Definition:Big-O Notation/Complex/Point
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Definition
Let $z_0 \in \C$.
Let $f$ and $g$ be complex functions defined on a punctured neighborhood of $z_0$.
The statement:
- $\map f z = \map \OO {\map g z}$ as $z \to z_0$
is equivalent to:
- $\exists c \in \R_{\ge 0}: \exists \delta \in \R_{>0}: \forall z \in \C : \paren {0 < \cmod {z - z_0} < \delta \implies \cmod {\map f z} \le c \cdot \cmod {\map g z} }$
That is:
- $\cmod {\map f z} \le c \cdot \cmod {\map g z}$
for all $z$ in a punctured neighborhood of $z_0$.