Suppose z is holomorphic in a bounded domain, continuous on the closure, and constant on the boundary. Prove that f must be constant throughout the domain.
Given , z is holomorphic in a bounded domain, continuous on the closure, and constant on the boundary .
By Maximum moduler principal moduler principal every holomorphic function attains its maximum and minimum on its boundary .
Now since the function is constant on its boundary so suppose ,
f (z) = c on its boundary where c is a constant .
by maximum moduler principal maximum of the function is c and also minimum is c
for all z belongs to the domain .
f(z) = c for all z belongs to the domain .
Hence f must be constant throughout the domain .
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