocycloid
• Isoperimetric inequality
• Pedal curve
• Visual angle

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