I came across a paper that uses the abbreviation "p.e.":
Khatri and Mardia, The Von Mises-Fisher Matrix Distribution in Orientation Statistics. 1976.
It's in Section 7 on page 105. I'm including a short excerpt:
Using the density function of $K^{\frac{1}{2}}XX'K^{\frac{1}{2}} = S$ given in Khatri (1975a), we find that the p.e. $X$ given $XX'=I_n$ is
What does the abbreviation "p.e." mean in this sentence?