What does it mean when one uses a comma inside an expectation $\mathbb{E}[a, b]$?
This is in my lecture notes and so want to confirm what this notation may mean as it doesn't seem to be standard (from a google search anyways).
Edit:
Context:
Consider a continuous time random walk. Let the length of each hop be distributed by $l \sim \lambda$.
Assume $\lambda(l)$ is isotropic, then $\mathbb{E}[l_i, l_j] = \mathbb{E}[(l_i)^2] \delta_{ij}$
If anyone knows why the statement itself is true, then please share as that would be equally useful!