= Polynomial hull in one complex variable
{title2=$\widehat K$}
For a <compact set> $K\subset\mathbb C$, its polynomial hull is $\widehat K=\{z:|P(z)|\le\sup_K|P|\text{ for every polynomial }P\}$. In one complex variable this is $K$ together with all bounded components of its complement. The <maximum modulus principle> proves inclusion of each filled hole. Conversely, polynomial approximation of an exterior pole by the <Runge approximation theorem> separates an exterior point from the filled compact set. The filled set has connected complement, which makes it the natural set for polynomial approximation of functions whose poles lie in the unbounded complementary component.
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