pi
abbr. 产品改进（product improve）；预估发票，形式发票（proforma invoice）；首席研究员（principal investigator）
例句
The key is the stylesheet processing instruction (PI) in the second line.
关键是第二行中的样式表处理指令（PI）。
Zu was best known for his calculation of pi (π).
祖以计算 pi（Π）而闻名。
In order to remember Zu, some mathematicians suggest calling pi "Zu Lv".
为了纪念祖，一些数学家建议称圆周率为“祖率”。
