Behnke–Stein theorem
In mathematics, especially several complex variables, the Behnke–Stein theorem states that a union of an increasing sequence G k ⊂ C n {\displaystyle G_{k}\subset \mathbb {C} ^{n}} (i.e., G k ⊂ G k + 1 {\displaystyle G_{k}\subset G_{k+1}} ) of domains of holomorphy is again a domain of holomorphy. It was proved by Heinrich Behnke and Karl Stein in 1938.