Estermann measure
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In plane geometry the Estermann measure is a number defined for any bounded convex set describing how close to being centrally symmetric it is. It is the ratio of areas between the given set and its smallest centrally symmetric convex superset. It is one for a set that is centrally symmetric, and less than one for sets whose closure is not centrally symmetric. It is invariant under affine transformations of the plane.[1]
If is the center of symmetry of the smallest centrally-symmetric set containing a given convex body , then the centrally-symmetric set itself is the convex hull of the union of with its reflection across .[1]
Minimizers
The shapes of minimum Estermann measure are the triangles, for which this measure is 1/2.[1][2] The curve of constant width with the smallest possible Estermann measure is the Reuleaux triangle.[3]