i and j are constant, ignore them when you take derivatives.

和 j 是常矢量,求导的时候可以忽略它们

耶鲁公开课 - 基础物理课程节选

But the mathematicians told Dirac, "You cannot ignore the negative energy solution because it tells you there's a second solution and you cannot throw them out."

但数学家们告诉狄拉克,你不能忽略负的能量,因为数学告诉我有两个根,你不能随便丢掉

耶鲁公开课 - 基础物理课程节选