yield，我觉得应该

应该是yield.

Y stands for Income, which is I.
However, I is often confused for I as in oneself so Y is simply used instead

By the way, when talking about budget constraint, we often use M instead of Income.

我觉得y不代表英文吧，应该是函数关系中因变量习惯用y表示而已。

yield