function f {\displaystyle f} from A ⊆ C {\displaystyle A\subseteq \mathbb {C} } to B ⊆ C {\displaystyle B\subseteq \mathbb {C} } is called holomorphic in domain A {\displaystyle A} if for every open domain E ⊆ A {\displaystyle E\subseteq A} there exist derivative f ′ ( z ) ∀ z ∈ E {\displaystyle f'(z)~\forall ~z\in E} .
