Angus Nicoll of the European Center for Disease Prevention says that at this point no one knows whether the virus will become more or less virulent in the months ahead.

VOA: standard.2009.04.30

So even though we see a nodal plane down the center, I just want to really point out that it's only when we have a nodal plane in the internuclear or the bond axis that we're calling that a pi orbital.

虽然在中间有个节面，我想要指出的是,只有节面在核间轴,或者键轴上时,我们才叫它π轨道。

麻省理工公开课 - 化学原理课程节选

er is a vector at each point of length one pointing radially away from the center.

r 是一个模长恒为 1 的矢量,方向沿半径向外

耶鲁公开课 - 基础物理课程节选