Let be an Open Disk, and let be a Harmonic Function on such that for
all . Then for all , we have
See also Liouville's Conformality Theorem
References
Flanigan, F. J. ``Harnack's Inequality.'' §2.5.1 in
Complex Variables: Harmonic and Analytic Functions. New York: Dover, pp. 88-90, 1983.