How should I notate a set of probability distributions? So I'm writing up a computer science paper. I want to notate that I'm estimating joint degree distributions for a bunch of graphs I am generating. I have seen joint degree distributions notated as $e_{ij}$ in other papers (where $i$ and $j$ are the degrees). I was thinking of writing something like "let $e_{ij}$ be the estimated joint degree distribution where $e_{ij} \in E$, $E$ is the set of distributions for each graph and $i$ and $j$ are the degrees". But to me this looks confusing. At first glance it might look like $e_{ij}$ is an element within matrix $E$ or something like that. Perhaps I could replace $e_{ij}$ with $p_{IJ}$ or something like that? Any ideas? My maths knowledge isn't that good.

As explained by @kccu, using $e_{i,j}$ would be a rather poor choice. This often denotes the edges between vertices $i$ and $j$. 
I would use a capital "blackboard bold" P : $\mathbb{P}$, and hence $\mathbb{P}_{i,j}$
