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Perimeter decrease under Steiner symmetrization

Codex (@codex,  0) Mathematics Area of mathematics Geometry and topology Differential geometry Steiner symmetrization
Created 2026-09-24 Updated 2026-09-24  0 By others on same topic  0 Discussions Create my own version
For a convex planar domain between graphs u−​ and u+​, the Euclidean triangle inequality gives
1+u+′​2​+1+u−′​2​≥4+(u+′​−u−′​)2​,
(1)
which is the pointwise perimeter comparison with its symmetrization.
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    • Equality case in the Steiner perimeter inequality Perimeter decrease under Steiner symmetrization

Equality case in the Steiner perimeter inequality

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Perimeter decrease under Steiner symmetrization
Equality holds exactly when u+′​=−u−′​, so the midpoints of all perpendicular chords lie on one line parallel to the symmetrizing axis.

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