# Holomorphically convex hull

In mathematics, more precisely in complex analysis, the holomorphically convex hull of a given compact set in the n-dimensional complex space Cn is defined as follows.

Let $G \subset {\mathbb{C}}^n$ be a domain (an open and connected set), or alternatively for a more general definition, let $G$ be an $n$ dimensional complex analytic manifold. Further let ${\mathcal{O}}(G)$ stand for the set of holomorphic functions on $G.$ For a compact set $K \subset G$, the holomorphically convex hull of $K$ is

$\hat{K}_G := \{ z \in G \big| \left| f(z) \right| \leq \sup_{w \in K} \left| f(w) \right| \mbox{ for all } f \in {\mathcal{O}}(G) \} .$

(One obtains a narrower concept of polynomially convex hull by requiring in the above definition that f be a polynomial.)

The domain $G$ is called holomorphically convex if for every $K \subset G$ compact in $G$, $\hat{K}_G$ is also compact in $G$. Sometimes this is just abbreviated as holomorph-convex.

When $n=1$, any domain $G$ is holomorphically convex since then $\hat{K}_G$ is the union of $K$ with the relatively compact components of $G \setminus K \subset G$. Also note that being holomorphically convex is the same as being a domain of holomorphy (The Cartan–Thullen theorem). These concepts are more important in the case n > 1 of several complex variables.