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(gè)引理，作者找到隨機變量獨立性的一個(gè)充要條件。 - Necessary and suffcient conditions for the existence of such strategies are established.
建立了關(guān)于這類(lèi)策略存在的充要條件。
