necessary and suffieient condition

• 充要条件

necessary and suffieient condition的用法和样例：

例句

1. Necessary and suffcient conditions for the existence of such strategies are established.
   建立了关于这类策略存在的充要条件。
2. The author of this article finds a necessary and sufficient condition of the independece of random variables by using the three lemmas.
   利用三个引理，作者找到随机变量独立性的一个充要条件。