定义
已知
- 函数 $f$ 和函数 $g$
则 $f\in \Omega(g)$ :=
$$ \exists C>0 \ \exists k \ \forall x>k \ (\left|f(x)\right| \geq C\left|g(x)\right|) $$
aka
$$ \exists C(C>0 \land \exists k \forall x(x>k \to \left|f(x)\right| \geq C\left|g(x)\right|)) $$
性质
- 若 $f_{1}\in O(g_{1}) \land f_{2}\in\Omega(g_{2})$ 则 $f_{1}/f_{2} \in O(g_{1}/g_{2})$