Harmonic mean value and maximum principle
A harmonic u equals its average on every circle whose closed disk lies in its domain.
Key knowledge
A harmonic u equals its average on every circle whose closed disk lies in its domain. On a connected domain, an interior maximum or minimum forces u constant. On a bounded domain with continuous boundary extension, extrema occur on the boundary; applying this to differences gives uniqueness of the Dirichlet solution.
Conditions and scope
- u is C² and harmonic; the circle's radius is positive.
- Uniqueness is for solutions continuous on the same compact closure and with the same boundary values.
Sources · 2
References
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Evidence and review
Section 5.5 harmonic mean-value and maximum/minimum properties; difference argument for uniqueness, complemented by UCSD Theorem 19.1.
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Entry details
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