The necessary and sufficient condition的用法和樣例：
例句
- The author of this article finds a necessary and sufficient condition of the independece of random variables by using the three lemmas.
利用三個引理，作者找到隨機變數獨立性的一個充要條件。 - Necessary and suffcient conditions for the existence of such strategies are established.
建立了關於這類策略存在的充要條件。
