Abstract
LetD be a bounded domain in the complex plane whose boundary consists of finitely many pairwise disjoint simple closed curves. GivebD the standard orientation, and letA(D) be the algebra of all continuous functions on\(\bar D\) which are holomorphic onD. We prove that a continuous functionf onbD extends to a function inA(D) if and only if for eachg∈A(D) such thatf+g≠0 onbD, the change of argument off+g alongbD is nonnegative.
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Globevnik, J. The argument principle and holomorphic extendibility. J. Anal. Math. 94, 385–395 (2004). https://doi.org/10.1007/BF02789056
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DOI: https://doi.org/10.1007/BF02789056